Temporary Circulatory Support

	INTRODUCTION

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The expansion of indications for circulatory support, development of better support devices, and improved results mandate that all surgeons acquire an understanding of the circulatory support devices currently available. Studies show that even smaller facilities that do not have cardiac transplantation may have improved patient survival if a device can be rapidly implemented and the patient transferred to a tertiary facility with expanded capabilities.⁶ In this chapter we describe the devices currently available, and discuss indications for use, patient management considerations, and the overall morbidity and mortality associated with temporary mechanical support. The goal of the use of temporary assist devices is to achieve improved function of the native heart allowing for removal of the device. If recovery is unlikely, then transition to heart transplantation (Ch. 60) or a chronic assist device (Chs. 62 and 63) may be the only solution for achieving long-term survival.

	COUNTERPULSATION

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Historical Notes

The concept of increasing coronary blood flow by retarding the systolic pressure pulse was demonstrated by Kantrowitz and Kantrowitz in 1953 in a canine preparation and again by Kantrowitz and McKinnon in 1958 using an electrically stimulated muscle wrap around the descending thoracic aorta to increase diastolic aortic pressure.^7–9 In 1961 Clauss et al used an external counterpulsation system synchronized to the heart beat to withdraw blood from the femoral artery during systole and reinject it during diastole.¹⁰ One year later Moulopoulos, Topaz, and Kolff produced an inflatable latex balloon that was inserted into the descending thoracic aorta through the femoral artery and inflated with carbon dioxide.¹¹ Inflation and deflation were synchronized to the electrocardiogram to produce counterpulsation that reduced end-systolic arterial pressure and increased diastolic pressure. In 1968 Kantrowitz reported survival of one of three patients with postinfarction cardiogenic shock refractory to medical therapy using an intra-aortic balloon pump.¹² These pioneering studies introduced the concept of supporting the failing circulation by mechanical means. Currently intra-aortic balloon counterpulsation is used in an estimated 70,000 patients annually.⁸

Physiology

The major physiologic effects of the intra-aortic balloon pump (IABP) are reduction of left ventricular afterload and an increase in aortic root and coronary perfusion pressure.^13–15 Important related effects include reduction of left ventricular systolic wall tension and oxygen consumption, reduction of left ventricular end-systolic and diastolic volumes, reduced preload, and an increase in coronary and collateral vessel blood flow.^16–19 Cardiac output increases because of improved myocardial contractility owing to increased coronary blood flow and the reduced afterload and preload, but the IABP does not directly move or significantly redistribute blood flow.^20,21 IABP reduces peak systolic wall stress (afterload) by 14% to 19% and left ventricular systolic pressure by approximately 15%.^16,20,22,23 Since peak systolic wall stress is related directly to myocardial oxygen consumption, myocardial oxygen requirements are reduced proportionately.^24–26 Coronary blood flow is subject to autoregulation, and in experimental animals the IABP does not increase flow until hypotension reduces flow to less than 50 mL/100 g ventricle/min.¹⁴ However, as measured by echocardiography and color flow Doppler mapping, peak diastolic flow velocity increases by 117% and the coronary flow velocity integral increases 87% with counterpulsation.²⁷ Experimentally, collateral blood flow to ischemic areas increases up to 21% at mean arterial pressures higher than 190 mm Hg.²⁸

Several variables affect the physiologic performance of the IABP. The position of the balloon should be just downstream to the left subclavian artery (Fig. 17-1). Diastolic augmentation of coronary blood flow increases with proximity to the aortic valve.^29,30 The balloon should fit the aorta so that inflation nearly occludes the vessel. Experimental work indicates that for adults balloon volumes of 30 or 40 mL significantly improve both left ventricular unloading and diastolic coronary perfusion pressure over smaller volumes. Inflation should be timed to coincide with closure of the aortic valve, which for clinical purposes is the dicrotic notch of the aortic blood pressure trace (Fig. 17-2). Early inflation reduces stroke volume, increases ventricular end-systolic and diastolic volumes, and increases both afterload and preload. Diastolic counterpulsation is visualized easily as a pressure curve in the arterial waveform and indicates increased diastolic perfusion of the coronary vessels (and/or bypass grafts).^31,32 Deflation should occur as late as possible to maintain the duration of the augmented diastolic blood pressure, but it must happen before the aortic valve opens and the ventricle ejects. For practical purposes deflation is timed to occur with the onset of the electrocardiographic R-wave. Active deflation of the balloon creates a suction effect that acts to decrease left ventricular afterload (and therefore myocardial oxygen consumption).

View larger version (57K): [in this window] [in a new window]   FIGURE 17-1 (A) Balloon inflation during left ventricular (LV) diastole occludes the descending thoracic aorta, closes the aortic valve, and increases proximal coronary and cerebral perfusion. (B) Balloon deflation during LV systole decreases LV afterload and myocardial oxygen demand.

View larger version (15K): [in this window] [in a new window]   FIGURE 17-2 Illustration showing the effect of the intra-aortic balloon on aortic pressure. After ejection produces the pulse (A), inflation of the balloon increases aortic diastolic pressure (B). At end diastole, sudden deflation reduces aortic end-diastolic pressure (C) below that of an unassisted beat and reduces afterload and myocardial oxygen demand.

By far the most important biological variables are heart rate and rhythm. Optimal performance requires a regular heart rate with an easily identified R-wave or a good arterial pulse tracing with a discrete aortic dicrotic notch. Current balloon pumps trigger off the electrocardiographic R-wave or from the arterial pressure tracing. Both inflation and deflation are adjustable, and operators attempt to time inflation to coincide with closure of the aortic valve and descent of the R-wave. During tachycardia the IABP usually is timed to inflate every other beat; during chaotic rhythms the device is timed to inflate in an asynchronous fixed mode that may or may not produce a mean decrease in afterload and an increase in preload. In unstable patients every effort is made to establish a regular rhythm, including a paced rhythm, so that the IABP can be timed properly.

Indications

The traditional indications for insertion of the intra-aortic balloon pump are cardiogenic shock, uncontrolled myocardial ischemic pain, and postcardiotomy low cardiac output.^33–36 In recent years indications for IABP have broadened to include patients with high-grade left main coronary artery stenosis, high-risk or failed percutaneous transluminal coronary angioplasty, atherectomy, or stents; poorly controlled ventricular arrhythmias before or after operation; and patients with postinfarction ventricular septal defect or acute mitral insufficiency after myocardial infarction.^37–40 In addition, the IABP occasionally is used prophylactically in high-risk patients with poor left ventricular function with either mitral regurgitation or preoperative low cardiac output owing to hibernating or stunned myocardium. Patients with these conditions benefit from temporary afterload reduction during weaning from cardiopulmonary bypass, particularly if myocardial contractility is not immediately improved by revascularization. In some institutions a femoral arterial catheter is inserted in anticipation of IABP use in patients undergoing complex procedures who have myocardial dysfunction.⁴¹ In exceptional patients, IABP is used with extracorporeal membrane oxygenation (ECMO) to unload the left ventricle and generate pulsatility while providing circulatory assistance for postcardiotomy patients.^42–44

Nearly 90% of patients who receive intra-aortic balloon counterpulsation have various manifestations of ischemic heart disease, with or without associated valvular heart disease.^45–47 Patients with valvular heart disease without coronary disease who receive an intraoperative IABP generally have mitral valve disease. A few patients have IABP for end-stage cardiomyopathy, acute endocarditis, or before or after heart transplantation.⁴⁸

Of 231 patients who had IABP insertions at the Cleveland Clinic, Eltchaninoff reports that 83 (34.6%) were for complications of acute myocardial infarction, 44 (18.3%) were owing to failed angioplasty, 48 (20%) were for high-risk angioplasty, and 31 (12.9%) were for stabilization before cardiac surgery.³⁵ Only 13 (5.4%) were for end-stage cardiomyopathy.

The timing of IABP insertion varies widely between reports. The percentage of IABPs inserted before cardiac surgery varies between 18% at St. Louis University Medical Center and 57% in a group of community hospitals that do cardiac catheterization.^49,50 The percentage of IABPs inserted intraoperatively varies from 42% to 72%, with a smaller number inserted early after operation (3% to 14%).

The overwhelming reason for intraoperative use of the IABP is failure to wean from cardiopulmonary bypass. Approximately 75% of intraoperative balloon insertions are for this reason. Preoperative low cardiac output and postinfarction angina are additional indications for intraoperative insertion of the IABP.

Techniques of Insertion

The intra-aortic balloon pump is usually inserted into the common femoral artery percutaneously.⁵¹ A cutdown is most often used during cardiopulmonary bypass when the pulse is absent. The superficial femoral artery is avoided because of its smaller size and increased possibility of leg ischemia. For patients with small vessels an 8.5F catheter is recommended; otherwise, the 9.5F catheter is used. The iliac and axillary arteries and, very rarely, the abdominal aorta are infrequently used alternative sites.^52,53 Direct insertion into the ascending aorta is used for intraoperative insertions in patients with severe aortoiliac or femoral occlusive disease that prevents passage of the balloon catheter.^54–56

Approximately two thirds to three quarters of all femoral arterial insertions utilize the percutaneous method.⁵⁷ Although percutaneous insertion was associated with a higher incidence of leg ischemia in the past, this is no longer true.^58,59 In the catheterization laboratory both the guidewire and balloon are monitored by fluoroscopy, but this is not essential if not available. The cutdown technique may be done with local anesthesia outside of the operating room, but preferably is done in the operating room with local or general anesthesia. After the femoral artery is exposed, a guidewire is introduced followed by dilating catheters and the balloon. The catheter can be inserted without the sheath in some instances.⁶⁰ The balloon catheter usually fits snugly in the arterial wound, so a pursestring suture is not needed. If bleeding is present around the entrance site, sutures are used for control. The wound is closed completely. Regardless of the method of insertion, whenever possible the tip of the balloon is visualized by fluoroscopy or transesophageal echocardiography to place the balloon just downstream to the left subclavian artery.⁶¹

The timing of inflation and deflation of the balloon must be monitored closely during counterpulsation. This is done by observing the continuously displayed arterial pressure tracing; a second systolic pulse should appear with every heartbeat and begin just after the smaller first pulse begins to decay. Timing the balloon for irregular rhythms is difficult and the circulatory support provided by the balloon is compromised; in these patients attempts are made to convert the patient to a sinus or paced rhythm or to slow (80–90 bpm) atrial fibrillation using appropriate drugs or cardioversion. For tachycardias over 110 to 120 bpm the balloon is timed to provide inflation on alternate beats if the machine is not able to reliably follow each beat. Generally patients are not given heparin for the IABP. The exit site of the catheter must be kept clean with antiseptics and covered in an effort to prevent local infection or septicemia.

A percutaneous IABP can be removed without exposing the femoral puncture site. The balloon catheter is disconnected from the pump and completely deflated using a 50-mL syringe. Using steady pressure over the femoral puncture site, the balloon catheter is withdrawn smoothly and removed, and pressure is maintained over the puncture site for 30 minutes. If the balloon is inserted via a cutdown, the balloon is preferably removed in the operating room. The puncture site is closed with sutures. If blood flow to the lower limb is impaired after removal, a local thromboembolectomy using Fogarty catheters and an angioplasty procedure using a vein patch is performed.

If the percutaneous needle punctures the iliac artery above the inguinal ligament intentionally or inadvertently in obese individuals, removal should be done through a surgical incision in the operating room, because the backward slope of the pelvis makes pressure difficult to maintain after withdrawal and substantial occult retroperitoneal bleeding may occur.

If the common femoral or iliac arteries cannot be used because of occlusive disease or inability to advance the guidewire, the axillary artery usually is exposed below the middle third of the clavicle for insertion.^52,53 This vessel is smaller than the femoral artery, but generally more compliant. Fluoroscopy or transesophageal (not transthoracic) echocardiography is recommended to ensure that the guidewire does not go down the ascending thoracic aorta into the heart.

Transaortic insertion of IABP may be done through an 8- or 10-mm woven Dacron or polyfluorotetraethylene graft that is beveled and sutured end-to-side to the ascending aorta using a side-biting clamp on the aorta.⁵⁶ The opposite end of the graft is passed through a stab incision in the chest wall below but near the xiphoid. The balloon is passed through this sleeve into the aorta and guided into the proximal descending thoracic aorta so that the balloon does not occlude the left subclavian orifice when inflated. The suture cuff of the balloon catheter is trimmed so that it can be inserted into the graft and tied tightly to achieve secure hemostasis. This connection is placed just beneath the skin so that none of the graft protrudes. The catheter is secured in place.

A simpler method uses two aortic pursestring sutures to secure the aorta around the balloon catheter. No graft is used, yet bleeding complications are minimal.^54,55 Regardless of the technique of insertion, balloon catheters inserted through the ascending aorta are removed in the operating room to secure closure of the aorta.

Pulmonary arterial counterpulsation is recommended for right heart failure but has not achieved wide use.^62,63 Because of the short length of the pulmonary artery either a prosthetic graft (20–25 mm) is sewn end-to-side to the main pulmonary artery and tied around the balloon catheter placed inside. There are little data regarding the amount of afterload reduction of the right ventricle.

Complications

Reported complication rates of the intra-aortic balloon pump vary between 12.9% and 29% and average approximately 20%.^36,57,64 Life-threatening complications are rare.⁶⁵ Leg ischemia is by far the most common complication (incidence 9% to 25%); other complications include balloon rupture, thrombosis within the balloon, septicemia, infection at the insertion site, bleeding, false aneurysm formation, lymph fistula, lymphocele, and femoral neuropathy.^66,67 There is no significant difference in limb ischemia in the five different types of IABPs clinically available.^64,68

Balloon rupture occurs in approximately 1.7% of patients and usually is indicated by the appearance of blood within the balloon catheter and only occasionally by the pump alarm. Rupture may be slightly more common with transaortic insertion. Although helium usually is used to inflate the balloon, gas embolism has not been a problem. If rupture occurs, the balloon should be deflated forcibly to minimize thrombus formation within the balloon and promptly removed. If the patient is IABP-dependent, a guidewire is introduced through the ruptured balloon, the original balloon is removed, and a second balloon catheter is inserted over the wire. If the ruptured balloon is not removed easily, a second balloon is inserted via the opposite femoral or iliac artery or through the axillary artery to maintain circulatory support.⁶⁹

Removal of a kinked or thrombosed ruptured balloon that cannot be withdrawn by firm traction requires operation. A thrombosed balloon can severely lacerate the femoral artery. The catheter should be withdrawn as far as possible with firm traction. The location of the tip should be determined by x-ray or ultrasound and an incision planned to expose that segment of the vascular system. In the operating room thrombolytic drugs may be considered if these drugs are not contraindicated by recent surgery.⁷⁰ The trapped balloon is removed through an arterotomy after control of the vascular segment is obtained.

Although the incidence of clinically significant lower leg ischemia varies from 9% to 25% of patients, up to 47% have evidence of ischemia during the time the IABP is used.^66,67 Thus the preinsertion status of the pedal pulses should be determined and recorded in every patient before the IABP is inserted. After insertion, the circulation of the foot is followed hourly by palpating pulses or by Doppler ultrasound. Foot color, mottling, temperature, and capillary refill are observed; the appearance of pain, dullness to sensation, and minimal circulation indicate severe ischemia that requires restoration of the circulation to the extremity as soon as possible.

There are three alternatives. If the patient is not balloon-dependent, it is removed immediately. In the majority of patients this relieves the distal ischemia; a few patients require surgical exploration of the puncture site, removal of thrombus and/or emboli, and reconstruction of the femoral artery. If the patient is balloon-dependent, a second balloon catheter can be introduced into the opposite femoral or iliac artery and the first removed. If this alternative is not available or attractive, circulation to the ischemic extremity is restored using a cross-leg vascular graft or, less commonly, an axillofemoral graft.^70,71 Prompt revascularization preempts development of the compartment syndrome (incidence 1% to 3%) and the need for fasciotomy. Prompt and aggressive treatment of leg ischemia has reduced the incidence of amputation to 0.5% to 1.5%, but if amputation is necessary the level often is above the knee. Several risk factors for development of leg ischemia have emerged. Female gender, peripheral vascular disease, diabetes, cigarette smoking, advanced age, obesity, and cardiogenic shock are reported to increase the risk of ischemic complications after IABP. Since the IABP is inserted for compelling indications, identification of risk factors does not influence management, except to encourage removal of the device as soon as the cardiac status of the patients allows. In some series longer duration of IABP counterpulsation is associated with an increased risk of complications.⁶⁶

Although most ischemic complications are owing to impairment of arterial inflow, severe atherosclerotic diseases of the descending thoracic aorta may produce embolization of atherosclerotic material that can cause toe ischemia and eventually require amputation. Emboli may also reach the renal and visceral arteries to produce ischemia of these organs. The presence of aortic atherosclerosis can be determined by echocardiography and if present, insertion through the axillary artery considered.⁷² The ischemic rate of axillary insertions is not known because of the low number of cases reported.

Approximately 1% of patients develop false aneurysms at the femoral puncture site either in the hospital or shortly after discharge, and rare patients develop an arterial-venous fistula. Both conditions are confirmed readily by duplex scanning and require elective operative repair; neglected false aneurysms can rupture. The rare complication of lymphocele or lymph fistula preferably is treated surgically by local exploration and suture control.

Bleeding produces a local hematoma that is not evacuated unless skin necrosis is likely. If bleeding occurs in the wound, the wound is explored, bleeding is stopped, part of the hematoma is evacuated without extending the dissection, and the wound is reclosed. Bleeding from transaortic insertion is uncommon (3% to 4%). Retroperitoneal bleeding from an iliac artery puncture may not be obvious, but may cause death.

Septicemia occurs in up to 1% of patients, but the risk increases with the duration of IABP. Septicemia is an indication for IABP removal, but if the patient is balloon-dependent, a replacement balloon catheter is inserted in a new site. Septicemia is treated aggressively after blood cultures are obtained with broad-spectrum antibiotics, which are switched to one or more specific antibiotics when the organism is known. Local infections occur in 2% to 3% of patients and usually are treated by drainage, packing, antibiotics, and secondary closure.

Acute aortic dissection from the catheter tip piercing the intima has been reported.³⁶ This problem is prevented preferably by not advancing the catheter against resistance and monitoring with fluoroscopy or transesophageal echocardiography. Occasional femoral neuropathies resolve over time, but can be disabling. Transaortic IABP is associated with a 2% to 3% incidence of cerebral vascular accidents.⁵⁵

Results

Very few complications of IABP cause death. Rare instances of bleeding (retroperitoneal or aortic), septicemia, central nervous system injury, or aortic dissection may cause or contribute to a patient's death. Mortality is higher in patients with leg ischemic complications than in those without.

Counterpulsation increases coronary arterial flow, reduces afterload and myocardial oxygen consumption, and experimentally reduces infarct size early after infarction.⁷³ Without revascularization IABP produces a marginal increase in survival, but with revascularization both short-term and long-term survival as well as quality of life are substantially improved.^74–76

However, mortality is high in patients who receive IABP because of the cardiac problems that led to the need for the device. Overall reported hospital mortality ranges from 26% to 50%, although it has been decreasing. (Fig. 17-3).^77–79 Risk factors for hospital mortality include advanced age, female gender, high NYHA class, preoperative nitroglycerin, operative or postoperative insertion, and transaortic insertion in one study and age and diabetes mellitus in another. A third study correlates hospital death with acute myocardial infarction, ejection fraction less than 3%, NYHA class IV, and prolonged aortic cross-clamp and bypass times.⁷⁸ Time of insertion affects hospital mortality: preoperative insertion is associated with a mortality of 18.8% to 19.6%;⁴⁸ intraoperative insertion, 27.6% to 32.3%;⁴⁸ and postoperative insertion, 39% to 40.5%. Mortality is highest at 68% for patients with pump failure; lowest at 34% for patients with coronary ischemia; and 48% for patients who had a cardiac operation.³³ Risk factors at the time of weaning from cardiopulmonary bypass associated with the likelihood of hospital death are heart block, advanced age, female gender, and elevated preoperative blood urea concentration.⁷²

View larger version (55K): [in this window] [in a new window]   FIGURE 17-3 Hospital mortality for this 5-year period was 26%.

Given the overall ease of IABP insertion, excellent physiologic augmentation of coronary blood flow, and LV unloading, this form of therapy should be considered as the first line of mechanical support in patients who do not have significant peripheral vascular disease. There is some suggestion that preoperative prophylactic IABP insertion in high-risk patients (left ventricular EF < 40%, unstable angina, left main stenosis > 70%, redo CABG) can improve cardiac index, reduce length of ICU stay, and decrease mortality.^81,82

	DIRECT CIRCULATORY SUPPORT

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Background

The need for acute cardiac support beyond cardiopulmonary bypass was clear from the early days of cardiac surgery. Spencer et al reported the first successful clinical use of a temporary device in 1965 after four patients were placed on femoral–femoral cardiopulmonary bypass. Only one patient survived to discharge. Subsequently, in 1966 the first successful use of a left ventricular assist device was reported after a double valve operation.⁸³ Debakey used an assist device that was implanted in an extracorporeal location between the left atrium and the axillary artery, marking the first use of an extracorporeal temporary device support system. The patient survived for 10 days on the pump and was eventually discharged home.

The excitement surrounding these events prompted the formation of the Artificial Heart Program in 1964, sponsored by the National Heart Institute, which encouraged the development of mechanical circulatory support systems.⁸⁴ One of the objectives of this program was to promote the development of support systems that would be used in cases of acute hemodynamic collapse.

Ideal Device

Despite recent advances in biotechnology, recognition of many of the problems and complications associated with extracorporeal circulation have delineated the limitations of these devices. The components of an ideal device must overcome some of the existing problems.

An ideal device should support adequate flow, maximize hemodynamics, and unload the ventricle for patients of all sizes. Although using different cannulation approaches can address some of these issues (see below), even under ideal conditions the currently available pumps are only able to support flows up to a maximum of 6 L/min, a limitation in obese patients.

At the other extreme is the need to support patients with small body surface areas. Current devices have addressed the problem associated with variations in patient size by being designed as extracorporeal systems. Therefore, by virtue of having small-diameter cannulas transversing the chest, the pumps can support patients with varying body surface area. The disadvantage of such a system is the potential for driveline and mediastinal infections. In addition, the length of the cannula between the heart and the device, particularly the inflow cannula, predisposes to areas of stasis and potential thrombus generation. Thromboemboli, which occur despite adequate anticoagulation, are one of the leading etiologies of death in patients supported on devices.

All current pumps require anticoagulation that increases the ever-present threat of early postoperative bleeding. In addition, requirements for transfusion of large amounts of coagulation factors and platelets enhance the inflammatory response that is induced by surgery and is further aggravated by the artificial circuit. Activation of the contact and complement systems, as well as release of cytokines by leukocytes, endothelial cells, and macrophages, further increases the potential negative and detrimental effects of use of temporary assist devices.^85,86 The ensuing inflammatory cascade and volume overloading can have detrimental effect on the pulmonary vascular resistance and right ventricular overload, often necessitating addition of a right ventricular assist device.

Current temporary assist devices all have the capability of biventricular support, provided that the lungs can support oxygenation and ventilation. In cases of acute lung injury superimposed on circulatory failure, ECLS (ECMO) is the only device currently approved that can support an in-line oxygenator.

The multitude of clinical scenarios that often lead to the need for mechanical support all require that support be instituted expeditiously. All current devices must therefore be easily implantable. In the postcardiotomy setting with access to the great vessels, the cannulas should allow the versatility of choosing any inflow or outflow site that is clinically indicated (see below). In an active resuscitative setting, such as cardiac arrest in the catheterization laboratory, in which time is critical and transport to the operating room often impractical, percutaneous cannulation must be an option.

Table 17-1 summarizes some of the components of an ideal temporary support device. At present, no single device is inclusive of all the components.

A wide range of indications exist for acute mechanical support, but the primary goal is rapid restoration of the circulation and stabilization of hemodynamics. The routine use of transesophageal echocardiography (TEE) has greatly helped in assessing the etiology of cardiogenic shock by allowing evaluation of ventricular function, regional wall motion abnormalities, and valvular mechanics. In a patient with mechanical complications secondary to myocardial infarction, such as acute rupture with tamponade, acute papillary muscle rupture, or postinfarction ventricular septal defect, emergent surgical correction may obviate the need for device support. Similarly in the postcardiotomy setting with failure to separate from cardiopulmonary bypass, TEE may direct the surgeon to the need for additional revascularization and reparative valve surgery and successful weaning from bypass.

If echocardiography fails to reveal a surgically correctable cause for cardiogenic shock, most surgeons use hemodynamic data to consider the need for mechanical assistance. These criteria include a cardiac index less than 2.2 L/min/m², systolic blood pressure lower than 90 mm Hg, mean pulmonary capillary wedge pressure or central venous pressure higher than 20 mm Hg, and concomitant use of high doses of least two inotropic agents.⁸⁷ These situations may be clinically associated with arrhythmias, pulmonary edema, and oliguria. In such circumstances, use of an intra-aortic balloon pump may be considered as the first step. In the postcardiotomy shock setting, without mechanical support, the mortality is greater than 50%.⁸⁸ In this setting, some believe that early implantation of an assist device capable of supporting high flows and allowing the heart to rest may improve results and allow for recovery of stunned myocardium.⁸⁹ Furthermore, new pharmacologic agents such as the phosphodiesterase inhibitor milrinone, nitric oxide, and vasopressin have helped to optimize hemodynamics during this critical initial period, reducing the need for concomitant right ventricular support.^90,91

Once mechanical assistance has been instituted, the stabilized patient can undergo periodic evaluation to assess native heart recovery, end-organ function, and neurologic status. We initiate evaluation for cardiac transplantation concomitantly. Patients who do not have occult malignancy, severe untreated infection, or neurologic deficit are selected for cardiac transplantation if all other criteria are met and there is no sign of cardiac recovery. In this subgroup we generally transition to a chronic ventricular assist device until an organ becomes available. In those patients with gradual improvement in myocardial pump function, the devices may be weaned and removed (see below).

	CONTINUOUS FLOW PUMPS

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There are two types of commercially available pumps for extracorporeal circulation: roller pumps and centrifugal pumps. Roller pumps are used rarely, if ever, for temporary circulatory support beyond routine cardiopulmonary bypass applications because of important disadvantages. Although inexpensive, roller pumps are insensitive to line pressure if the outflow line becomes obstructed, and also require unobstructed inflow. Additionally, they may cause spallation of tubing particles and are subject to tubing failure at unpredictable times. These systems require constant vigilance and are difficult to operate for extended periods. Use of roller pumps beyond 4 to 5 hours is associated with hemolysis, and for this reason roller pumps are inappropriate for mechanical assistance that may involve several days to weeks of support.⁹²

Centrifugal Pumps

Centrifugal pumps are familiar assist systems because of their routine use in cardiopulmonary bypass. Although many different pumphead designs are available, they all work on the principle of generating a rotatory motion by virtue of moving blades, impellers, or concentric cones. These pumps can generally provide high flow rates with relatively modest increases in pressure. They require priming and de-airing prior to use in the circuit, and the amount of flow generated is sensitive to outflow resistance and filling pressures. The differences in design of the various commercially available pumpheads are in the numbers of impellers, the shape and angle of the blades, and the priming volume. The only exception is the Medtronic BioPump (Medtronic Bio-Medicus, Eden Prairie, MN), which is based on two concentric cones generating the rotatory motion. The pumpheads are disposable, relatively cheap to manufacture, and are mounted on a magnetic motorized unit that generates the power. Despite design differences, in vitro and in vivo testing has shown no clear superiority of one pump over the other.^93–95 Although earlier designs caused mechanical trauma to the blood elements, leading to excessive hemolysis, the newly engineered pumps are less traumatic and can be used for longer periods. Studies have documented that centrifugal pumps have a superior performance with regards to mechanical injury to red blood cells when compared to roller pumps.⁹⁶

COMPLICATIONS

Complications with temporary mechanical assistance are high and very similar for patient on centrifugal pump support or ECLS (see below). The major complications reported by a voluntary registry for temporary circulatory assistance using primarily left ventricular assist devices (LVAD), right ventricular assist devices (RVAD), and biventricular assist devices (BVAD) are bleeding, low cardiac output with BVAD, renal failure, infection, neurologic deficits, thrombosis, and emboli, hemolysis, and technical problems (Table 17-2). The incidence of these complications in 1279 reported patients differed significantly between continuous perfusion systems and pneumatically driven systems (see the following section) with respect to bleeding, renal failure, infection, and hemolysis. Neurologic deficits occurred in approximately 12% of patients, and in Golding's experience noncerebral emboli occurred equally often.⁹⁷ Golding also found that 13% of patients also developed hepatic failure. An autopsy study found anatomic evidence of embolization in 63% of patients even though none had emboli detected clinically.⁹⁸

RESULTS

Although a meaningful comparison of results of centrifugal support from different institutions is not possible, in general overall survival has been in the range of 21% to 41% (Table 17-3). The voluntary registry reported the experience with 604 LVAD, 168 RVAD, and 507 BVAD experiences; approximately 70% were with continuous flow pumps and the remainder with pulsatile pumps.¹ There were no significant differences in the percentage of patients weaned from circulatory assistance or the percentage discharged from the hospital according to the type of perfusion circuitry. Overall 45.7% of patients were weaned and 25.3% were discharged from the hospital.¹ The registry also reports that long-term survival of patients weaned from circulatory support is 46% at 5 years.¹ Most of the mortality occurs in the hospital before discharge or within 5 months of discharge.

Data from the University of Missouri Hospital are also very similar.⁹⁹ From the 91 patients with postcardiotomy heart failure, 46% were weaned from the device and 21% survived to hospital discharge. Although weaning was more successful with RVAD support alone compared to LVAD or biVAD support (100% for RVAD vs. 48.5% for LVAD, vs. 44.9% for biVAD), survivals were not significantly different (RVAD, 22%; LVAD, 24.3%; biVAD, 18.4%). Joyce reports that 42% of patients supported by Sarn impeller pumps were eventually discharged.¹⁰¹ This is the highest reported survival and probably reflects the fact that some of these patients received transplants, which is known to improve overall survival.

Extracorporeal Life Support

By the 1960s it was clear that CPB was not suitable for patients requiring circulatory support for several days to weeks. The development of extracorporeal life support (ECLS) as a temporary assist device (also referred to as extracorporeal membrane oxygenation, or ECMO) is a direct extension of the principles of cardiopulmonary bypass and follows the pioneering efforts of Bartlett et al in demonstrating the efficacy of this technology in neonatal respiratory distress syndrome.¹⁰²

There are a number of key differences between CPB and ECLS. The most obvious difference is the duration of required support. Whereas CPB is typically employed for several hours during cardiac surgery, ECLS is designed for longer duration of support. With ECLS a lower dose of heparin is used, and reversal of heparin is not an issue because a continuous circuit is used and areas of stasis have been minimized (such as cardiotomy suction or venous reservoir). In addition, the membrane oxygenator allows for longer duration of support. These differences are thought to reduce the inflammatory response and the more pronounced coagulopathy that can be seen with CPB.⁸⁶

A typical ECLS circuit is demonstrated in Figure 17-4. The system is comprised of the following:

1. Hollow-fiber membrane oxygenator with an integrated heat-exchange system: The microporous membrane provides the necessary gas transfer capability via the micropores where there is direct blood-gas interface with minimal resistance to diffusion. By virtue of the membranes being close to each other, the diffusion distance has been reduced without a significant pressure drop across the system.¹⁰³ Control of oxygenation and ventilation is relatively easy. Increasing the total gas flow rate increases CO₂ removal (increasing the "sweep") by reducing the gas phase CO₂ partial pressure and promoting diffusion. Blood oxygenation is simply controlled by changing the fraction of O₂ in the gas supplied to the oxygenator.¹⁰³
   
2. Centrifugal pump: These pumps are totally nonocclusive and afterload-dependent. An increase in downstream resistance, such as significant hypertension, will decrease forward flow to the body. Therefore, flow is not determined by rotational flow alone, and a flow meter needs to be incorporated in the arterial outflow to quantitate the actual pump output. If the pump outflow should become occluded, the pump will not generate excessive pressure and will not rupture the arterial line. Similarly, the pump will not generate significant negative pressure if the inflow becomes occluded. This protects against cavitation and microembolus formation.
   
3. Heat exchanger: The heat exchanger allows for control of blood temperature as it passes through the extracorporeal circuit. Generally the transfer of energy occurs by circulating nonsterile water in a countercurrent fashion against the circulating blood. Use of water as the heat exchange medium provides an even temperature across the surface of the heat exchanger without localized hot spots.¹⁰³
   
4. Circuitry interfaced between the patient and the system: The need for systemic anticoagulation on ECLS and the complications associated with massive coagulopathy and persistent bleeding during the postcardiotomy period led to the development of biocompatible heparin-bonded bypass circuits. In 1991, the Carmeda Corporation in Stockholm, Sweden, released a heparin-coating process that could be used to produce an antithrombotic surface.¹⁰⁴ This process was applied to extracorporeal tubing and the hollow-fiber microporous oxygenator surface.¹⁰⁵ Initial experience suggested that the need for systemic anticoagulation had been eliminated. In addition, heparin coating has been associated with a decrease in the inflammatory response with reduced granulocyte activation¹⁰⁶ and complement activation.¹⁰⁷ Bindsler¹⁰⁸ and Mottaghy¹⁰⁹ reported excellent hemodynamic support with minimal postoperative blood loss in experimental animals for up to 5 days. Magovern and Aranki reported similar excellent results with clinical application.^110,111 Although these heparin-bonded circuits were initially thought to completely eliminate the need for heparinization, thrombus formation without anticoagulation remains a persistent problem. In a study of 30 adult patients with cardiogenic shock who underwent ECLS using the heparin-bonded circuits and no systemic anticoagulation, 20% of patients developed left ventricular thrombus shown by transesophageal echocardiography and an additional 6% had visible clot in the pumphead.¹¹² Protamine administration after starting ECLS can precipitate intracardiac clot. If the left ventricle does not eject and blood remains static within the ventricle, clot formation is more likely. Intracavity clot is more likely in patients with myocardial infarction due to expression of tissue factor by the injured cells. Protamine may bind to the heparinized coating of the new circuit and negate an anticoagulant effect.¹¹³
   

View larger version (29K): [in this window] [in a new window]   FIGURE 17-4 Percutaneous ECMO support is attained via femoral vessel access. Right atrial blood is drained via a catheter inserted into the femoral vein and advanced into the right atrium. Oxygenated blood is perfused retrograde via the femoral artery. Distal femoral artery perfusion is not illustrated.

A key difference between the centrifugal pump and ECLS is the presence of an in-line oxygenator. As a result, ECLS can be used for biventricular support by using central cannulation of the right atrium and aorta or by peripheral cannulation. Intraoperatively, the most common application of ECLS has been for patients who cannot be weaned from cardiopulmonary bypass after heart surgery. In these cases, the existing right atrial and aortic cannulas can be used. An alternative strategy, and one that we prefer, is to convert the system to peripheral cannulas and to cannulate both the proximal and distal femoral artery. Conversion permits chest closure and removal of perfusion catheters at the bedside in intensive care.

The cannulation is done by surgical cutdown in the groin for exposure of the common femoral artery and vein. The entire vessel does not need to be mobilized and exposure of the anterior surface of the vessels is sufficient. A purse-string suture is placed over the anterior surface of the vessel. The largest cannula that the vessel can accommodate is selected. Typically arterial cannulas are 16F to 20F and venous cannulas are 18F to 28F in size. The cannulation is performed under direct vison using Seldinger's technique. A stab incision is made in the skin with a #11 blade knife, a needle is inserted through the stab incision into the vessel, and a guidewire is gently advanced. Dilators are then sequentially passed to gently dilate the tract and the insertion point in the vessel. The cannulas are then inserted, the guidewire is removed, and a clamp is applied. For venous drainage we typically employ a long 2-stage cannula (Fem-Flex II; Research Medical, Midvale, UT) and direct it to the level of the right atrium under TEE guidance.

To minimize limb complication from ischemia, one strategy is to place a 10F perfusion cannula in the superficial femoral artery downstream to the primary arterial inflow cannula to perfuse the leg (Fig. 17-5). This cannula is connected to a tubing circuit that is spliced into the arterial circuit with a Y-connector.¹¹⁴ The distal cannula directs continuous flow into the leg and significantly reduces problems with leg ischemia. An alternative strategy is to completely mobilize the common femoral artery and sew a 6- or 8-mm short Dacron graft to its anterior surface as a "chimney." The graft serves as the conduit for the arterial cannula and no obstruction to distal flow exists. This strategy also allows for a more secure connection and avoids problems with inadvertent dislodgement of the cannulas because of loosening of the purse strings. In general, complete percutaneous placement of arterial cannulas is avoided to prevent iatrogenic injury during insertion and ensure proper positioning of the cannulas. However, when venovenous bypass is the only mode of support needed, percutaneous cannulation is performed. Surgical exposure is not necessary and bleeding is less with this technique. Although traditionally the perfusion circuit involves atrial drainage and femoral reinfusion (atriofemoral flow), a recent prospective study has shown the reverse circuit (femoroatrial flow) to provide higher maximal extracorporeal flow, and higher pulmonary arterial mixed venous oxygenation.¹¹⁵

View larger version (24K): [in this window] [in a new window]   FIGURE 17-5 Surgical exposure of the femoral vessels facilitates cannulation for ECMO. A small 10F cannula is used to perfuse the distal femoral artery.

An alternative central cannulation site is the axillary artery. Direct cannulation of this artery has been associated with progressive edema of the arm.¹¹⁶ Therefore, the best strategy to maintain arm perfusion is to expose the axillary artery and sew a 6- or 8-mm graft to the vessel as a "chimney." The cannula is then placed in the graft and tied securely with several circumferential umbilical tapes.

Once instituted, the system can be monitored by trained ICU nurses and maintained by a perfusionist. Evidence of thrombus in the pumphead requires a change. Leakage of plasma across the membrane from the blood phase to the gas phase continues to be a problem, gradually decreasing the efficiency of the oxygenator and increasing resistance to flow, necessitating oxygenator exchanges. Using this system, ECLS flows of 4 to 6 L/min are possible at pump speeds of 3000 to 3200 rpm. Higher pump speeds are avoided to minimize mechanical trauma to blood cells. Other means of improving flow include transfusion of blood, crystalloid, or other colloid solutions to increase the overall circulating volume. Physiologically, ECLS will unload the right ventricle, but will not unload the ejecting left ventricle, even though left ventricular preload is reduced.¹¹⁷ If the heart is dilated and poorly contracting, the marked increase in afterload provided by the ECLS system offsets any change in end-diastolic left ventricular volume produced by bypassing the heart. The heart remains dilated because the left ventricle cannot eject sufficient volume against the increased afterload to reduce either end-diastolic or end-systolic volume. ECLS, therefore, may theoretically increase left ventricular wall stress and myocardial oxygen consumption unless an intra-aortic balloon pump or other means is used to mechanically unload the left ventricle and reduce left ventricular wall stress.¹¹⁸ We routinely use IABP in the majority of patients to decrease the increased afterload imposed by ECLS, and add pulsatility to the continuous flow generated by the centrifugal pump. Kolobow has devised a spring-loaded catheter introduced through the femoral vein to render the pulmonary valve incompetent to decompress the left ventricle during ECLS, but this has not been used clinically.¹¹⁸ Others use atrial septostomy to decompress the left ventricle if the pulmonary artery pressures remain elevated.¹¹⁹

An RVAD is rarely indicated in the postcardiotomy setting because in general these patients have global biventricular dysfunction. ECLS as an RVAD (with outflow to the pulmonary artery via the right ventricular outflow tract) may be only used in patients with good function of the left ventricle. Exclusive right ventricular dysfunction is rare but may occur if retrograde cardioplegia is used and fails to protect the right ventricle, in cases of pulmonary thromboendarterectomy, or in patients with right ventricular infarction. Note that if significant pulmonary hypertension is present, this configuration may not adequately load the left ventricle.

COMPLICATIONS

The experience in adults with ECMO for postoperative cardiogenic shock is more limited because of nearly universal bleeding problems associated with the chest wound in combination with heparin anticoagulation with the ECMO circuit.¹²⁰ Pennington reported massive bleeding in six of six adults supported by ECMO following cardiac surgery. Even without the chest wound, bleeding was the major complication in a large study of long-term ECMO for acute respiratory insufficiency.¹²¹ Muehrcke reported experience with ECMO using heparin-coated circuitry with no or minimal heparin.^117,122 The incidence of reexpoloration was 52% in the Cleveland Clinic experience; transfusions averaged 43 units of packed cells, 59 units of platelets, 51 units of cryoprecipitate, and 10 units of fresh frozen plasma. Magovern reported somewhat fewer uses of blood products, but treated persistent bleeding by replacement therapy and did not observe evidence of intravascular clots; two patients developed stroke after perfusion stopped. Other important complications associated with ECMO using heparin-coated circuits included renal failure requiring dialysis (47%), bacteremia or mediastinitis (23%), stroke (10%), leg ischemia (70%), oxygenator failure requiring change (43%), and pump change (13%).¹¹⁷ Nine of 21 patients with leg ischemia required thrombectomy and one amputation. Half of the patients developed marked left ventricular dilatation and six patients developed intracardiac clot detected by transesophageal echocardiography.⁹⁰ Intracardiac thrombus may form within a poorly contracting, nonejecting left ventricle or atrium because little blood reaches the left atrium with good right atrial drainage.^40,82,89,108 We have observed intracardiac thrombus in heparinized patients and those perfused with pulsatile devices and a left atrial drainage cannula. The problem, therefore, is not unique to ECMO or the location of the left-sided drainage catheter, but is related to left ventricular function. In patients on ECMO with a left ventricular thrombus, we have removed the thrombus at the time a HeartMate LVAD was implanted for a bridge to transplantation.¹²²

More recent reports documenting the high incidence of complications with ECLS have continued to plague temporary support mechanisms based on continous flow. Kasirajan reported an 18.9% incidence of intracranial hemorrhage with female gender, heparin use, elevated creatinine, need for dialysis, and thrombocytopenia as important associated risk factors.¹²³ Smedira recently reported on 107 postcardiotomy patients supported on ECLS with a 48% rate of infection, 39% need for dialysis, 29% neurologic events, 5% pump thrombus formation, and 27% limb complications.¹²⁴

RESULTS

Table 17-4 summarizes some of the reported results with ECLS for postcardiotomy circulatory support. Magovern reported improved results in 14 patients supported by a heparin-coated ECMO circuit after operations for myocardial revascularization.¹¹⁷ Eleven of 14 patients (79%) with revascularization survived, but none of three patients with mitral valve surgery and none of four patients who underwent elective circulatory arrest survived. Overall, 52% of the whole group survived, but two developed postperfusion strokes that were probably from thrombi produced during perfusion. Although the Cleveland Clinic experience with heparin-coated ECLS circuits produced a survival rate of 30%, the patient population was more diversified and represented only 0.38% of cardiac operations done during the same time period.¹¹² In a recent report on 82 adult patients supported with ECMO for a variety of indications, survival for postcardiotomy was 36%, whereas none of the patients who had acute cardiac resuscitation survived, and survival for cardiac allograft failure was 50%.¹²⁵

	PULSATILE PUMPS

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ABIOMED BVS 5000

In 1992, the ABIOMED device became the first extracorporeal pump designed to provide pulsatile univentricular or biventricular support that was approved by the Food and Drug Administration. It has been used in Europe and the United States for the purpose of postcardiotomy pump failure with more than 850 patients currently reported to the registry. The system is a simple, user-friendly, extracorporeal pulsatile pump that is available in over 450 centers in United States, with the majority being utilized in nontransplant centers. The pump is configured as a dual chamber device containing an atrial chamber that fills passively by gravity and a ventricular chamber that pneumatically pumps the blood to the outflow cannula (Fig. 17-6). The two chambers and the outflow tract are divided by trileaflet polyurethane valves, which allows for unidirectional blood flow.

View larger version (44K): [in this window] [in a new window]   FIGURE 17-6 The ABIOMED BVS 5000. (Left panel) The atrial chamber empties through a one-way valve into the ventricular chamber (diastole). (Right panel) The pneumatically driven pump compresses the ventricular chamber and blood flows through a one-way valve into the patient (systole). The atrial chamber fills by gravity during pump systole.

The ventricular chamber, on the other hand, requires active pulsatile pumping by a pneumatic driveline. Compressed air is delivered to the chamber, causing bladder collapse and forcing blood out of the pump to the patient. During diastole the air is vented to the atmosphere, allowing refilling of the chamber during the next cycle. The rate of pumping and the duration of pump systole and diastole are adjusted by the pump microprocessor that operates asynchronously to the native heart rate. The pump automatically makes adjustments to account for preload and afterload changes and delivers a constant stroke volume of approximately 80 mL. The maximum output is approximately 5 to 6 L/min with the newer BVS 5000i console. This design requires minimal input by personnel except during periods of weaning. Medical management should include optimizing patients' hydration status and outflow resistance as the pump's performance depends on these parameters.

The main advantage of the device is the ability to provide independent univentricular or biventricular support as needed. The device has not demonstrated any significant hemolysis and the pulsatile flow may have some degree of physiological benefit. As opposed to the centrifugal pump and ECLS, patients can be extubated and can have limited mobility, such as transfer from bed to chair or dangling of the legs from the bed.

CANNULATION

The cannulas are constructed from polyvinyl chloride and have a velour body sleeve that is tunneled subcutaneously and is designed to promote hemostasis and tissue ingrowth at the exit sites. Three sizes of wire-reinforced inflow cannulas are commercially available (Fig. 17-7). These include a 32F right-angle light-house tip, a 36F malleable cannula with an adjustable backbone, and a 42F right-angle light-house tip cannula.

View larger version (87K): [in this window] [in a new window]   FIGURE 17-7 Inflow and outflow cannulas for the ABIOMED system. The arterial grafts have a Dacron graft at the end for direct end-to-side anastomosis to the aorta or pulmonary artery. The inflow cannulas are either in a right-angle configuration or with a malleable backbone that can be adjusted to the desired angle.

Careful cannula insertion is important for optimal performance. Venous inflow must be unimpeded and outflow grafts must not be kinked. In addition, careful consideration must be given to cannula position when bypass grafts are crossing the epicardial surface of the heart. Depending on the location of these grafts, these cannulas must be placed such that graft compression does not occur. The three-dimensional layout of this geometry needs to be visualized and thought out in advance, particularly if chest closure is planned. Any graft compression will make recovery unlikely if at all possible.

It is technically much easier to use cardiopulmonary bypass for placement of these cannulas, although off-pump insertion is possible and may be preferable in certain clinical situations, particularly for isolated right-sided support.¹²⁶ A side-biting clamp is typically used on the aorta to perform the outflow anastomosis. If the patient is on cardiopulmonary bypass, the pulmonary artery anastomosis can be done without the need of a partial cross-clamp. The length of the graft is measured from the anticipated skin exit site to the site of anastomosis, and the Dacron graft is cut to the appropriate length so that there is no excessive tension or any kinking. The cutaneous exit site is planned so that approximately 2 cm of the velour cuff is extending from the skin and the remainder is in the subcutaneous tunnel. The cannula is not tunneled subcutaneously until after completion of the anastomosis. For the aortic anastomosis, incorporation of a Teflon or pericardial strip will help control suture-line bleeding.

Although cannulation for right ventricular outflow is most commonly constructed so that the cannula with the 10-mm Dacron graft is sewn to the pulmonary artery, we prefer an alternative quicker technique that has been recently reported.¹²⁷ Two concentric pledgeted purse-string sutures are placed along the right ventricular outflow tract anteriorly. The 36F straight atrial cannula is then introduced in the right ventricular outflow tract through a cruciate incision and directed through the pulmonary valve into the main pulmonary artery. Cannula position is confirmed by palpation and purse strings are tightened and secured with snares. This is an easy, reproducible, and quick technique for RVAD outflow placement and is a useful technique in situations where access to the pulmonary artery is difficult because of scarring or poor visualization. In addition, RVAD placement can be performed without the need for CPB. If this technique is used, the cannula must be externalized from the skin prior to insertion into the pulmonary artery.

For inflow cannulation, a double-pledgeted purse-string suture using 3-0 polypropylene is placed concentrically for cannula placement. Tourniquets must be firmly secured to prevent inadvertent loosening of the purse string and bleeding from the insertion sites. In addition, the heart is generally volume loaded to prevent air embolism during insertion.

For right-sided support, we usually use the 42F right-angle cannula for drainage from the midatrial wall with the cannula directed to the IVC. For left-sided drainage, several options are available. The 36F malleable cannula is used because it provides the versatility to accomodate variations in anatomy and clinical conditions. Left atrial cannulation can be achieved via the interatrial groove, the dome of the left atrium, or the left atrial appendage. Alternatively the body of the right ventricle or the left ventricular apex may be cannulated.¹²⁸ There is no need to excise a core of the ventricular apical muscle as is common with other VADs.¹²⁹ Ventricular cannulation offers the advantages of excellent ventricular decompression, which may improve ventricular recovery,¹³⁰ but bleeding from around the cannula may also become a problem, particularly in the setting of recent myocardial infarction.

One of the advantages of the ABIOMED is that the perfusionist can prepare and de-air the circuit while the cannulas are being placed. Connecting the cannulas to the externalized circuit is easy and can be done expeditiously.

The drive console for the ABIOMED BVS 5000 is simple to operate. The control system automatically adjusts the duration of pump diastole and systole, primarily in response to changes in preload. Pump rate and flow are visible on the display monitor. During automated operation, the device can be managed by the bedside nurse and almost never needs additional adjustments.

COMPLICATIONS

As with all patients who require postcardiotomy mechanical support, complications are frequent. Guyton¹³¹ reported 75% bleeding complications, 54% respiratory failure, 52% renal failure, and 26% permanent neurologic deficit. Infection occurred in 13 patients (28%) while on the device, but only 3 cases were considered device-related. Other complications included embolism in 13%, hemolysis in 17%, and mechanical problems related to the atrial cannula site in 13% of patients.¹²⁴ No major changes in platelet count or blood chemistries occurred during the period of circulatory support.

Jett et al¹²⁸ reported on 55 patients supported on the ABIOMED for a variety of indications including postcardiotomy failure (28), failed transplant allograft (8), acute myocardial infarction (2), and myocarditis (1). They reported a 40% incidence of bleeding, 50% respiratory complications, and 25% neurologic complications. Marelli¹²⁶ also reported a similar incidence of complications in 19 status I patients with 6 developing renal failure, 9 reexplored for bleeding, and 3 dying of sepsis and multisystem organ failure. As with all acute mechanical support systems, these relatively high complication rates are a reflection of the significant preexisting hemodynamic insult that occurs, necessitating implementation of mechanical support. Early device insertion should be considered and may improve overall outcome.⁸⁷

RESULTS

The ABIOMED system is available in over 550 U.S centers, with over 5000 patients supported to date. Results from several reports are summarized in Table 17-5. In a multicenter study, Guyton¹³¹ reported 55% of postcardiotomy patients were weaned from support and 29% were discharged from the hospital. However, 47% of patients who had not experienced cardiac arrest before being placed on circulatory support were discharged. Of 14 patients who had presupport cardiac arrest, only 1 (7%) was discharged. In another report of 500 patients treated with the BVS 5000 system, which included 265 (53%) who could not be weaned from cardiopulmonary bypass, 27% of patients were discharged from the hospital.¹³² Recent data utilizing this device in a wide range of clinical situations, including postcardiotomy failure, have reported successful weaning in 83% and discharge to home for 45% of patients.¹²⁹ These excellent results are also repoted by Marelli¹²⁶ in 14 of 19 patients who were weaned or transplanted with a 1-year survival of 79%. Korfer¹³³ also recently reported 50% hospital discharge in 50 postcardiotomy patients supported with the ABIOMED, and 7 of 14 patients transplanted with a 1-year survival of 86%. The ABIOMED Worldwide registry experience suggests that that better results can be expected from experienced heart transplant programs.¹³⁴ In fact, early transfer of patients from smaller facilities (the "spokes") to transplant centers (the "hub") has been shown to result in improved overall survival.⁶

The Thoratec VAD (Thoratec Laboratories Corp., Berkeley, CA) was introduced clinically in 1976 under an investigational device exemption (IDE) and was approved as a bridge to heart transplantation in 1996. Although it was first used clinically in 1982 for postcardiotomy support, only recently has it been approved for use as a temporary cardiac assist device. As such, it is the only device currently available that bridges the gap between short- and long-term devices. The advantage of this concept is that it allows the device to be implanted initially with the intent of myocardial recovery, particularly in the postcardiotomy heart failure setting. If myocardial recovery does not occur, then the device can be used long term until a suitable heart becomes available for transplantation. Although this concept is desirable and offers the advantage of avoiding a second operation for transitioning from a temporary support device (bridge to bridge) to a more chronic device (see below), it comes at increased expense (because of the higher price of the pumps) and can also create ethical dilemmas if the patient recovers but is not a transplant candidate.

The device is a pneumatically driven pulsatile pump that contains two polyurethane, seamless bladders within a rigid housing.¹³⁵ The inlet and outlet ports contain Bjork-Shiley convexo-concave tilting disc valves to provide unidirectional flow. The effective stroke volume of each prosthetic ventricle is 65 mL. The pneumatic drive console applies alternating negative and positive pressures to fill and empty each prosthetic bladder. Driveline vacuum and positive pressures can be adjusted to improve filling and to improve systemic arterial pressure. The pump eject time (equivalent of ventricular systole) also can be adjusted depending on preload and afterload conditions. One or both pumps may be used to provide univentricular (LVAD or RVAD) or biventricular support (BVAD).

The prosthetic ventricles are placed on the upper abdomen (Fig. 17-8) and are connected to the heart and great vessels by large bore, wire-wrapped, polyurethane cannulas that traverse the chest wall.^136,137 The cannulas are connected to a large console via pneumatic drivelines. The pumps can be operated in three control modes. The fixed rate mode operates independently of the patient's heart, and the operator sets the rate. In the external synchronous mode, the pump empties when triggered by the patient's R-wave on the electrocardiogram. The usual mode for most patients is the full to empty mode in which ejection occurs when the device senses that the prosthetic ventricle is filled. In this volume mode, heart rate is determined by the rate of prosthetic ventricular filling.

View larger version (116K): [in this window] [in a new window]   FIGURE 17-8 The Thoratec device shown here can be placed in a variety of positions for support of one or both ventricles. (A) Cannulation of the left atrial appendage. (B) Cannulation of the left ventricular apex. (C) Cannulation of the interatrial groove. (Courtesy of Nancy E. Olson, Thoratec Corporation.)

CANNULATION

The device implantation is typically performed on cardiopulmonary bypass. Recently, a method has been described for off-pump insertion as a right ventricular assist device.¹³⁸ As with the ABIOMED, it is important to carefully select the cannula position and cutaneous exit sites. The pump should be planned to rest on the anterior abdominal wall. Lateral placement may lead to excessive tension at the skin exit sites and prevent a seal from being formed. Approximately 1.5 to 2 cm of the felt covering of the cannulas must extend beyond the skin exit site with the remainder in the subcutaneous tunnel to promote ingrowth of tissue and create a seal. The length of the cannulas extending out must be adjusted based on the length of the atrial cannula (if one is used). The end of the atrial cannula widens out to connect to the inflow port of the device and therefore cannot be trimmed. On the other hand, ventricular or outflow cannulas can be trimmed to adjust the length.

The outflow cannula is generally attached first. The arterial cannulas are available with a 14-mm Dacron graft (for the pulmonary artery) or an 18-mm Dacron graft (for the aorta), and must be cut to length after the appropriate exit site has been selected. They come in two lengths, 15 cm and 18 cm, which are again selected based on the patient's anatomy and the planned exit site. The graft is generally sewn on the aorta or pulmonary artery after applying a partial occluding clamp, and sewn with 4-0 polypropylene suture with or without a strip of pericardium or Teflon felt for reinforcement. An alternative technique for pulmonary outflow has been described.¹³⁸ This technique involves placement of pledget-reinforced 3-0 polypropylene suture anteriorly over the right ventricular outflow. The right-angle atrial inflow cannula is then immersed in hot water and straightened. This cannula is inserted through a stab incision in the right ventricular outflow tract and directed through the pulmonary valve into the main pulmonary artery. This method of cannula insertion has not been widely reported.

Inflow can be accomplished by cannulation of the atria or the ventricles.¹³⁹ All cannulations are generally reinforced with a double layer of pledgeted concentric purse strings. For atrial cannulation, a 51F right-angle cannula is available in two lengths, 25 cm and 30 cm. For the left atrium, the cannula is inserted throught the atrial appendage, the interatrial groove, or the superior dome of the left atrium. For right atrial cannulation, the cannula is inserted into the middle right atrial wall and directed towards the inferior vena cava. If this cannula is inserted towards the tricuspid valve, the leaflets may interfere with proper inflow as they get sucked into the cannula during negative pressure. In addition, if the tip is not advanced to reside properly in the right atrium, negative pressure can intermittently cause the compliant right atrial wall to collapse around the cannula and interfere with proper filling of the pump.

An alternative approach can use the ventricles for inflow cannulation.^138,139 This provides better drainage, higher flows, and perhaps improves the chance of myocardial recovery. This is achieved by placing a concentric layer of pledgeted horizontal mattress sutures at the apex of the left ventricle or the acute margin of the right ventricle (superior to the posterior descending artery). The cannulas used for this purpose can be either the blunt-tip 27-cm straight ventricular tube or the smooth-tipped, beveled 20-cm ventricular tube. The previously placed sutures are sequentially passed through the cuff, the apex of the heart is elevated, and the mean arterial pressure at the aorta is maintained over 70 mm Hg to prevent air embolization during cannula insertion. Either a core of tissue is removed or a cruciate incision is made with care not to cut any of the mattress sutures. The cannula is then inserted and secured by tying the sutures. The free end can then be directed out through the previously planned cutaneous exit site and a tubing clamp is placed on it.

INITIATION OF SUPPORT

Connecting the cannulas to the pump is difficult and must be done with care. The connections to the pump have a sharp beveled edge that should be carefully directed under gentle pressure to fit the cannulas without damaging the inner surface of the tube. In addition, if this tip bends, it may provide a nidus for thrombus formation. The inflow cannula is typically connected first. Prior to connecting the outflow cannula, a purse string is placed on the outflow Dacron graft and a 5F to 7F vascular catheter with an aspiration port is introduced through it and directed into the pump chamber through the outflow valve. This maneuver is critical because de-airing can be difficult as a result of the way the pump sits on the abdomen. The outflow cannula is then connected. Prior to releasing the tubing clamps and flooding the chamber with blood, the air in the chamber is aspirated. Once all the air is evacuated, the de-airing catheter and the inflow tubing clamp are removed, allowing the chamber to fill with blood. Then gentle hand pumping can be performed to ensure complete air evacuation through the opening in the Dacron graft prior to removing the outflow tubing clamp.

We slowly come off cardiopulmonary bypass before starting the pump. This ensures complete filling of the chambers to minimize the possibility of air being introduced into the circuit by the negative pressure of the pump. If a BVAD is in place, we start the LVAD first and then the RVAD. The pump is begun in the fixed rate mode and negative pressure is set low at –5 to –10 mm Hg. The inflow sites should also be covered with saline. Driveline pressure and and systolic duration can be adjusted to optimize pump flows. Additional refinements can be done once the chest is closed.

COMPLICATIONS

The complications reported for the bridge to transplantation are similar to those reported for postcardiotomy patients. In a multicenter trial the most common complications were bleeding in 42%, renal failure in 36%, infection in 36%, neurologic events in 22%, and multisystem organ failure in 16% of patients.¹³⁵ Similar complications have been reported from other centers.^135,140,141

RESULTS

Most temporary use of the Thoratec VAD is for postcardiotomy patients, but the device has also been used for patients after myocardial infarction and during cardiac transplant rejection.^135,142,143 The Thoratec pump has also been used to support a patient with myocarditis who eventually recovered adequate native heart function.¹⁴⁴ After cardiotomy, results are similar to those obtained with continuous flow devices and the ABIOMED BVS 5000. In a review of 145 patients with nonbridge use of the Thoratec device, 37% of patients were weaned and 21% were discharged.⁷¹ More experienced centers have achieved hospital survival rates over 40%.^142,143 Renal failure and myocardial infarction are poor prognostic events for survival.¹³¹

The Thoratec pre–market approval experience (Table 17-6) for the treatment of 53 patients with postcardiotomy heart failure had an in-hospital survival of 28%. The majority of these patients were supported with a BVAD. The Bad Oeynhausen group, however, has reported a 60% survival for postcardiotomy patients supported with the Thoratec device.^133,145 Clearly the greatest advantage that the Thoratec device offers is that it can be applied for longer duration of support than any other temporary device mentioned previously. This feature may be uniquely advantageous particularly because the duration of support necessary is usually unclear in advance. All other devices mentioned have an increasing complication rate with a longer duration of support. Furthermore, the Thoratec device allows for physical rehabilitation and ambulation of the patients during the recovery period.

	BRIDGE TO BRIDGE OR BRIDGE TO HEART TRANSPLANTATION

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Similar improved results have been reported from the Cleveland Clinic, in which 18 of 107 postcardiotomy patients who were appropriate transplant candidates were converted to an LVAD.¹²⁴ Of these, 72% survived to transplantation and 92% were alive at 1 year. The successful use of LVADs for postcardiotomy support has also been reported by DeRose in a group of 12 patients.¹⁵³ LVAD support was converted to a HeartMate Assist device at a mean of 3.5 days. Of these 8 were transplanted, and 1 was explanted with an overall survival of 75%.

Korfer et al also reported on 68 patients supported with the ABIOMED BVS 5000 system.¹³³ The majority of these patients were postcardiotomy, with 32 being weaned, and 13 transplanted. Overall survival was 47%. More recently, Korfer reported on 17 patients with postcardiotomy shock who received the Thoratec device. In this group 7 were transplanted and 1 successfully weaned for an overall survival of 47.¹⁴⁵

Device Selection

To date, insufficient data exist to recommend one device over the other for patients who require temporary mechanical support. The particular device used is often based on availability rather than science. Currently, the majority of heart centers use the ABIOMED BVS 5000 as their primary means of short-term cardiac support.

For centers with multiple devices, patient presentation and cardiopulmonary status will determine the device selected. Patients undergoing cardiopulmonary resuscitation are best serviced by urgent femoral cannulation. This avoids the time delay of transportation and sternotomy. Patients with severe hypoxia and lung injury either from aspiration or pulmonary edema benefit from the oxygenation and lung rest provided with ECMO. Oxygenators have been added to the ABIOMED system but they add substantial afterload to the system and reduce the flow.

For postcardiotomy support all devices have been used with similar success. The Thoratec system is the most versatile, providing both short- and long-term support. If a bridge to bridge strategy is utilized, we prefer ECLS followed by implantation of the TCI HeartMate. Typically, patients are supported on ECLS for 48 to 72 hours while transplant evaluation is completed. They are then transitioned to the HeartMate if myocardial recovery has failed. This approach avoids the high-risk emergency heart transplantation and provides the time necessary for improved organ function.

The best device for acute myocardial infarctions or myocarditis remains uncertain. For fulminant myocarditis the device least traumatic to the heart is advisable, as recovery is likely. Transplantation is more likely for more indolent myocarditis or for giant cell myocarditis, and a chronic implantable system or the Thoratec device may be the best choice. Whether earlier support or direct LV drainage will improve recovery in the setting of an acute MI is unknown.

Patient Management

The ultimate goal is to maintain optimal perfusion of all end-organs, to allow time for recovery from an acute hemodynamic insult, and to prevent further deterioration of organ function. Ideally, pump flow would achieve mixed venous saturation greater than 70%. Low-flow states can often be corrected by intravascular volume expansion. With centrifugal pumps and ECMO, pump speed can be adjusted to control flow and allow some degree of cardiac ejection to decrease the likelihood of stasis and intracardiac thrombus formation. Increasing flow rates by using excessive pump speeds can also cause significant hemolysis. Fluid administration to expand intravascular volume is the best way to increase flow. However, right heart failure may also manifest as a low-flow state in presence of low pulmonary artery pressures. This condition usually requires the institution of right-sided circulatory support and is associated with a lower overall survival.

VENTILATORY SUPPORT

We use pressure-controlled ventilation to maintain peak inspiratory pressures below 35 cm H₂O at tidal volumes of 8 to 10 mL/kg. Inspired oxygen is set initially at 100% with a positive end-expiratory pressure of 5 cm H₂O. Fractional inspired oxygen is then gradually decreased to less than 50% with partial pressure of oxygen maintained between 85 and 100 mm Hg. These measures are instituted to diminish the deleterious effect of barotrauma and oxygen toxicity in the setting of lung injury.

ANTICOAGULATION

Anticoagulation should be done judiciously to balance excessive risk of bleeding against clot formation in the pump. Platelet counts decrease within the first 24 hours of support; therefore, counts are monitored every 8 hours and we routinely transfuse platelets to maintain counts over 50,000/mm³ during routine support and above 100,000/mm³ if bleeding is present. Fresh frozen plasma and cryoprecipitate are given to control coagulopathy and maintain fibrinogen greater than 250 mg/dL and also replace other coagulation factors consumed by the circuit. Although other institutions have reported on the use of plasminogen inhibitors such as aminocaproic acid and aprotinin to decrease fibrinolysis,¹⁵⁴ we have not routinely used these drugs because of concern for thrombus formation. In most cases heparin infusion is started soon after adequate hemostasis is present. Anticoagulation is achieved by systemic heparinization with a continuous infusion starting at 8 to 10 U/kg/h and titrated to maintain PTT between 45 and 55 seconds. For ECMO, in most cases heparin infusion is started within 24 hours if used in the postcardiotomy setting but sooner in patients without a sternotomy.

FLUID MANAGEMENT

Patients are aggressively diuresed while on support to minimize third space fluid accumulations. If response to diuretic therapy is suboptimal because of renal insufficiency, we use a hemofilter/dialyzer spliced into the arterial or venous limb of the circuit if feasible. Otherwise, we use continuous venovenous hemodialysis (CVVHD). This system permits control over fluid balance by continuous ultrafiltration that can be adjusted for volume removal and also allows for dialysis as needed.

NEUROLOGIC MONITORING

Patients are sedated with fentanyl or propofol infusion to maintain comfort. Muscle paralysis is utilized as needed to decrease the energy expenditure and to decrease chest wall stiffness to allow for optimal adjustment of the ventilation parameters. All patients are periodically assessed off sedation to establish neurologic function. Response to simple commands, ability to move all extremities, and spontaneous eye movements are used as gross indications of intact sensorium. A low threshold of obtaining CT scans of the head is exercised if any change is noted or index of suspicion is high.

WEANING

A weaning trial is usually attempted after 48 to 72 hours of support. It is critical not to rush weaning and to allow time for myocardial as well as end-organ recovery. The principle of weaning is common to all devices and all have various controls available that allow reduction of flow, thereby enabling more work to be performed by the heart. Flow is gradually reduced at increments of 0.5 to 1 L/min. Adequate anticoagulation is critical during this low-flow phase to prevent pump thrombosis, and in general it is not recommended to reduce flow to less than 2.0 L/min for a prolonged period. We add additional heparin during this period to maintain ACT above 300 seconds. With optimal pharmacologic support, and continuous TEE evaluation of ventricular function, flows are reduced while monitoring systemic blood pressure, cardiac index, pulmonary pressures, and ventricular size. Maintenance of cardiac index and low pulmonary pressures with preserved LV function by echo suggests weaning is likely. A failed attempt at weaning results in resumption of full flow. Absence of ventricular recovery after several wean attempts is a poor prognostic sign. Patients who are transplant candidates undergo a full evaluation and subsequently are staged to a long-term ventricular assist device as a bridge to cardiac transplantation. We and others have found that early conversion to chronic ventricular support is beneficial and improves the low survival that is associated with cardiogenic shock, particularly in the postcardiotomy setting.^{119,124,133,153}

	SUMMARY OF COMPLICATIONS AND RESULTS

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Duration of Support

Complications tend to increase with increasing length of support. Therefore, in general, these devices are used for less than 2 weeks but longer durations have been reported (Table 17-7). An exception to this general rule is the Thoratec device, which can be used for a longer period until cardiac transplantation. The longest reported duration of support with this device is 365 days. Table 17-8 summarizes the indications for which temporary support has been implemented.

At present all current devices are thrombogenic and require anticoagulation. The delicate balance between overanticoagulation resulting in bleeding versus inadequate anticoagulation and thromboembolism is a major determinant of morbidity.

BLEEDING

During the acute phase, bleeding remains a significant problem, occurring at suture lines and cannulation sites, or often consists of a diffuse coagulopathy that becomes difficult to localize. The high incidence is partly because of the hemostatic disarray associated with the operation, the low-flow physiologic state that necessitates pump placement, and the need for anticoagulation early in the course of support. In general, this translates into a large transfusion requirement that can be detrimental in terms of problems with transfusion reactions, increases in pulmonary vascular resistance, and importantly increased sensitization of patients who may later require transplantation. The need for rapid transfusion often prohibits the use of filters that generally slow the rate of infusion. Golding has reported severe bleeding in 87% of patients supported with centrifugal pumps with a mean transfusion requirement of 53 units of blood.⁹⁷ We have seen a median transfusion requirement of 14 units (range 1 to 99) using ECMO.⁸⁵

The use of heparin-coated circuits has failed to reduce the coagulopathy and bleeding associated with ECMO effectively. However, peripheral cannulation with ECMO for acute support is associated with less bleeding than transthoracic approaches in the postcardiotomy setting. Similarly, with the ABIOMED and Thoratec devices, the incidence of bleeding has been as high as 27%.^89,131

THROMBOEMBOLISM

Despite the development of heparin-coated systems, the incidence of thromboembolism remains a constant threat. Thrombin deposition in centrifugal pumps with increasing duration of support is a well-known phenomenon. Golding reported thromboembolism in 12.7% of 91 patients supported with centrifugal pump for postcardiotomy pump failure.⁹⁷ In 202 adult patients supported with ECMO, pumphead thrombus was noted in 5% and neurologic complications occurred in 29%.⁸⁵ Both factors were found to have a profound negative impact on survival or ability to be weaned from support. Similarly, thromboembolic incidence of 8% and 13% has been reported for the Thoratec and ABIOMED, respectively. These numbers may underestimate the actual number of thromboembolic episodes. Curtis et al reported autopsy results in 8 patients who had no clinical evidence of thromboembolism. In this group, 5 (63%) were found to have evidence of acute thromboembolic infarction in the cerebral, pulmonary, and system territories.¹⁵⁵

Weaning and Survival

There are currently no data to indicate that one device is superior over another in terms of weaning and survival. Published reports suggest that weaning can be accomplished in approximately 45% to 60% of patients; however, survival overall is less than 30% with only 50% of weaned patients discharged alive from the hospital. Reports on long-term follow-up in this group are unavailable. The Cleveland Clinic recently demonstrated in a cohort of patients supported on ECMO that the high early attrition rate diminishes rapidly within 6 months of ECMO removal, and 65% of patients discharged are alive at 5 years.⁸⁵ Risk factors associated with increased mortality have included age older than 60 years, emergency operations, reoperations, renal insufficiency, and preexisting left ventricular dysfunction. In all series, sepsis, multisystem organ failure, and neurologic complications stand out as the causes of death.

This overall static survival rate in reported series over the last decade has seen significant improvement at transplant and assist centers where appropriate transplant candidates are bridged to transplantation after a period of support. In the Cleveland Clinic experience, ECMO support has been converted to an implantable LVAD in 18 patients.⁸⁵ Of these, 72% survived to transplant with 92% 1-year survival. DeRose et al have described the successful use of an implantable LVAD for postcardiotomy support in a group of 12 patients after elective or emergency coronary artery grafting requiring IABP, Bio-Medicus, or ABIOMED LVAD support.¹⁵³ All were converted to the TCI HeartMate at a mean of 3.5 days. Of these, 9 were transplanted, 1 was explanted, and all discharged for an overall survival of 75%. Similar results have been described by Korfer.¹³³ In their experience with 68 patients supported with the ABIOMED BVS 5000, the majority with postcardiotomy failure, 32 were weaned and 13 patients transplanted with an overall survival of 47%. Thoratec VADS was used in another 17 patients at their institution for postcardiotomy support with 8 survivors (47%), 7 patients transplanted, and 1 successfully weaned.

	CONCLUSION

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Currently a number of options exist for temporary circulatory support, and with advances in technology the number of devices will expand. Each device has advantages and disadvantages and to date none satisfy all the requirements of an ideal device. We have clearly learned many lessons that should direct the development of systems and strategies that maximize survival and reduce complications. In this arena, better understanding of the host inflammatory response, appreciation of the induced derangement in the coagulation cascade, and development of systems that do not require anticoagulation should improve overall outcomes. In addition, development of therapies that alter the reperfusion injury and preserve organ function is important. Agents that affect the inflammatory response in general, such as steroids, aprotinin, and plasmapheresis, or more specific blockades such as leukocyte depletion or direct cytokine inhibition, will need evaluation.

Risk analysis has also taught us that patients requiring postcardiotomy support generally fit into a particular profile. Specifically, these are patients who require emergency operations, have poor ventricular reserve, are older, and have extensive atherosclerotic coronary disease and preexisting renal dysfunction. Preoperative awareness should prompt maximization of medical pharmacologic support and the readiness to implement mechanical devices early in the face of cardiac pump failure.