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Satti SD, Epstein LM. Cardiologic Interventional Therapy for Atrial and Ventricular Arrhythmias.
In: Cohn LH, Edmunds LH Jr, eds. Cardiac Surgery in the Adult. New York: McGraw-Hill, 2003:12531270.

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Chapter 52

Cardiologic Interventional Therapy for Atrial and Ventricular Arrhythmias

S. Dinakar Satti/ Laurence M. Epstein

HISTORICAL EVOLUTION
????Surgical Ablation
????Catheter Ablation
BIOPHYSICS OF RADIOFREQUENCY ABLATION
ELECTROPHYSIOLOGY STUDY PROCEDURAL PROTOCOL
????Patient Screening
????Patient Monitoring
????Diagnostic Electrophysiology Study
DIAGNOSTIC ELECTROPHYSIOLOGY TECHNIQUES
????Activation Mapping
????Pace Mapping
????Anatomic Mapping
????Entrainment Mapping
????Advanced Mapping Techniques
CLINICAL APPLICATIONS
????Atrioventricular Nodal Reentrant Tachycardia
????Atrioventricular Reentrant Tachycardia
????Atrial Tachycardia
????Atrial Flutter
????Surgical ScarRelated Atrial Arrhythmias
????Atrial Fibrillation
????Ventricular Tachycardia
????Cost-Effectiveness
FUTURE DIRECTIONS
????Imaging Technology
????Catheter Technology
????Alternative Energy Sources
CONCLUSION
REFERENCES

?? INTRODUCTION
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Cardiologic intervention for tachyarrhythmias has evolved from pharmacologic therapy to surgically based elimination of arrhythmogenic foci and circuits to transvenous catheter-based ablation procedures. It has dramatically changed the management of tachyarrhythmias. Catheter-based ablation has become the standard form of therapy for many tachyarrhythmias, surpassing pharmacologic and surgical methods.

This chapter will review the development of catheter-based ablation methods. It will focus on radiofrequency (RF) ablation, from the physics of RF energy as an ablation tool to clinical application to specific tachyarrhythmias. It will also review mapping techniques for localization of tachyarrhythmias to appropriately target the energy delivery. Lastly, future techniques and technologies will be discussed.


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Techniques for catheter-based ablation of tachyarrhythmias began with the development of methods for intracardiac recording of electrical signals. This led to the procedures for programmed stimulation of cardiac tissue for the induction and termination of tachyarrhythmias. In 1967, Durrer first described initiation and termination of tachycardia in a patient with WPW syndrome.1 In 1969, the His bundle was first reproducibly recorded using a transvenous electrode catheter.2 This led to a variety of tachyarrhythmias to be studied using intracardiac catheters. Catheters were used to identify mechanisms of tachycardia initiation and maintenance.

The idea emerged that critical regions of cardiac tissue were necessary for the initiation and propagation of tachyarrhythmias. If these regions could be interrupted, the tachyarrhythmia could then be clinically cured. Once catheter mapping could localize arrhythmogenic foci, surgical excision was explored. In 1968, such a surgical procedure for the elimination of an accessory pathway was first published.3 This heralded an era of nonpharmacologic treatment of tachyarrhythmias.

Surgical Ablation

A variety of arrhythmogenic foci and circuits were successfully mapped and ablated using surgical techniques in the 1970s. Resection of an atrial focus was described in 1973 to cure atrial tachycardia.4 Surgical ablation became more refined and precise as mapping techniques localized tachyarrhythmias with greater detail. With the elucidation of reentry circuits within the atrioventricular (AV) node, surgical dissection was performed to treat AV nodal reentrant tachycardia without causing AV block.5

Although surgical ablation was therapeutic for a variety of tachyarrhythmias, the morbidity and mortality associated with thoracotomy and open heart surgery limited its application. The risk of the procedure was not justified for most patients with tachyarrhythmias because they were not life threatening. Rather, ablation procedures were an option of last resort in highly symptomatic patients refractory to medical therapy. With these limitations of the surgical approach, catheter-based ablation procedures were explored.

Catheter Ablation

Once the His bundle could be recorded with consistency using catheters and thus localized, the idea evolved to using the catheter to delivery energy to cardiac tissue to achieve local tissue injury. In 1981 Scheinman et al reported the first catheter-based ablation procedure, describing the ablation of the His bundle in dogs.6 This same group, in March of 1981, performed the first closed-chest ablation procedure in man. They described a patient with atrial fibrillation refractory to medical therapy. The patient was placed under general anesthesia and a catheter was advanced to the His bundle region. Using a standard external direct current (DC) defibrillator and by attaching one of the defibrillator pads to the intracardiac catheter, energy was delivered between the distal electrode of the catheter and a cutaneous grounding pad. A series of DC shocks were then delivered, achieving complete heart block.7

Induction of complete AV nodal blockage was then extended to the treatment of other supraventricular tachycardias (SVTs).8 This necessitated the implantation of a pacemaker. As experienced was gained and specific catheters were developed, energy could be more precisely directed to target a variety of tachyarrhythmias, including ablation of accessory pathways, atrial tachycardia, single limb of AV nodal reentrant tachycardia, and ventricular tachycardia. Although DC shock ablation advanced the field of catheter-based ablation beyond surgical ablation, it had its own limitations. High-energy discharges resulted in the formation of a plasma ball at the distal electrode. Energy delivery was not titratable and lesions were patchy in nature. Cardiac rupture and perforation were associated risks with this procedure. In addition, since direct current energy stimulated skeletal muscle, general anesthesia was required for these procedures.

The development of the use of radiofrequency (RF) energy as an ablative energy source heralded a new era in the nonpharmacologic, nonsurgical treatment of tachyarrhythmias. RF energy had been used for decades by surgeons for surgical cutting and cautery and had a long history of safety and efficacy. Animal studies using RF energy were first described in 1987.9 RF energy produces controlled lesions at the catheter tip over a period of 40 to 90 seconds. The improved safety and efficacy of RF catheter-based ablation procedures superseded the extensive and imprecise lesions created by DC shock ablation techniques.


?? BIOPHYSICS OF RADIOFREQUENCY ABLATION
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Radiofrequency (RF) ablation involves the delivery of sinusoidal alternating current between the catheter tip at the endocardial surface and a large grounding pad on the skin. The current has a frequency of 350 to 700 kHz. The use of frequencies below 350 kHz results in direct skeletal and cardiac muscle stimulation. This results in perception of pain by the patient as well as induction of polymorphic tachyarrhythmias. The use of frequencies above 700 kHz results in loss of energy during transmission as well as a decrease in resistive heating. Limiting energy delivery to the range of 350 to 700 kHz makes the procedure relatively painless and eliminates the need for general anesthesia.

The principal method of tissue injury with RF delivery is thermal. As the RF energy passes through the tissue at the distal electrode of the ablation catheter, resistive heating produces coagulation necrosis. The lesions produced are well demarcated and are 5 to 6 mm wide by 2 to 3 mm deep when a standard catheter tip is used. No significant heating occurs at the tissue-grounding pad interface due to the large surface area of contact. To achieve irreversible tissue injury, a temperature of about 55?C to 58?C is required.10 At temperatures above 100?C, plasma reaches the boiling point and coagulates on the catheter tip. Coagulum and desiccated tissue act as an insulating barrier to energy delivery, resulting in a rise in impedance and the prevention of tissue heating. If subendocardial tissue temperature exceeds 100?C, steam may be formed within the tissue, resulting in a rapid expansion and crater formation and an audible "pop." Such lesions can cause unpredictable injury, resulting in thromboembolic risk and rupture of thin-walled structures. Thus it is important to regulate the temperature of the catheter tissue interface either manually or automatically by regulating the energy delivered.

The morphology of the lesion formed by RF energy is distinct. There is a centralized region of necrosis with surrounding inflammation. As the lesion matures in a matter of weeks to months, the surrounding region of inflammation may progress to necrosis or inflammation may resolve without permanent damage. Thus it is critical to place RF lesions precisely. Tachyarrhythmias that initially disappear with RF delivery may recover if the lesion is peripheral to the site of the tachyarrhythmia and the temporary inflammation resolves. On the contrary, a lesion may extend as the border zone of inflammation becomes necrotic, resulting in unintentional injury to surrounding tissue.

RF lesion size is well suited for focal endocardial tachyarrhythmias. Tachyarrhythmias utilizing a reentrant circuit with a wide isthmus or localized deep in the myocardium may be more difficult to ablate. Advancements in catheter technology have improved the success rates of RF ablations. Primary of these was the inclusion of a thermistor or thermocouple within the ablation electrode to monitor catheter-tissue interface temperature. This allows the maximal energy delivery without the buildup of excessive temperature that results in coagulum and increases in impedance. Other advances have been the development of larger catheter tips, which increase the surface area, reducing current density and increasing cooling by surrounding blood pool. Irrigated catheters bathe the catheter tip internally or externally with a saline solution to cool the catheter-tissue interface, preventing coagulum formation and enhancing energy delivery. Catheters themselves have been designed to be steerable and allow precise positioning of the tip to the region of interest. Future advances in catheter technologies will be discussed later in the chapter.


?? ELECTROPHYSIOLOGY STUDY PROCEDURAL PROTOCOL
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Catheter ablation of tachyarrhythmias is typically an elective procedure with the patients presenting on an outpatient basis. As it is an invasive procedure and thus carries some procedural risk, patients need to be thoroughly evaluated and informed regarding the procedure. Occasionally electrophysiology procedures are performed on an emergency basis in patients with recurrent or incessant hemodynamically significant arrhythmias.

Patient Screening

Patients are typically seen on a consultative basis. A complete history and physical examination are performed. An ECG at baseline and recordings of the tachyarrhythmia are crucial to planning the procedure. A patient may present to a physician's office or to an emergency room with the tachyarrhythmia, allowing the recording of a 12-lead ECG. A loop recorder is sometimes helpful to record tachyarrhythmias that are infrequent or of short duration. The recording of onset and termination of the tachyarrhythmia, either spontaneously, with vagal maneuvers, or with drugs, is also very helpful in determining the mechanism of the tachycardia.

Several adjunctive studies may be necessary to fully assess the patient. An echocardiogram is useful in evaluating for the presence of structural heart disease that will sway the differential diagnosis of an unknown tachyarrhythmia. A treadmill exercise stress test is useful in evaluating the relationship of catecholamine state in the induction of tachyarrhythmias. If clinical history indicates, right heart catheterization, stress testing with imaging, or coronary angiography may be performed to evaluate for volume status and coronary artery disease. This is useful to rule out ischemia in an atypical presentation and to assure that the patient will tolerate a prolonged period of supine posture, sustained tachycardias, and possible hypotension during the procedure.

Pharmacologic considerations include stopping all antiarrhythmic medications at least four half-lives prior to the procedure to allow for induction of tachyarrhythmias. In most cases, AV nodal blocking medications should also be stopped when the AV node may be involved in the reentrant circuit. Anticoagulation medications should also be stopped and, depending on the indication, the patient may be bridged for the procedure with subcutaneous low molecular heparin or admitted into the hospital for conversion to intravenous heparin. Because of the need for systemic anticoagulation for procedures that require left heart access, the menses cycle should be taken into consideration in timing the procedures in premenopausal women. Routine complete blood count, electrolyte status, and coagulation profile should be obtained. Additional laboratory tests that are sometimes useful are thyroid function tests in patients with history suggestive of hyperthyroid state and beta HCG serum levels in women of childbearing age.

Patients should present on the day of the procedure in a fasting state. Intravenous access is obtained for administration of conscious sedation and for emergency access. Procedures are typically performed using short-acting benzodiazepines and narcotics in combination. General anesthesia is rarely required. It may be used in patients with idiosyncratic reaction to benzodiazepines, those undergoing prolonged procedures, or in unstable patients, such as those undergoing ablation of ventricular tachycardia.

Patient Monitoring

Continuous monitoring is performed using 12-lead surface ECG, noninvasive or invasive blood pressure monitoring, continuous pulse oximetry, and capnography. An external defibrillator is available and attached to the patient with "hands-free" patches throughout the procedure. Intubation and resuscitation supplies should be readily available. Electrophysiological procedures are typically divided into diagnostic and ablative phases. The diagnostic phase involves obtaining venous access, passage of multiple catheters into the heart to record intracardiac electrograms, and induction and mapping of tachyarrhythmias. Once tachyarrhythmias are localized, ablation follows.

Diagnostic Electrophysiology Study

Diagnostic localization of tachyarrhythmias involves positioning catheters in strategic locations within the heart to obtain intracardiac recordings from all four chambers of the heart as well as from the His bundle. Venous access is typically obtained in the bilateral groins via the right and left femoral veins. Catheters of 4F to 6F in size are passed into the right atrium and right ventricle as well as positioned just across the tricuspid value to obtain His bundle recordings under fluoroscopic guidance. To obtain recordings of the left atrium and ventricle, a catheter is guided into the coronary sinus, which passes posteriorly in the atrioventricular groove and drains into the right atrium. Because of the angle of the entrance into the coronary sinus, cannulation is easier via the superior vena cava and thus typically venous access has been obtained from the right internal jugular, left subclavian, or left antecubital vein. However, steerable catheters allow for reliable access from the inferior approach and are now being used more frequently (Fig. 52-1).



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FIGURE 52-1 Radiograph in the right anterior oblique projections showing catheters positioned for a standard diagnostic electrophysiology procedure. Three nonsteerable diagnostic catheters are introduced from the inferior vena cava into the right heart. Two 4F catheters with 4 electrodes are positioned in the region of the right atrial appendage (RA) and right ventricular apex (RV apex). A 5F catheter with 6 electrodes is positioned across the tricuspid annulus to obtain a His bundle recording (His). A nonsteerable 6F catheter is introduced via the right internal jugular vein into the coronary sinus to obtain left atrial and ventricular recordings. Finally a deflectable 7F ablation catheter is positioned in the region of the low right atrium.

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Direct recordings of the left heart are sometimes necessary and accomplished either by trans-septal cannulation via the intra-atrial septum from the right atrium or via a retrograde approach from the femoral artery and across the aortic valve. Systemic anticoagulation with heparin is maintained during catheter manipulation in the left heart because of the risk of thromboembolic events due to platelet and thrombin aggregation on the catheters, coagulum formation with RF delivery, and fibrinolytic activation.

Once diagnostic catheters are positioned, programmed electrical stimulation is performed to induce and study the tachyarrhythmia. Sometimes modulation of the autonomic nervous system is required to induce tachyarrhythmias with the infusion of atropine or isoproterenol. As will be described later, there are specific pacing maneuvers to initiate and evaluate the mechanism of a variety of tachyarrhythmias. Once an optimal site for ablation is assessed, steerable ablation catheters are positioned at the target site. These catheters come with a variety of curvatures. Specifically shaped long vascular sheaths may also be used to direct catheters. Once RF energy is delivered to the target site, reinduction of the tachycardia is attempted.

At the end of the procedure, all catheters and sheaths are removed and manual pressure held to achieve hemostasis. If the patient was heparinized for the procedure, sheath removal is delayed until anticoagulation reverses. The patient is placed on bed rest for 4 or more hours. During this recovery, the patient is monitored for hemodynamical stability and recovery from sedation, and to assess for bleeding from puncture sites. The patient may be discharged to home the same day or be observed overnight in the hospital. Routine follow-up studies are not warranted unless to assess for a complication. Patients may be placed on 4 to 6 weeks of aspirin to reduce the risk of embolic events due to thrombus formation on ablated myocardium, especially those undergoing ablations in the left heart.

In referring patients for catheter ablation, it is important to weigh the risks and benefits of the procedure for the individual patient. Most tachyarrhythmias, although causing a variety of symptoms, are generally hemodynamically well tolerated and are not life threatening. Thus an awareness of the potential complications of catheter ablation is necessary prior to referring a patient. Complications can be divided into those involving access, catheter manipulation within the heart, and ablation.

Access-related complications include pain, adverse drug reaction from anesthesia and sedation, infection, thrombophlebitis, and bleeding at the site of access. Associated with bleeding are hematoma or arteriovenous fistula formation. Arterial damage or dissection may also result. Systemic or pulmonary thromboembolism can occur, most seriously resulting in transient ischemic attack or stroke.

Complications associated with placement of intracardiac catheters can be more life threatening. These include trauma of a cardiac chamber or the coronary sinus resulting in myocardial infarction, perforation, hemopericardium, and cardiac tamponade. Programmed electrical stimulation can result in the induction of life-threatening tachyarrhythmias such as ventricular tachycardia or fibrillation. Catheter manipulation can also result in usually transient but sometimes permanent damage to valvular apparatus or to the conduction of the right or left bundles due to mechanical trauma.

RF delivery within cardiac structures carries with it its own set of risks. Inadvertent ablation of the normal conduction system could result in complete heart block requiring permanent pacing. Perforation of a cardiac chamber or vascular structure can also occur with RF delivery. Collateral damage to coronary circulation could result in myocardial infarction, heart failure, or cardiogenic shock. Phrenic nerve paralysis can occur. Ablation near the pulmonary veins within the left atrium can result in venous stenosis and pulmonary hypertension. Finally, new tachyarrhythmias can arise from the scar induced by RF ablation.

An 8-year prospective study of 3966 procedures found an overall complication rate of 3.1% for ablative and 1.1% for diagnostic procedures. Complications are more likely to occur in elderly patients and those with systemic disease.11 No deaths were reported in this series and other studies have shown very low mortality rates directly attributable to the electrophysiology study. Body dosimetry studies have shown that the lifetime excess risk per 60 minutes of fluoroscopy exposure is 294 per million cases or 0.03% per patient. The risk is higher for obese patients and the lungs are the most susceptible organs.12 Technology is evolving and advances will continue to improvethe safety and efficacy of this procedure.


?? DIAGNOSTIC ELECTROPHYSIOLOGY TECHNIQUES
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A variety of techniques have been developed to elucidate the origin and mechanism of tachyarrhythmia propagation. These techniques are necessary to reveal targets for ablation. They involve pacing in a specific chamber at particular intervals to initiate a tachyarrhythmia, assess its response to pacing maneuvers, or terminate it. Thus some of these techniques involve pacing in sinus rhythm while others are performed during the tachyarrhythmia. Occasionally pharmacologic modulation of the autonomic nervous system is used in conjunction with pacing maneuvers.

Activation Mapping

Activation mapping is a technique of localizing focal tachycardias and accessory pathways. It involves positioning the mapping/ablation catheter during the tachyarrhythmia in such a way that activation at the catheter tip precedes any other intracardiac activation or corresponding surface P wave or QRS. The earliest site of activation during a focal tachycardia must by definition be the source of the tachycardia.

Accessory pathways can also be mapped using this technique by tracing the tricuspid and mitral annulus with the ablation catheter. The atrial insertion can be localized by determining the earliest site of retrograde atrial activation during atrioventricular reciprocating tachycardia or in sinus rhythm during ventricular pacing. The ventricular insertion can be localized in patients with preexcitation by determining the site of earliest ventricular activation during sinus rhythm.

Pace Mapping

Pace mapping is performed during sinus rhythm after obtaining a 12-lead surface ECG during tachycardia. By pacing at different sites and comparing the resulting paced ECG to the tachycardia ECG, the disparity can be assessed and the catheter can be repositioned until a perfect 12-lead match is obtained. These sites are then the targets for ablation. This technique is typically used for focal ventricular tachycardias, especially those of right ventricular outflow origin.

Mapping of local electrogram potentials requires recording an intracardiac signal from the structure targeted for ablation. For instance, to achieve complete heart block, the His bundle is the target and the ablation catheter is positioned such that the distal tip is recording a His bundle potential. Similarly for bundle branch reentry, the right bundle potential is used to target the right bundle to achieve right bundle branch block. Sometimes an accessory pathway potential can be recorded and used to localize a target for ablation.

Anatomic Mapping

Anatomic mapping is yet anther way to localize potential targets for ablation. This does not involve any electrical signal but instead uses fluoroscopy to localize anatomical landmarks for ablation. Catheters placed through the inferior vena cava into the coronary sinus and across the tricuspid valve delineate these structures. In typical atrial flutter that is dependent on the inferior vena cava and tricuspid annulus isthmus, these anatomical landmarks are used for the delivery of a line of lesions to prevent conduction across this isthmus. Bidirectional conduction block across this isthmus terminates flutter and prevents its reinitiation.

Entrainment Mapping

Entrainment mapping is a technique of localizing reentrant circuits for ablation. It involves positioning the catheter in a region thought to be involved in a reentrant tachycardia. This is confirmed by pacing during the tachycardia slightly faster than the tachycardia rate. If the pacing site is within the circuit, then activation should proceed orthodromically around the circuit resulting in an identical activation pattern. The resultant QRS or P wave should match that of the tachycardia. At the termination of the pacing the activation wavefront proceeds around the circuit. Therefore, if pacing was in the circuit, the first beat should be close to the cycle length of the tachycardia.

In clinical situations a combination of these mapping techniques is used to localize an arrhythmia for ablation. Usually an anatomical approach is used to find the general region and then more precise mapping used to specifically localize the focal arrhythmia or reentrant circuit.

Advanced Mapping Techniques

The success of ablation is very much dependent on localization of arrythmogenic foci and circuits. The previously mentioned mapping techniques are useful for arrhythmias that originate from specific anatomical locations or have characteristic endocardial electrograms. Conventional fluoroscopy and the use of a single roving mapping catheter have limited success in ablation of complex arrhythmias that may originate from sites without characteristic fluoroscopic landmarks or have variable electrograms as recorded from the catheter tip. Advanced mapping techniques have been developed as adjuncts to conventional methods to improve the efficacy of catheter ablation for arrhythmias that are transient, focal, or hemodynamically unstable and thus require rapid mapping.

Multielectrode "basket" catheters (Constellation, Boston Scientific Corp., Natick, MA) consist of eight collapsible arms at the distal end that deploy within a cardiac chamber to provide recordings from 64 electrodes. Multielectrode catheters allow acquisition of electrograms from multiple sites simultaneously to enable reconstruction of activation maps such that a single ectopic beat can be mapped online rapidly. Although these catheters are still fluoroscopically guided, multiple data points are acquired simultaneously, limiting procedure times, especially in patients with hemodynamically unstable arrhythmias.13 The disadvantage of these catheters is the low spatial resolution (approximately 1 cm) due to the spacing between the electrodes and arms. As RF lesions are on the order of 0.5 cm in size, this limits the use of this technology to macro reentrant arrhythmias.

An electroanatomical mapping system (CARTO, Biosense, Diamond Bar, CA) uses a magnetic field to localize the mapping catheter tip in three-dimensional space. Three coils in a locator pad located beneath the patient's chest generate ultra-low-intensity magnetic fields in the form of a sphere that decays in strength. A sensor in the catheter tip measures the relative strength and hence the distance from each of the coils. This allows for the recording of the spatial and temporal location of the catheter. Electrodes at the catheter tip record local electrograms and this information is displayed on screen as a three-dimensional map of the local activation times relative to a reference catheter in a color-coded fashion. Data from multiple single mapping points acquired during tachycardia can be reconstructed to show animated sequences of arrhythymia propagation. Voltage maps can be obtained to delineate regions of scar and diseased myocardium.14

The advantages are that this system is nonfluoroscopic, and it allows for precise location of the catheter tip with a resolution of less than 1 mm. The catheter can be positioned independent of fluoroscopic landmarks. In addition, recordings of catheter position allow for the repositioning of the catheter to previously mapped sites of interest. A limitation of this mapping technique is that data acquisition is sequential and point to point. Construction of an activation sequence can be time consuming and difficult in nonsustained arrhythmias or those originating from multiple sites.15

A noncontact endocaridial mapping system (EnSite 3000, Endocardial Solutions, Inc, St Paul, MN) consists of a catheter with a woven braid of 64 insulated 0.003-mm-diameter wires with 0.025-mm breaks in insulation that serve as electrodes. Using a ring electrode on the shaft of the catheter as a reference, raw far field signals from the multielectrode array can be used to construct a virtual electrogram of the chamber of interest. A locator signal is also generated between the noncontact array and a standard mapping catheter to permit nonfluoroscopic localization of the catheter to regions of interest. The position of the roving catheter is acquired over time to construct a three-dimensional map of the endocardial surface. Landmarks can be tagged and labeled such as the His catheter or the coronary sinus.

Over 3360 virtual electrograms are simultaneously acquired by this system, allowing high-density maps to be acquired from a single beat. Thus this system is very useful for mapping in unstable or transient, nonsustained arrhythmias. It allows for nonfluoroscopic navigation of the mapping catheter, return to points of interest, and logging of ablation sites.16

Intracardiac echocardiography (ICE) has extended the principles of intravascular ultrasound (IVUS) for electrophysiological use.17 In contrast to IVUS catheters, ICE catheters use lower-frequency (5.510 MHz) transducers to extend the imaging range. Newer ICE catheters are steerable and have Doppler capability (Acuson, Mountain View, CA), allowing for hemodynamic evaluation of intracardiac structures. For electrophysiological use, ICE catheters are useful for visualization of endocardial structures such as the fossa ovalis and guiding trans-septal catheterization. As the recognition of the importance of anatomy in the genesis of arrhythmias has grown, so has the importance of imaging. ICE catheters allow the accurate targeting of anatomical sites such as the crista terminalis and pulmonary vein ostia. They are useful for imaging diagnostic and ablation catheter positions and visualization of tissue contact for optimal ablation. Through visualization of pericardial effusions, rapid diagnosis of perforation and other complications is feasible.


?? CLINICAL APPLICATIONS
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Using the techniques described above, a variety of tachyarrhythmias can be targeted for percutaneous catheter-based ablation, including both atrial and ventricular arrhythmias that are either focal or that utilize reentrant circuits.

Atrioventricular Nodal Reentrant Tachycardia

The most common supraventricular arrhythmia is atrioventricular nodal reentrant tachycardia (AVNRT). Of patients with supraventricular tachycardia, AVNRT represents up to 60% of cases that present to tertiary centers for electrophysiological studies. This tachycardia can present at any age although most patients that present for medical attention are in their 40s and the majority are female.18,19 Advances in RF catheter ablation of this tachycardia has made it a first-line therapy for those symptomatic patients not wishing to take medications.20

This tachycardia has a reentrant mechanism utilizing two pathways within the AV nodal tissue. Controversy remains about whether atrial tissue is an integral part of the circuit. The pathways are the "slow pathway" and "fast pathway" based on their relative conduction velocities. The anatomical location of these pathways is variable but generally located within the triangle of Koch. Koch's triangle is bounded by the tricuspid annulus and the tendon of Todaro with the coronary sinus at the base. The apex of the triangle is the His bundle at the membranous septum where it passes through the central fibrous body. The anterior third of the triangle contains the compact AV node and the fast pathway, and the middle and posterior portion, near the coronary sinus os, contains the slow pathway (Fig. 52-2).21 Recent studies suggest the left atrial extension of the AV node may be involved in the reentrant circuit.22



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FIGURE 52-2 (A) Diagrammatic representation of typical atrioventricular nodal reentrant tachycardia. Surface ECG shows narrow complex tachycardia with no clear P waves. The reentrant circuit (gray arrows) consists of the posterior slow pathway region acting as the antegrade limb and the anterior fast pathway region acting as the retrograde limb. The slow pathway target site is located between the coronary sinus os (CS) and the tricuspid valve annulus (TV). SVC = superior vena cava; RA = right atrium; IVC = inferior vena cava; RV = right ventricle. (B) Surface ECG showing precordial leads in AVNRT. This demonstrates that the retrograde P waves are barely discernable in some leads. In V1, it forms a pseudo r' wave (arrow). P waves are also visible in the terminal portions of QRS complexes in V2 and V3 but not in the lateral leads.

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In the typical form of AVNRT, antegrade conduction from the atrium to the ventricle occurs over the slow pathway and the retrograde conduction from the ventricle to the atrium occurs over the fast pathway. Since conduction in the retrograde direction is fast, the atria and ventricle are depolarized almost simultaneously. Thus the electrocardiographic feature of this tachycardia is P waves that are inscribed within the QRS and thus not seen or barely discernible at the termination of the QRS complex.23

In less then 10% of cases, the circuit is reversed. In atypical AVNRT, the antegrade limb occurs over the fast pathway and retrograde VA conduction occurs over the slow pathway. Thus the ECG of this tachycardia shows inverted P waves in the inferior leads denoting retrograde activation of the atria with short PR segment due to rapid antegrade conduction.

Ablation of either of these tachycardias involves disruption of the reentrant circuit. In the early days of catheter ablation therapy, highly symptomatic patients who had failed all medical therapy underwent complete atrioventricular junctional ablation utilizing DC current with the insertion of a permanent pacemaker. While this greatly improved these patients' quality of life, it made them pacemaker dependent. It was therefore not indicated for patients with less symptomatic arrhythmia.

Initial attempts to selectively eliminate AVNRT while leaving antegrade AV nodal conduction intact were performed with DC energy.24 It was difficult to choose a specific target and in some patients the fast pathway and in others the slow pathway was ablated. With the advent of RF energy, more selective ablation could be performed. Early procedures utilizing RF energy targeted the fast pathway. While successful in up to 90% of patients, there was still a 5% to 10% incidence of inadvertent complete heart block.25 In addition, ablation of the fast pathway left the patients with a prolonged PR interval. In some, this led to symptomatic atrial contraction during ventricular contraction (pseudopacemaker syndrome) especially during sinus tachycardia.

Subsequently it was found that the slow pathway could be successfully targeted in the posterior triangle of Koch.26 Slow pathway ablation has a high degree of success with recurrence rate in the range of 2% to 7% with the complication of complete AV block occurring about 1% (range 0%3%) of the time.27 The 1992 NASPE self-reported survey on 3052 patients who underwent slow pathway ablation showed a success rate of 96% with complication rate of 0.96%,28 and the 1998 NASPE survey reported a total of 1197 with a success rate of 96.1% and a complication rate of 1% incidence of AV block.29

Atrioventricular Reentrant Tachycardia

The next most common type of supraventricular tachycardia is atrioventricular reentrant tachycardia (AVRT). About 30% of supraventricular tachycardias are due to AVRT. This is a reentrant tachycardia utilizing the AV node and an accessory pathway. These accessory pathways are remnants of conductive tissue from embryonic development that span the normally electrically inert tricuspid and mitral valve annulus and provide an independent path of conduction outside of the AV node between the atria and the ventricles. The most common form of AVRT is part of the Wolf-Parkinson-White (WPW) syndrome of ventricular preexcitation and symptomatic arrhythmias. The most common accessory pathways connect the atrium to the ventricle. Other accessory pathways may connect the atria or AV node to the His-Purkinje system. In sinus rhythm, antegrade conduction over the accessory pathway results in preexcitation of the ventricles eccentric to the AV node and is manifested by a short PR segment and slurring of the onset of the QRS, the delta wave. Absence of these findings does not exclude an accessory pathway as the degree of preexcitation may vary or conduction may only occur in the retrograde direction (approximately 30% of accessory pathways).

Patients with WPW typically present with palpitations due to rapid heart rate. This may be the result of AVRT or due to any supraventricular tachycardia with resulting rapid atrioventricular conduction via the accessory pathway. Associated symptoms may be mild such as palpitations and shortness of breath or as severe as syncope and sudden death. Sudden death may be due to ventricular fibrillation resulting from the extremely rapid ventricular activation over the accessory pathway during atrial fibrillation in some patients.

Indications for ablation of accessory pathways include patients with symptomatic AVRT or those with atrial tachyarrhythmias with rapid ventricular conduction who fail or do not wish medical therapy. Relative indications for ablations include asymptomatic patients in high-risk professions, those with family history of sudden death, or those mentally distraught over their condition.15

In the typical or orthodromic form of AVRT, antegrade conduction from the atrium to the ventricle occurs over the AV node and retrograde conduction occurs over the accessory pathway. In this form of AVRT, the P wave in the tachycardia closely follows the preceding QRS complex with a long PR segment (Fig. 52-3). In the rare antidromic form of AVRT, antegrade conduction occurs over the accessory pathway with retrograde conduction over the AV node. This results in eccentric depolarization of the ventricle producing a wide complex tachycardia with retrograde P waves that can be easily mistaken for ventricular tachycardia with one-to-one VA conduction.



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FIGURE 52-3 (A) Diagrammatic representation of atrioventricular reentrant tachycardia. This macro-reentrant circuit (gray arrows) utilizes the AV node and an accessory pathway (AP), in this case a right lateral pathway. In orthodromic AVRT, antegrade conduction occurs over the AV node and retrograde conduction occurs over the AP. Because of the conduction delay from the His-Purkinje system through the ventricular myocardium to reach the AP, retrograde P waves are discernable after the QRS complexes (arrow). In antidromic AVRT, the reentrant circuit is reversed and surface ECG shows P waves that closely precede the QRS complexes. SVC = superior vena cava; RA = right atrium; IVC = inferior vena cava; CS = coronary sinus, TV = tricuspid valve; RV = right ventricle. (B) Intracardiac recording of atrioventricular reentrant tachycardia with termination of eccentric conduction over the accessory pathway during RF ablation. The tracing at 50 mm per second speed shows four surface leads (V1, II, I, and a VF) and intracardiac recording from catheters: ablation (ABL), His distal, mid and proximal as well right ventricular apex (RVA). The first three beats of the tracing show evidence of eccentric conduction over an accessory pathway: short PR segment and delta wave. With onset of RF energy from the ablation catheter positioned in the region of shortest AV conduction, conduction becomes normal within two beats, with normalization of PR segment and loss of delta wave.

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Patients with highly symptomatic WPW syndrome or asymptomatic patients in high-risk professions who had failed medical therapy were the initial population to undergo surgical interruption of accessory pathways. Although this procedure evolved with success rates near 100% and mortality rates of 1%,30 it still involved a major surgical procedure. Catheter-based ablation of accessory pathways has success rates approaching surgical ablation with lower morbidity and mortality.

The first catheter ablation procedures were performed with direct current energy in patients with posteroseptal accessory pathways.31 With the development of RF energy for ablation, accessory pathways in all locations could be treated.

Ablation of right-sided accessory pathways is performed by accessing the right heart via the femoral vein or occasionally via the antecubital, subclavian, or internal jugular veins. Left-sided accessory pathways are ablated via a trans-septal approach or via a retrograde approach from the femoral artery. The initial procedures to ablate left-sided accessory pathways were via a retrograde aortic approach. While successful in more than 95% of patients, there was a small risk of aortic valve perforation or dissection of a coronary artery.32 Most centers now perform ablation of left-sided accessory pathways via a trans-septal approach.

Rarely, left-sided accessory pathways can be ablated from within the coronary sinus if they are on the epicardial surface, which is unusual. Due to the more sloping architecture of the tricuspid annulus, it is more difficult to achieve a stable catheter position for right-sided pathways. While the use of long curved vascular sheaths may be helpful, the success rates for right free wall accessory pathways are lower then for other locations.

The major challenge that remains is the ablation of accessory pathways near the normal conduction system and those that are epicardial in location. Ablation of pathways that are anteroseptal and midseptal in location carries a high risk of causing complete heart block. It is hoped that newer ablative energies such as cryoablation may offer a safer alternative. Recently, elimination of epicardial accessory pathways via a pericardial approach has been attempted.33

Unusual accessory pathways include atriofasicular (Mahaim) pathways. These show decremental conduction and the atrial insertion is typically located in the right free wall. These can often be successfully ablated via an endocardial catheter approach.34

The 1998 NASPE prospective catheter ablation registry reported on 654 patients with a 94% success rate.29 Success rates are lower (in the range of 84%88%) for septal and right free wall pathways. Other pathways have success rates in the range of 90% to 95%.3537

Complications associated with ablation of accessory pathways include those associated with all ablation procedures. The 1998 NASPE registry reports major complications of cardiac tamponade in 7 patients, pericarditis in 2 patients, and acute myocardial infarction, femoral artery pseudoaneurysm, AV block, and pneumothorax in 1 patient each. A specific complication associated with trans-septal catheterizations for left-sided pathways is persistent intra-atrial shunt. Although acute shunt may be present in up to 50% of patients, long-term sequelae and persistence beyond 3 weeks are rare.38,39 Mortality rates are less than 1% and nonfatal complications are about 4%.

Atrial Tachycardia

Atrial tachycardias depend wholly on atrial tissue for initiation and maintenance of the tachycardia. Ectopic atrial tachycardia, sinoatrial nodal reentrant tachycardia, inappropriate sinus tachycardia, atrial flutter, and atrial fibrillation fall into this category. Atrial flutter and atrial fibrillation will be considered separately below. Focal atrial tachycardias, a less common type of supraventricular tachycardia, forms about 10% of all SVTs referred for electrophysiological studies. Multifocal atrial tachycardia is due to multiple foci of abnormal automaticity or triggered activity and is not amenable to catheter ablation.

Patients with atrial tachycardia present with palpitations and associated symptoms may include chest discomfort, shortness of breath, and dizziness. Symptoms most often are benign and not life threatening. These arrhythmias are more common in patients with structural heart disease. Indications for ablation include failure or intolerance of medical therapy. Rarely, incessant tachycardias can lead to cardiomyopathy. With ablation and control of heart rate, myocardial dysfunction can be reversed.40,41

Surface ECG features of atrial tachycardia include abnormal P-wave morphology or axes that are close to the following QRS complexes. Mapping and ablation of atrial tachycardias can be more difficult as they can originate from anywhere within the right or left atrium. But there are specific anatomical regions that have a high incidence of foci and these are primary targets. They include the crista terminalis, atrial appendages, valve annulus, and pulmonary ostia.42 Intracardiac echocardiography has been used to localize these anatomic regions of interest. Mapping is facilitated by other techniques.

Activation mapping by means of a roving mapping catheter is used to localize a region of earliest activation. Successful targets for ablation usually have intracardiac activation 30 milliseconds or more prior to any surface P wave. Sometimes two catheters are used to triangulate a target by moving one catheter at a time toward sites of earlier activation. A caveat to this mapping technique is that an early right atrial activation site may in fact originate from the left atrium as the posterior septum of the right atrium overlies the left atrium.43

Other techniques that have aided focal ablation are the use of noncontact44 and electroanatomical mapping systems. These newer mapping catheters have increased the success rate of ablation of focal tachycardias, especially in patients with altered atrial anatomy such as those with congenital anomalies and postatrial surgery.45

Inappropriate sinus tachycardia and sinoatrial nodal reentry tachycardias occur more infrequently and experience with catheter ablation of these tachycardias is more limited. Inappropriate sinus tachycardia is also difficult to ablate due to the variability and diffuse location of sinoatrial tissue. Medical therapy is the preferred method of therapy and catheter ablation is attempted only after drug failure. Catheter ablation may result in complete loss of sinoatrial node function and resulting junctional rhythm, requiring insertion of a pacemaker. Even if resting heart rate is reduced with nodal modification, symptoms may continue with episodes of tachycardia. Sinoatrial nodal reentrant tachycardia is targeted for ablation using techniques similar to those used for other atrial tachycardias.

Success with ablation of atrial tachycardia is quite variable depending upon the location of the arrhythmogenic foci and the experience of the operator. The 1998 NASPE survey showed a success rate of 80% for right-sided versus 72% for left-sided versus 52% for septal foci in 216 cases of atrial tachycardia ablation.29 Another large review examined the frequency of arrhythmias as a predictor of success. In 105 patients, the overall initial success rate was 77%, and 10% had recurrence over a 33-month follow-up period. There was an 88% success rate for paroxysmal form versus 71% for permanent and 41% for repetitive forms of atrial tachycardia.46

Atrial Flutter

Atrial flutter is a type of atrial tachycardia that utilizes a macro-reentrant circuit contained within the atria. A variety of natural and surgical barriers to conduction can create a reentrant circuit within the atria. Typical atrial flutter is due to a right atrial circuit, bound anteriorly by the tricuspid valve (TV) annulus. Posteriorly, it is confined by the superior vena cava, crista terminalis, inferior vena cava (IVC), eustachian ridge, and coronary sinus (CS) (Fig. 52-4).



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FIGURE 52-4 Diagrammatic representation of typical or counterclockwise right atrial flutter. Surface ECG shows large inverted P waves in the inferior leads. Lead III above shows 2:1 AV conduction with "sawtooth" flutter waves. The reentrant circuit (gray arrows) is confined to the right atrium by the tricuspid valve annulus (TV) and barriers to conduction within the right atrium. These include the superior vena cava (SVC), crista terminalis (CT), inferior vena cava (IVC), eustacian ridge (ER), and coronary sinus (CS). The isthmus between the IVC and TV is the preferred target for ablation.

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In the typical and more common form of atrial flutter, the circuit transverses the right atrium in a counterclockwise manner in the frontal plane. In the inferior leads, the P waves are negative and have a "sawtooth" appearance. In V1, the P wave is usually upright and in V6 it is inverted. Clockwise flutter utilizes the same circuit but in a reversed manner. The ECG also shows a reversed pattern. In the inferior leads the P waves are upright, with inverted P waves in V1 and upright in V6. This surface ECG morphology is suggestive of the circuit but needs intracardiac confirmation.47 These two forms of atrial flutter have been termed "isthmus dependent" due to the use of the IVC-tricuspid annular isthmus. Other types of atrial flutter are called "atypical."

A macro-reentrant circuit can be cured by lesions that transect the circuit between two anatomical barriers. In the case of isthmus-dependent flutter, the target for ablation is the isthmus between the IVC and TV. This is a relatively narrow target and easily reachable by ablation catheters introduced from the IVC. Success rates for ablation of this form of atrial flutter are high. Initial success rates are up to 90% with recurrence rates of 10% to 15%.48 Given these results, ablation has become the first line of therapy for recurrent isthmus-dependent atrial flutter. In addition, studies have shown that the incidence of atrial fibrillation is markedly reduced in patients with atrial flutter treated with ablation as compared to medications.49

In some patients with atrial fibrillation, treatment with class IC antiarrhythmics or amiodarone results in the development of typical atrial flutter. A hybrid approach using ablation of the IVC-TV isthmus and continued use of the antiarrhythmic medication has been shown to be highly successful in reducing the incidence of atrial fibrillation.50

While the right atrial circuit described above is the most common, a variety of other circuits in the right and left atrium are possible. These are more common in patients with underlying heart disease. Although initially thought not to be amenable to ablative therapy, mapping and ablation of these arrhythmias are now routinely performed. However, the success rate are somewhat lower than that for typical isthmus-dependent atrial flutter. An electroanatomic map of a patient with left atrial flutter can be seen in Figure 52-5. Ablation in the isthmus between these scars resulted in termination of the flutter.



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FIGURE 52-5 An electroanatomical map of a patient with left atrial flutter is shown in the RAO projection. Two large areas of scar can be seen in gray. Gradation in color shows activation sequence, with lighter being the earliest and darker being late in relation to a reference catheter, in this case positioned in the coronary sinus. The circulating wavefronts describe a figure-8 pattern around these two areas of scar but are confined by the narrow region (isthmus) between them. Ablation in the isthmus resulted in termination of the flutter.

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Surgical ScarRelated Atrial Arrhythmias

Incisional scars from prior cardiac surgery can be the substrate for reentrant atrial arrhythmias. The most common is an atypical atrial flutter related to a lateral right atrial incision. Mapping demonstrates a circuit circling the incision. Ablation from the end of the incision to either the superior vena cava or more commonly the inferior vena cava is often curative.51

It had been thought that there was conduction block between the donor and recipient atria in patients who have undergone heart transplantation. Recent reports have demonstrated reentrant arrhythmias due to donor-recipient atrial conduction. Mapping the connection between the atria can successfully ablate these arrhythmias.52 Atrial arrhythmias have also been reported in a not insignificant number of patients who have undergone the surgical Maze procedure for atrial fibrillation. These treatment failures are most often due to reentrant circuits involving gaps in the Maze lesion or through alternative pathways such as the musculature surrounding the coronary sinus.53 These arrhythmias can now be successfully mapped and ablated. Multipole noncontact and electroanatomical mapping systems are useful in improving success of these ablations. The principle of interrupting these circuits by placing lesions to connect conduction barriers remains the same.54

Atrial Fibrillation

Atrial fibrillation is another difficult to treat atrial tachycardia with variable targets for ablation. Atrial fibrillation is often symptomatic for patients due to irregular and/or rapid ventricular rates. Patients can also be completely asymptomatic and present with stroke or be diagnosed on routine examination. The prevalence of atrial fibrillation and the associated risk of stroke makes this arrhythmia a prime target for catheter-based ablation. Medical therapy for atrial fibrillation is of limited efficacy and can be associated with significant proarrhythmia in some patients. In addition, sustained rapid ventricular rates can lead to a tachycardia-related cardiomyopathy. When medical therapy aimed at maintaining sinus rhythm or blocking AV nodal conduction to slow ventricular response fails, ablation can be considered.

In the past, AV nodal or His ablation, with placement of a permanent pacemaker, was considered in patients with difficult to control ventricular rates and symptomatic palpitations. The advantages of the approach are the relative ease and speed at of the procedure. The downside is that it renders the patient pacemaker dependent. Success rates of this procedure are nearly 100%.55 Complications of this procedure include the same complications as other ablation procedures. A unique complication associated with creation of complete heart block is bradycardia-related ventricular tachycardia (torsades de pointes). The incidence this complication is reduced with high-rate atrial pacing at 80 to 90 bpm for up to a month following ablation. In highly symptomatic patients, this approach to the treatment of atrial fibrillation has been associated with improvement in quality of life and left ventricular function, and with a reduction in hospitalizations.56 With up to 3 years of follow-up, no long-term sequelae have been noted.57 Patients with CHF and atrial fibrillation may particularly benefit from this approach. This population is at particular risk of proarrhythmia due to antiarrhythmic medications and has been shown to experience improved left ventricular function following this treatment.

Surgical experience with the Maze procedure to create lines of conduction block in the atrium has led the way for catheter-based ablation procedures to cure atrial fibrillation. Studies have attempted to replicate the success of the surgical procedure using a catheter-based approach. Atrial fibrillation consists of multiple reentrant circuits within the atrium, around the vena cava, pulmonary veins, and appendages, and around areas of functional block. Creation of multiple lines of block between these nonconducting structures prevents development of reentry.

A variety of catheter techniques have been employed including a "point-by-point" approach, a "drag" approach, and a multielectrode approach. Right atrial lesions alone were shown to be ineffective.58 Attempts at left atrial or biatrial lesions have met with limited success due to the prolonged procedure times, high risk of complications, and limited efficacy.59,60 There is also concern about reduced mechanical function of the atria following extensive ablation. This limits the hemodynamic benefit of the restored sinus rhythm.

As understanding of the mechanism of atrial fibrillation has evolved, attempts to block propagation of atrial fibrillatory circuits have been abandoned in favor of attempted ablation of the fibrillatory triggers. Data suggest that atrial fibrillation may in some cases be triggered by an organized supraventricular tachycardia. About 12% of patients with AVNRT or AVRT will develop symptomatic atrial fibrillation in 1 year of follow-up.61 Thus ablation of the initiating trigger arrhythmia may prevent atrial fibrillation.

In a series of patients undergoing a left-sided catheter Maze procedure, it was discovered that rapidly firing foci arising from the musculature of the pulmonary veins were driving atrial fibrillation.62 Ablation of these foci eliminated atrial fibrillation in some patients. This procedure has evolved to empiric electrical isolation of the pulmonary veins. One approach is the complete encircling of the pulmonary veins.63 Another approach is segmental isolation of each vein by mapping the location of the connecting fibers. Electroanatomical mapping and intracardiac echocardiography have been employed to facilitate ablation. Data on the long-term efficacy and complications of this procedure are limited and it is still considered to be a procedure in evolution. In one series of 251 patients, the short-term success over 10.4 months of follow-up was 80%. There was an 85% success rate in those with paroxysmal and 68% success in those with permanent atrial fibrillation. There was a continued need for antiarrhythmic medications in some patients.64

A major complication of ablation within the pulmonary veins is focal pulmonary vein stenosis. In 102 patients undergoing pulmonary vein focal ablation, 39% with right upper vein ablation and 23% with left vein ablation developed focal pulmonary vein stenosis by transesophageal echocardiography 3 days after the procedure.65 In this series, only 3 patients experienced symptoms of dyspnea on exertion and only 1 had mild increase in pulmonary pressure. Although most cases are asymptomatic, severe cases have been reported that progress to pulmonary hypertension and lung transplant. The ablation procedure is evolving in an attempt to prevent this complication. Changes have included limiting ablation to the vein ostia, limiting power, and using ultrasound imaging during ablation.

In some patients, triggering foci arise from outside the pulmonary veins. Reported sites have included the superior vena cava, ligament of Marshall, crista terminalis, posterior wall of the left atrium, tricuspid/mitral valve annulus, and limbus of the fossa ovalis. Pulmonary vein isolation failures in some cases may be due to the triggers from these alternative sites.

In patients with persistent and/or chronic atrial fibrillation, there may be a combination of triggers and substrate responsible. Successful ablation may require elimination of triggers combined with modification of substrate. The future role of focal ablation for atrial fibrillation will depend on advances in catheter technologies to improve efficacy, reduce procedure times, and reduce complications.

Ventricular Tachycardia

Ablation techniques can also target ventricular tachycardia (VT). Over 90% of life-threatening ventricular arrhythmias originate from myocardium with structural abnormalities. Regions of scarred or aneurysmal myocardium create channels for reentrant circuits. Initial successes with resection of ventricular arrhythmogenic foci and reentrant circuits surgically have led to advancements in catheter-based ablation techniques. Beginning with DC ablation, techniques have advanced with the use of radiofrequency ablation catheters and electroanatomical mapping systems. Despite these advances, ablation for VT in patients with coronary artery disease has a limited role. Given the life-threatening potential of VT in patients with structural heart disease, even a single recurrence can be disastrous. Therefore, implantable cardiac defibrillators have become the primary therapy. Indications for ablation in this population are failure of antiarrhythmic medication to suppress symptomatic, sustained monomorphic VT, or more often frequent shocks from an implanted defibrillator despite optimal medical therapy.15

There are several factors limiting the role of catheter ablation of ischemic VT. The hemodynamic instability of patients with coronary artery disease and depressed left ventricular function limits mapping during the tachycardia. A patient may have multiple reentrant circuits, not all of which are clinically significant. The VT reentrant circuits involve scarred myocardium or can be epicardial in locations that may be out of reach for radiofrequency energy to penetrate. Finally short-term success may be eclipsed by development of new ventricular tachycardias with further myocardial injury.

Success of ischemic VT ablation is variable due to the heterogeneity of the population. Reported studies have shown efficacy in the range of 60% to 90% using the criteria of reduced defibrillator shocks and decreased need for antiarrhythmic medications. Recurrence rate is as high as 40%. Recently, success has been reported with an approach employing "substrate mapping." This technique defines the potential arrhythmic substrate using electroanatomical voltage maps. Ablation is targeted to eliminate potential reentrant circuits. Complications are in the range of 2%, with concern for perforation and cardiac tamponade due to the thin, scarred ventricles that are the substrates for ablation and thromboembolic events in those undergoing extensive ablation.6668 One reason for failure of ischemic VT ablation is the deep or epicardial location of circuits. Catheter ablation has been directed at these epicardial circuits using a pericardial approach with some success.69

In patients with dilated cardiomyopathy and His-Purkinje system disease, sustained monomorphic VT can occur due to a macro-reentrant circuit using the bundle branches. Patients typically present with syncope or sudden death or can present with palpitations. The most common circuit is down the right bundle branch and up the left bundle branch resulting in a wide complex tachycardia with a left bundle branch block pattern. Treatment involves ablation of one of the fascicles involved in the reentrant circuit. The right bundle is most commonly targeted to interrupt the reentrant circuit. Long-term success is good for prevention of recurrent bundle branch reentry. Due to intrinsic conduction disease, patients may develop heart block. Patients may also develop other VTs due to other structural abnormalities, requiring further ablation, antiarrhythmic therapy, or defibrillator implantation.70,71

Other cardiac disorders can be associated with VT and are potential candidates for catheter ablation. This includes right ventricular dysplasia,72 infiltrative disorders (sarcoid), and tumors. As in patients with atrial arrhythmias due to surgical incisions, patients with prior ventricular surgery can develop incision-related VT. This has occurred most often in patients who have undergone repair of congenital abnormalities such as tetrology of Fallot.

Ventricular tachycardia that presents in patients with no structural heart disease is termed idiopathic and represents up to 10% of all VTs that present to tertiary referral centers. Patients may be asymptomatic or present with palpitations, dizziness, or syncope. Idiopathic VT may be focal or a micro-reentrant circuit utilizing the Purkinje fibers.

Focal VT can originate from either the right or left ventricle. Right ventricular tachycardia typically originates from the outflow tract, on the septal or free wall sites. It has a typical left bundle branch morphology with leftward, inferior axis. This occurs more often in women than men, and patients typically present in their 30s to 50s. Idiopathic left ventricular tachycardia typically originates from the left posterior fascicle. It has a right bundle branch morphology with rightward, superior axis, and may be verapamil sensitive. It occurs more often in men. These VTs are localized by activation or pace mapping. Ablation is facilitated by the lack of other cardiac pathology and the presence of only one VT. Success rates for idiopathic VT are in the range of 70% to 90% with recurrence rates in the range of 15%. Complication rates are consistent with other ablative procedures.73,74

Cost-Effectiveness

Several studies have shown the cost-effectiveness of catheter ablation compared to medical therapy and surgical ablation. Catheter ablation has lower procedural costs than surgical ablation and reduces the need for further medical care and emergency room visits in comparison to drug therapy. A U.S. study comparing lifelong medical therapy versus catheter ablation for monthly episodes of supraventricular tachycardia showed a cost savings of $27,900 for ablation. There was also improvement in quality of life.75 An Australian study compared 50 patients who underwent RF ablation to 20 who underwent surgical therapy and 12 who were treated medically. Costs over 20 years were estimated to be (in Australian dollars) $2911, $17,467, and $4959, respectively, for these groups.76


?? FUTURE DIRECTIONS
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After the advent of steerable catheters, newer imaging techniques have been the most useful in directing the positioning of catheters to specific areas of interest within the heart. The techniques of nonfluoroscopic mapping discussed above have extended the efficacy of catheter-based ablation. Newer imaging techniques are in development that will continue to push the field of electrophysiology.

Imaging Technology

One novel technology in development is the use of magnetic resonance imaging (MRI) technology to visualize cardiac structures as well as catheter location with acquisition up to 10 frames per second. This would allow high-resolution three-dimensional mapping of the heart as well as localization of catheters in three-dimensional space in real time. A catheter has been developed that both acts as a receiver, allowing for visualization of its entire length by MRI, and functions as a traditional mapping and ablation catheter.77

Catheter Technology

Another area of active research is that of catheter design. A limit to tissue energy delivery with radiofrequency is the rise in temperature at the tissue-electrode interface leading to an impedance rise and limitation of energy delivery deep into the tissue. Coagulum begins to form at the catheter-tissue interface at temperatures above 100?C. This results in an increase in impedance and limits lesion size. This limitation in lesion size places some epicardial foci and arrhythmia circuits out of reach of endocardial ablations.

Cooling the ablation catheter tip with saline irrigation either through the catheter or external to the catheter prevents coagulum formation at the tissue interface. This prevents a rise in impedance and allows for more energy delivery deep into the tissue, resulting in deeper and larger lesions.78 Catheters have been designed that are saline irrigated with infusion of saline through the catheter and then into the body through a porous tip. Catheters are also available that are completely contained, with recirculation of saline within the catheter.

Another method of increasing lesion size and depth also uses the principle of limiting coagulum size by increasing the electrode-tissue interface by increasing tip size. In vitro studies have shown that 8-mm catheter tips produced lesions that are twice as deep and four times larger than lesions produced by standard 4-mm tips.79 Catheter tips of 12-mm size were also tried but this resulted in smaller lesions, perhaps due to poor tissue contact. A limitation of larger catheter tips is that the larger surface area makes it difficult to regulate power delivery and achieve even temperatures. High current densities develop at the edges of the electrode resulting in higher temperatures and coagulum formation. Larger lesions are particularly useful for higher successful rates for atrial flutter and ventricular tachycardia ablations that involve thicker myocardium. They also reduce procedure times and limit radiation exposure for patients.80

Hypothermia has been the preferred method of delivering linear lesions in surgical ablation. Cryoablation has been shown to be safe and effective. Cryothermal energy delivery via transvenous catheters is under development. This technique utilizes nitrogen or nitrogen oxide pressurized flow through a catheter tip nozzle. As the gas expands beyond the obstruction, there is a temperature drop to as much as -90?C. The blood pool forms an ice ball with resulting cooling of the underlying tissue. The advantages of cryoablation are that limited tissue cooling results in reversible injury causing transient conduction block. This allows for placement of test lesions, or "ice mapping," without causing permanent damage, reducing the chance of collateral damage. This may be especially true for high-risk ablations that are close to a normal conduction pathway. Once a successful safe site is localized, irreversible lesions can be placed by cooling further and freezing the tissue. Another advantage is that the resulting ice ball adheres the catheter tip to the myocardium, preventing catheter movement during lesion formation. This allows for ablation delivery during tachycardia when rapid heart movement may result in catheter movement.81

Alternative Energy Sources

Other energy sources for ablation are currently under development. These include laser, microwave, and ultrasound. The advantages of these systems are that direct tissue contact is not necessary for lesion formation. Theoretically, deeper and larger lesions could be formed with these energies. Ultrasound has the advantage of offering both imaging and ablation in a single catheter. Currently, catheter-based delivery systems for these energies are under development. The potential adverse effects of these systems remain the same as other ablation techniques such as proarrhythmic effects of lesions, inadvertent collateral damage, and perforation.82 Energy delivery from a distance may someday allow noninvasive ablation of cardiac tissue.


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The past 35 years have seen the development of intracardiac recording, programmed stimulation, and catheter ablation. The field of interventional electrophysiology is relatively young. Over the past two decades, great advances have been seen in the interventional management of atrial and ventricular tachyarrhythmias. Where the surgeons have gone with their scalpels, electrophysiologists have followed with their catheters. Transvenous RF ablation has become the standard of care for the treatment of many arrhythmias. These procedures have been proven to be safe and efficacious. Improved treatments for arrhythmias such as atrial fibrillation and ventricular tachycardias will come with further understanding of the mechanisms underlying these arrhythmias. Future advances in catheter design, energy delivery, and imaging techniques will continue to advance the field of electrophysiology.


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