Catheter Ablation Therapy



Catheter Ablation Therapy


Darshan Krishnappa

Kalyanam Shivkumar



INTRODUCTION

Cardiac rhythm disorders are a frequent cause of morbidity and mortality occurring in 2% to 3% of the general population, with the prevalence increasing with advanced age and comorbidities such as hypertension, diabetes mellitus, chronic kidney disease, heart failure, and other structural heart diseases.1 Although pharmacotherapy has long been the mainstay in the management of cardiac arrhythmias, its use is restricted by limited efficacy and side effects leading to an increasing interest in surgical and interventional therapies. Although the era of invasive arrhythmia management was heralded by open surgical approaches, the high rates of morbidity and mortality associated with these procedures led to the development and widespread adoption of percutaneous catheter-based strategies. The safety and efficacy of catheter-based ablation therapies for cardiac arrhythmias has been established in numerous observational, registry-based and randomized controlled studies involving patients of diverse racial and ethnic groups conducted at multiple centers across the world and has become the first-line therapy for several arrhythmias.2,3

In this chapter, we will first discuss catheter-based mapping for the diagnosis and localization of arrhythmias followed by a discussion on the energy sources used for catheter ablation of arrhythmias. We will then discuss the current literature on the efficacy and safety of catheter ablation in various cardiac arrhythmias before highlighting the complications associated with these procedures.


FUNDAMENTALS OF CATHETER-BASED MAPPING OF CARDIAC ARRHYTHMIAS

Catheter ablation of cardiac arrhythmias begins with the induction and precise localization of the arrhythmia under investigation so as to maximize procedural success while limiting the amount of cardiac tissue that is ablated/damaged.

Catheters used for electrophysiology (EP) studies and mapping of arrhythmias have electrodes (ranging from four to multiple) at their distal end, which are capable of recording intracardiac electrical signals when placed in contact with excitable cardiac tissue. These diagnostic catheters are capable not only of sensing and recording signals from the heart but also stimulating or pacing the heart. An EP study begins with the placement of these diagnostic catheters (usually quadripolar and decapolar) at strategic locations within the heart—most commonly the high right atrium close to the superior vena cava—right atrial junction, at the atrioventricular (AV) junction where the bundle of His is located, at the right ventricular apex/base or right ventricular outflow tract and within the coronary sinus from where coronary sinus musculature and left atrial signals can be recorded. The electrogram characteristics (timing and morphology) at each of these sites along with a knowledge of fluoroscopic cardiac anatomy aid in the precise placement of these diagnostic catheters (Figure 59.1). Signals recorded from these catheters also aid in assessing the conduction properties of the AV conduction axis of the heart.

Pacing maneuvers from these intracardiac catheters are used to induce clinical arrhythmia, and interpretation of information recorded from these catheters placed at several vantage points within the heart (based on the timing of signals recorded by these catheters) during the induced arrhythmia—along with additional diagnostic pacing maneuvers performed during the arrhythmia—aid in its precise diagnosis. Although information from these catheters approximates the likely location of origin of cardiac arrhythmias (focus of origin or location of accessory pathway [AP] or reentrant circuit), a roving diagnostic or ablation catheter is then used for more precise localization.


Three-Dimensional Electroanatomic Mapping Systems

More recently, advances in technology have led to the development of three-dimensional (3D) electroanatomic mapping (EAM) systems—CARTO® (Biosense, Diamond Bar, CA), EnSite NavX® (Abbott, Abbott Park, IL), Rhythmia® (Boston Scientific, Cambridge, MA)—which enable in situ catheter visualization and reconstruction of cardiac chambers (atrial, ventricular, and vascular structures) with the ability to mark or tag the location of various cardiac structures (including the AV, aortic and pulmonary valves; papillary muscles; and the location of the AV conduction axis). The use of 3D EAM systems helps minimize fluoroscopic use and radiation exposure both for the patient and the interventional EP team. These 3D EAM systems are impedance- and/or magnetic-based and utilize electrical fields and magnetic fields, respectively, to precisely localize the position of the diagnostic and ablation catheters. Simultaneously, electrical signals are recorded using these catheters from the different cardiac chambers during sinus rhythm (voltage and activation mapping), during pacing (pace mapping and voltage mapping), and/or during the arrhythmia
(activation, voltage, and entrainment mapping). In addition, multielectrode catheters (with electrodes ranging from 20 to 64 per catheter) that are capable of rapidly recording electrical signals simultaneously from multiple areas of the heart permitting the construction of high-density EAMs are now available, thus improving the precision of arrhythmia mapping. The use of preprocedural computerized tomography (CT) or magnetic resonance imaging (MRI) and intraprocedural intracardiac echocardiography and image integration into 3D EAM systems further helps improve the precision of anatomical mapping particularly in patients undergoing ablation of complex arrhythmias including atrial fibrillation (AF) and ventricular arrhythmias (VAs).







Voltage Mapping

Voltage mapping consists of measuring the amplitude of electrical signals recorded from different sites within the chamber of interest to help identify areas of scarring which are often the site of location of critical isthmus of reentrant arrhythmias. Thresholds for normal voltage in the atria and ventricles have been suggested by prior studies, which serve as a guide to identify scarred tissue, though variations are often observed based on differences in myocardial thickness and activation wavefront directionality4,5,6 (Figure 59.2). Identification of such abnormal tissue helps focus further mapping efforts on specific areas of the heart in an attempt to identify critical sites responsible for cardiac arrhythmia.


Activation Mapping

During activation mapping, signals are recorded from the chamber of interest—the atrium during atrial arrhythmias, atrioventricular nodal reentrant tachycardia (AVNRT) or orthodromic AV reentrant tachycardia (AVRT), and the ventricle during VAs or antidromic AVRT—to determine the pattern of chamber activation during the arrhythmia to identify the site of earliest chamber activation and/or the reentrant circuit responsible for
the arrhythmia. Activation mapping during focal arrhythmias helps identify the site of origin of tachycardia whereas in orthodromic AVRT and antidromic AVRT, the site of earliest chamber activation helps localize the site of attachment of APs which are then targeted for ablation. Activation at successful ablation sites usually precedes the P-wave or QRS complexes (during atrial arrhythmias and VAs) by 30 milliseconds or more. In reentrant tachycardias (atrial flutter and scar-based reentrant ventricular tachycardia [VT]) activation mapping helps identify reentrant circuits, with localization of areas critical for the tachycardia (critical isthmus—typically areas of low-voltage fractionated signals with slow conduction) that are then targeted for ablation (Figure 59.3). Although activation mapping helps achieve precise localization of the site of origin of tachycardia, it is not always feasible because several tachycardias (especially VT) are hemodynamically unstable. In such scenarios, other mapping techniques such as pace mapping and voltage mapping are useful.







Pace Mapping

Another useful mapping technique in VAs is pace mapping. During pace mapping, pacing is performed from different sites within the heart, and the QRS morphology during pacing is then compared to that during the arrhythmia to help localize the site of origin (focal arrhythmias) or site of exit (reentrant arrhythmias) of arrhythmias. A pace match greater
than or equal to 11/12 ECG leads or greater than or equal to 95% using clinically available pace match software (CARTO® PASOTM, Biosense Webster, Diamond Bar, CA; Score Map, Abbott, Abbott Park, IL) is usually seen at sites of successful ablation. Although pace mapping is useful, it has several limitations including the inability to capture (1) local myocardium within areas of scarring at lower outputs and (2) neighboring tissue at higher outputs, thus reducing its accuracy. Further, pace mapping is not useful for atrial arrhythmias because surface P-waves are often not clearly discernible during atrial tachycardia, which limits any comparisons.






Entrainment mapping of arrhythmias is performed during mapping of hemodynamically stable reentrant arrhythmias to aid in confirming areas that have been identified as potential isthmuses during activation mapping. This is particularly useful in patients with large areas of scar with abnormal fractionated low-voltage signals seen at multiple sites which may represent either areas critical to the reentrant arrhythmia or passive bystander areas. During entrainment, pacing is performed from suspected areas during the tachycardia at a cycle length 20 to 30 milliseconds shorter than the tachycardia cycle length (ie, at a rate slightly faster than the tachycardia rate) and the response of the tachycardia to such a pacing maneuver is analyzed to determine if the site of pacing is a part of the reentrant circuit.7,8


ENERGY SOURCES FOR CATHETER ABLATION OF ARRHYTHMIAS

Catheter ablation of cardiac arrhythmias was first achieved by delivering direct current energy, via an intracardiac electrode catheter, to the His bundle region so as to disrupt AV conduction in patients with supraventricular arrhythmias refractory to medical therapy. Although successful, the degree of control over tissue ablated with this form of catheter ablation was limited and resulted in damage to a large mass of cardiac tissue and was associated with a high rate of complications.9 Several other energy sources, with a higher efficacy and more favorable safety profile, have been developed and adopted into clinical use.


Radiofrequency Ablation

Radiofrequency ablation (RF) was first used in 1987 for the treatment of cardiac arrhythmias and has since become the most common form of energy used for the ablation of arrhythmias. RF energy achieves tissue injury via resistive and conductive heating of tissue, with irreversible injury resulting from heating of tissues to a temperature greater than 50 °C. RF ablation uses energy in the 500 to 1000 kHz spectrum. RF energy from an RF generator is delivered between a platinum-iridium electrode at the tip of the ablation catheter (that serves as the cathode) and a dispersive grounding patch (that serves as the anode) usually applied on the abdomen, thigh, or calf. The small surface area of the distal end of the ablation catheter results in a high current density leading to resistive heating of tissue in contact with the catheter tip. With current density being inversely proportional to the square of the distance from the energy source, the degree of resistive tissue heating decreases exponentially as one moves away from the energy source. Thus, only tissue that is in immediate contact with the catheter tip undergoes resistive heating, thereby limiting and controlling tissue injury. Heat then radiates away from this area of resistive heating to neighboring tissue, resulting in conductive heating and damage to a small area of adjacent tissue in 3D space.
The current then flows through the body and returns via the grounding patch to the RF generator completing the circuit. The larger surface area of the dispersive electrode prevents heating and injury to the skin in contact with the grounding patch. Since its inception, the RF ablation catheter has undergone several iterations including changes in size, electrode type (platinum-iridium vs gold), and use of tip irrigation (closed-loop vs open-loop irrigation) in an attempt to achieve relatively larger and deeper lesions as compared to the initial nonirrigated catheters. Monitoring of the electrode tip-tissue interface temperature via thermocouples at the electrode tip is useful to titrate power in nonirrigated 4-mm tip catheters. However, they are not useful with larger tip (8 mm) and irrigated catheters owing to the greater passive and active cooling of the catheter tip, respectively, thus lowering the tip temperature although the temperature of tissue just beneath the surface is much greater.


Cryoablation

Cryoablation consists of inflicting tissue injury by cooling tissues to subzero temperatures. Cooling of tissue is achieved using a steerable catheter with a hollow shaft through which a hollow tube delivers a compressed cryorefrigerant such as nitrous oxide to the distal electrode tip. Expansion of the compressed nitrous oxide as it is released to the catheter tip leads to a liquid to gaseous phase transition with resulting cooling of the electrode tip via the Joule-Thompson effect. This cooled catheter tip subsequently extracts heat from the surrounding tissue resulting in tissue cooling. Whereas cooling tissue to 0 to -5 °C (probe temperature of 0 to -30 °C) results in reversible loss of function, freezing to lower temperatures of -20 to -30 °C (probe temperature of -70 to -80 °C) for longer durations of 2 to 3 minutes will result in permanent damage and loss of function. Cryoablation produces tissue injury via two mechanisms: direct cellular injury through intra- and extracellular ice crystal formation, which results in mechanical and osmotic cellular damage during both the freezing and thawing phase; and ischemic cell death caused by microcirculatory damage during the thawing phase. Cryoablation has certain advantages over RF ablation. The efficacy and safety of ablation at a site can be assessed by cooling to temperatures between 0 and -5 °C, which results in reversible loss of function. If the desired effect is not seen or significant collateral damage is seen, ablation at this site can be stopped, thereby preventing irreversible damage. Further, with freezing, the catheter tip adheres to the adjacent myocardium, resulting in assured catheter stability during ablation as opposed to the respirophasic and cardiophasic catheter motion during RF ablation. However, cryoablation is associated with a higher recurrence rate as compared to RF ablation in patients with AVNRT and AVRT, and hence in such patients is preferred only when ablation is required close to the conduction axis such as during ablation of superoparaseptal and midseptal APs which lie close to the bundle of His and AV node, respectively.10,11 Although cryoablation was first used to ablate AVNRT using steerable electrode tip catheters, more recently steerable cryoballoon catheters have been developed to aid in catheter ablation of AF. Several observational and randomized studies have shown the noninferiority (and even superiority in some studies) of cryoballoon ablation to RF ablation in patients with paroxysmal AF, while being associated with lower rates of pulmonary vein reconnection and shorter procedural times. These advantages have led to the increased use of cryoballoon ablation in preference to RF ablation in patients with paroxysmal AF.12,13,14,15


Laser Ablation

Laser ablation has only been studied for the management of paroxysmal AF. Endoscopic laser ablation uses a 980-nm diode laser to cause tissue heating and injury. Similar to cryoballoon ablation, multilumen balloon tip catheters have been developed for use in laser catheter ablation of AF. Initial studies with newer generation ablation catheters have shown noninferiority of laser ablation to RF ablation in patients with paroxysmal AF.16,17


Other Sources of Energy

Despite the considerable progress made in the field of catheter ablation of arrhythmias, the relatively high recurrence rate seen with some arrhythmias, such as AF and VT, and the potential for injury to neighboring structures during ablation has led to continued interest in identifying newer safer sources of energy to further improve efficacy rates and minimize complications. Electroporation is a newer form of ablation that uses pulsed electrical fields (created with high-voltage direct current) delivered locally to cardiac tissue via steerable sheaths. Electroporation works by creating pores in the cell membrane, thus increasing membrane permeability and causing irreversible cell injury. The tissue selectivity of different current pulses has led to considerable interest in this modality of ablation. Initial preclinical and clinical studies are promising with short procedural times and minimal collateral injury.18,19,20


INDICATIONS FOR CATHETER ABLATION


Supraventricular Arrhythmias Focal Atrial Tachycardia

Focal atrial tachycardia (FAT) is an organized tachycardia arising from a discrete region within the atria with atrial rates ranging from 100 to 250 bpm with variable ventricular rates. Although FAT may arise from any site within the atria, they most commonly arise from the crista terminalis, pulmonary veins, coronary sinus, superior vena cava, atrial appendages, perimitral/tricuspid annulus, and the parahisian location. Although pharmacotherapy has long been the mainstay of management of these patients, the high efficacy rates and low rates of complications of catheter ablation have led to it becoming a Class I indication for FAT especially if incessant or causing tachycardia-induced cardiomyopathy.2,3 Activation mapping during the tachycardia helps localize the focus of origin which can then be targeted for ablation. Several studies have demonstrated an acute success rate of 80% to 95%, with a recurrence rate of 4% to 20% and a low complication rate of 1% to 2%.21,22,23 The site of origin of the tachycardia considerably influences outcomes, with FAT arising from the parahisian location, superior vena cava, and pulmonary veins being
associated with higher recurrence rates. The development of pulmonary vein isolation procedures for FAT arising from the pulmonary veins and AF has further helped refine the ablation of these arrhythmias with improvements in outcomes (Table 59.1).


Atrioventricular Nodal Reentrant Tachycardia

AVNRT refers to reentry occurring at the AV junction and involving the different extensions of the AV node (fast pathway located posterosuperiorly and/or one or more of the slow pathways—right inferior extension, left inferior extension, or the inferolateral extension). The most common form of AVNRT—the so-called typical AVNRT—involves the right inferior slow pathway as the antegrade limb of the circuit whereas the fast pathway serves as the retrograde limb of the circuit. Other atypical forms of AVNRT involve different permutations of the different AV nodal extensions as parts of the circuit. Although beta-blockers and calcium channel blockers are useful in preventing and terminating episodes of AVNRT, their low efficacy, need for long-term medication, and the high success rates with catheter ablation have made catheter ablation a Class I indication in the management of patients with AVNRT. Although initial strategies of catheter ablation of AVNRT targeted the fast pathway, current approaches to catheter ablation involve ablating the slow pathway—most frequently the right inferior extension located anterior to the coronary sinus ostium—which is associated with a lower risk of complete AV block. Catheter ablation is highly successful with a reported success rate of 97% and a recurrence rate of 2% to 5% with a less than 1% risk of complete AV block (Table 59.1).2,3,24,25,26 Cryoablation carries a lower risk of AV block but a higher risk of recurrence.11







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May 8, 2022 | Posted by in CARDIOLOGY | Comments Off on Catheter Ablation Therapy
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