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).

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 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 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

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

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).

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