4 Atrial tachycardias (AT) can be either focal or macroreentrant arrhythmias, with the latter often being referred to as AFLs. In 2001, a joint expert working group established the current consensus for the classification of AT according to the underlying EP mechanism. In this classification, focal AT was defined as atrial activation originating from a discrete focus, arbitrarily defined as < 2 cm in diameter, radiating centrifugally. Macroreentrant AT, on the other hand, is due to activation encircling a “large” central obstacle, typically several centimeters in diameter, which may be functional or fixed. Activation mapping of macroreentrant AT defines continuous activity as 100% of tachycardia cycle length, as opposed to a maximum of 75% recorded in focal AT. Catheter ablation for AF is rapidly becoming the most commonly performed ablation procedure worldwide, and more centers are tackling persistent long-standing AF. To achieve higher success rates in patients with persistent AF, a more extensive biatrial ablation strategy is adopted, often involving a combination of linear ablation and the targeting of complex fractionated atrial electrograms (CFAEs). This can result in areas of slowed conduction and scar, which may create an EP milieu for both focal (microreentrant) and macroreentrant AT. It is becoming increasingly apparent that the incidence of postablation AT is becoming more common and is determined by the duration of AF and extent of ablation during the index procedure. This chapter will deal exclusively with mapping and ablation of focal ATs. Focal AT is the least common type of SVT, accounting for approximately 10 to 15% of patients referred for EP evaluation of SVT. Focal AT occurs equally in men and women.1 The EP mechanisms underlying focal AT include abnormal automaticity, triggered activity, and microreentry. Elucidating the mechanism underlying a focal AT in the EP laboratory is often difficult and of limited practical value for the ablation of these arrhythmias. Typically, the diagnosis of focal AT can be made based upon the clinical characteristics of the arrhythmia and the surface ECG. Differential diagnoses include sinus tachycardia, AVNRT, AVRT, and macroreentrant AT. The ECG of a focal AT is normally that of a long R-P tachycardia, but is not universally so. Unlike both typical AVNRT and AVRT, where the R-P relationship is fixed, in focal AT, there may be a variable or unhooked R-P relationship, and at rapid rates, decremental AV nodal conduction may result in an apparent short R-P tachycardia. Distinguishing focal AT from sinus tachycardia is straightforward when the P-wave morphology is different. AT arising from the superior crista terminalis may have a morphology indistinguishable from that of sinus tachycardia. In this setting the presence of a “warm-up” over 3 to 4 beats at onset and “cool down” over 3 to 4 beats at termination supports a diagnosis of AT over sinus tachycardia, which typically accelerates and decelerates over more than 30 seconds.2 The ECG of a macroreentrant AT is distinct in that it typically shows an undulating baseline with no clear isoelectric segment. The same can be true for focal ATs when the cycle length is very short or the atria are heavily scarred with slowed conduction, but this is less common. It is now well recognized that de novo focal ATs do not occur randomly throughout the atria but cluster at specific anatomic sites.3 In the right atrium (RA), common locations include the crista terminalis, tricuspid annulus, CS ostium, right atrial appendage (RAA), and perinodal region. In the left atrium (LA), the PV ostia,4 the aortomitral continuity, and the left atrial appendage (LAA)3 are frequent locations (Figures 4.1 and 4.2). Figure 4.1 Anatomical distribution of de novo focal ATs in the left and right atria. The atrioventricular valves have been removed from this view and are shown in Figure 4.2. CS, coronary sinus; CT, crista terminalis; LA, left atrium; LAA, left atrial appendage; MA, mitral annulus; PV, pulmonary veins; RA, right atrium; RAA, right atrial appendage; TA, tricuspid annulus. (Reprinted with permission from Kistler et al., J Am Coll Cardiol. 2006:48;1010-1017.) Figure 4.2 Anatomical distribution and representative P-wave morphologies of focal ATs arising from the tricuspid and mitral annuli. HBE, His bundle EGM; MV, mitral valve; TV, tricuspid valve. (Reprinted with permission from Kistler et al., J Am Coll Cardiol. 2006:48;1010-1017.) The anatomic location of ATs occurring after AF ablation is largely determined by the index ablation strategy. When prior ablation was limited to PV isolation, then postablation ATs are uncommon and typically arise from one or more reconnected PV. The addition of linear and/or CFAE ablation further increases the incidence of postablation AT, with the majority of focal ATs arising close to sites previously targeted for ablation.5 Detailed knowledge of the sites of prior ablation is of great assistance to the electrophysiologist attempting to localize the likely site of origin of a postablation AT. Figure 4.3 An algorithm for using P-wave morphology of an AT to localize the focus. Abbreviations as in Figures 4.1 and 4.2. LPV, left pulmonary veins; LS, left septum; RPV, right pulmonary veins; SMA, superior mitral annulus. (Reprinted with permission from Kistler et al., J Am Coll Cardiol. 2006:48;1010– 1017.) The surface ECG can provide a useful guide to the likely anatomic site of origin for de novo focal AT. Kistler et al published an algorithm (Figure 4.3) for the localization of ATs based on surface ECG P-wave morphology that was able to identify the correct anatomic location with an accuracy of 93%.3 However, it should be remembered that the spatial resolution of P-wave morphology has been estimated at 17 mm. Distinguishing left and right atrial foci is of particular importance when planning an ablation procedure, as transseptal access may be anticipated based on the P-wave morphology. A left-sided focus is suggested by a positive P wave in lead V1 and negative P wave in leads I and aVL. Successful analysis of P-wave morphology is contingent on the P wave being clearly visible and unencumbered from the preceding T wave. Prior to EP study, vagal maneuvers or adenosine may be useful in separating the P wave from the preceding T wave. Within the EP lab this is readily achieved by ventricular pacing at a faster cycle length than the AT (Figure 4.4). Examples of P-wave morphology of focal ATs are shown in Figures 4.5 and 4.6. Figure 4.4 A 12-lead ECG showing ventricular pacing during sustained AT in order to unencumber the P wave (indicated by the arrow) from the preceding T wave, allowing analysis of the P-wave morphology and its onset. Figure 4.5 Representative examples of P-wave morphologies of focal ATs arising from structures within the RA. Abbreviations as per Figure 4.1. Figure 4.6 Representative examples of P-wave morphologies of focal ATs arising from structures within the LA. Abbreviations as per Figure 4.1. LIPV, left inferior pulmonary vein; LSPV, left superior pulmonary vein; RIPV, right inferior pulmonary vein; RSPV, right superior pulmonary vein.
The Ablation of Atrial Tachycardia
Patrick M. Heck, MD, DM; Peter M. Kistler, MBBS, PhD; Andrew W. Teh, MBBS, PhD; Jonathan M. Kalman, MBBS, PhD
Introduction
Focal Atrial Tachycardia
Confirming the Diagnosis
Anatomic Locations
Surface ECG and P-Wave Morphology
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