Ablation of Focal Atrial Tachycardias

Crista Terminalis

In seminal work by Kalman et al. using intracardiac echocardiography, the CT was noted to be the location of two-thirds of focal AT arising from the RA. The superior CT was the most common site followed by the mid CT. Foci originating from the inferior CT were uncommonly observed. Focal AT from this site is more common in women particularly above the age of 40 years, frequently has features to suggest microreentry (onset and termination with programmed stimulation), and may have multiple foci. Although the reason for this characteristic distribution is unclear, there are certain electrophysiologic (EP) properties of the CT that may be important. The CT is a region of conduction anisotropy with poor transverse and rapid linear conduction, potentially creating a substrate for microreentry in the context of atrial remodeling. Furthermore, the CT is the anatomic site along which the sinus pacemaker complex is distributed, and therefore richly innervated with automatic tissue.

In a discussion of tachycardias arising from the CT, it is worthwhile considering whether sinus node reentry represents a distinct clinical entity. This tachycardia has been defined as an arrhythmia that can be induced and terminated with programmed stimulation with P wave morphology (PWM) identical or similar to that of the sinus P wave. However, it is difficult to apply this definition strictly because of the variability in sinus PWM (e.g., sinus arrhythmia), and it can be argued that this arrhythmia is synonymous with an AT arising from the CT. As such, the use of this term should probably be discontinued.

Tricuspid Annulus

In a series of consecutive patients with focal right atrial AT, 13% were reported to originate from the TA. In this study, foci arose from either the superior TA or the inferoanterior TA with the latter the most common site. An alternative series of patients with focal AT only observed foci arising from the superior TA. Isolated case reports have reported focal AT originating from the inferolateral TA and the anterolateral TA. In two animal studies, McGuire et al. described the presence of cells with atrioventricular (AV) nodal–type characteristics around the entire TA. These cells were histologically similar to the atrial cells but resembled nodal cells in their cellular electrophysiology, response to adenosine, and lack of connexin 43. One may speculate that these cells serve as the substrate for AT originating from around the TA.

Coronary Sinus Ostium

Kistler et al. reported in approximately 7% of patients undergoing radiofrequency ablation (RFA) for focal AT, the AT focus originated from the CS ostium. The CS ostium is characterized by the abrupt change in myocardial fiber orientation in the region of the Thebesian valve. Most focal ATs originating from the CS ostium demonstrate properties suggestive of reentry. It is speculative whether this change in fiber orientation may provide the anisotropic conduction necessary to initiate reentry.

Midline Atrial Structures: Parahissian (Perinodal), Atrial Septal (Left or Right), and Noncoronary Cusp

There is considerable confusion about the origin and nomenclature of focal AT originating from the abovementioned anatomic regions. The terms parahissian and perinodal have been used interchangeably. In addition, tachycardias described as arising from either the left or right side of the septum have also included tachycardias originating in the perinodal region. Furthermore, others have suggested that parahissian tachycardias are a subset of TA tachycardias because of their similar EP properties. In addition, ATs originating from the noncoronary cusp have been viewed either as a distinct entity or, alternately, as an early exit region for a tachycardia that originates from the perinodal region or anterior septum. In the latter of these hypotheses, the noncoronary cusp is thought to simply provide a safe approach to ablation of an AT that actually originates from the perinodal area. It is the authors’ opinion that tachycardias arising from each of the abovementioned regions should be viewed as a distinct clinical entity with careful mapping in each region. Rarely, AT has been described as arising from the left or right coronary cusps.

Right Atrial Appendage

The hallmark of these uncommon tachycardias is that they are frequently incessant and therefore may be associated with TMC. In a series of patients with RAA focal AT, Roberts-Thompson et al. observed that these arrhythmias either arose from the inferolateral base or the tip of the RAA. The inferolateral base of the RAA was the most common location in this study. On the other hand, in a series by Freixa et al., the tip of the RAA was the most common site with 60% of foci arising from this location. Both these studies noted that this tachycardia was more common in men.

Pulmonary Veins

The role of PV foci in the initiation of paroxysmal AF has been extensively documented. These patients frequently demonstrate a spectrum of arrhythmias ranging from focal atrial ectopy and nonsustained AT to paroxysms of atrial macroreentry or AF. In this patient population, there are multiple PV foci within each PV and from multiple PVs. As such, focal ablation is universally unsuccessful, and an approach that encompasses isolation is required. However, there is a group of patients with PV foci who demonstrate only focal AT as the clinical arrhythmia and in these patients the fundamental mechanism appears to be truly focal in the majority. Foci responsible for isolated AT tend to have longer cycle lengths and be ostially located rather than deep within the PV compared with foci associated with AF initiation. In addition, the foci arise more commonly from the superior PVs than the inferior PVs. AT originating from the PVs are virtually always spontaneous and rarely inducible with programmed stimulation, suggesting abnormal automaticity or triggered activity. In rare cases, PV AT has been induced by speech and deglutition. In some patients with PV AT, the arrhythmia may be incessant and result in TMC. Although this appears to be fully reversible following catheter ablation, recent studies have suggested that subtle ventricular abnormalities may persist. Given the focal nature of PV AT, these arrhythmias are curable in the long term with focal ablation. In a study by Teh et al. evaluating the long-term outcomes of patients with PV AT following catheter ablation, 96% were free from recurrence of AT off antiarrhythmic drugs at a mean follow-up 7.2 ± 2.1 years. None of the patients developed AF. In those patients with recurrences, in almost all instances this was from the original focus.

Mitral Annulus

The MA is the second most common location of LA focal AT. In a series of patients with LA focal AT by Hachiya et al., approximately one-third of foci originated from the MA. The superior region of the MA in close proximity to the aorto-mitral continuity (AMC) and the left fibrous trigone has been reported as the most common site of mitral annular focal AT. Gonzalez et al. hypothesized using a murine model that embryologic remnants of specialized conducting tissue located in this region may provide the substrate for focal AT. Furthermore, Wit et al. observed that muscle fibers of the anterior mitral valve leaflet were continuous with the LA myocardium. These fibers exhibited AV nodal–type characteristics with both spontaneous automaticity and anisotropic conduction properties potentially providing the substrate for abnormal automaticity or microreentry.

Left Atrial Appendage

LAA focal AT has been reported to most commonly arise from the base of the LAA. In a series by Yamada et al., all LAA foci arose from the base of the LAA, most commonly the medial side. Rarely, LAA focal AT has been described as originating from the tip of the LAA or the epicardium.

Coronary Sinus Musculature

Badhwar et al. described an uncommon type of focal AT originating from within the CS musculature in approximately 3% of patients undergoing catheter ablation. The mean distance from the CS ostium to the site of earliest endocardial activation recorded within the CS was 3.6 ± 0.5 cm. This site always preceded the earliest LA endocardial site during tachycardia. All patients were noted to have a sharp potential in the CS that preceded the CS atrial signal during tachycardia. In all patients, the tachycardia was successfully ablated within the CS.