Transition

Hypothetically, during the evolution from AF initiation to sustenance, an obligatory "transitional" condition occurs. The condition may involve a relatively uniform rhythm and/or a confined anatomical region.

The notion of a transitional rhythm is not new. For example, multiple studies have documented patients with both spontaneous paroxysmal supraventricular tachycardia (SVT) and AF, in whom the latter was believed to occur consistently during the former (32,33) (Fig. 6).

Mechanistically, AF occurring in the setting of ongoing SVT could be related to atrial electrophysiologic alterations induced by factors including rapid stimulation, pressure overload, or changes in autonomic tone (34-36). Several studies have demonstrated a low incidence of AF recurrence in patients with apparent SVT-mediated AF who undergo SVT ablation (37,38). However, this data is largely limited to those with Wolff-Parkinson-White syndrome (WPW); there is little data in patients with other types of SVT (39). AF in WPW patients may involve more complex issues regarding atrial vulnerability independent of SVT (40-45). Nevertheless, this has resulted in the

Orthodromic With Concealed

Fig. 6. Surface (leads I, II, top) and intracardiac (right atrial appendage: HRA; coronary sinus: CS; left ventricular lateral base: LV) electrograms during orthodromic AV reentrant tachycardia (ORT) utilizing a concealed left free-wall accessory pathway. Spontaneous degeneration of ORT to AF is observed (arrow).

Fig. 6. Surface (leads I, II, top) and intracardiac (right atrial appendage: HRA; coronary sinus: CS; left ventricular lateral base: LV) electrograms during orthodromic AV reentrant tachycardia (ORT) utilizing a concealed left free-wall accessory pathway. Spontaneous degeneration of ORT to AF is observed (arrow).

unsubstantiated but common practice of electrophysiologic testing to exclude any SVT in patients with AF, particularly those who are young with no structural heart disease.

More recently, the potential role of type I atrial flutter as a transitional rhythm has been evaluated. It has been demonstrated in animal models that atrial flutter can trigger AF (46). It has long been known that the spontaneous occurrence of atrial flutter is a marker for advanced atrial electrophysiologic pathology, and frequently coexists with AF (47). However, data from several reports evaluating the incidence of AF after catheter ablation of atrial flutter strongly suggest that atrial flutter does not function as an obligatory transitional rhythm for AF (Table 1). In fact, in most cases, atrial flutter appears to transition from AF, occurring principally as a result of the anatomical features of the right atrium and persisting when other wavelets spontaneously extinguish (55). This may help to explain the dominance of counterclockwise reentry in spontaneous atrial flutter (Fig. 7).

The examples of SVT and type I atrial flutter discussed here both insinuate the "degeneration" of an ongoing uniform atrial tachyarrhythmia to AF. There is very little data on the issue of transitional rhythms that appear early after APD initiation. In this regard, one type of atrial tachycardia to consider is so-called "repetitive firing," sometimes seen emanating from the same site as AF-initiating APDs (Fig. 8). Defining repetitive firing is problematic in clinical studies because of the scarcity of electrodes, which limits designation of the moment of degeneration to multiple wavelet reentry. The majority of repetitive firing appears to be (effectively) transient, either spontaneously

Table 1

Studies Evaluating the Incidence of Electrocardiographic AF Prior To and After Successful Radiofrequency Catheter Ablation of Type I Atrial Flutter

AF Incidence

Author N Pre-ablation Post-ablation

terminating or cloaked by supervening multiple wavelet reentry. Continuous repetitive firing with varying dependent atrial activation appears to be rare (3). Repetitive firing may have more than one mechanism among patients, but reentry is almost certainly common. The scale of these reentrant circuits is unclear; this may have significant ramifications for catheter ablation. Because of its transience, repetitive firing has generally not been targeted for ablation.

However, it may be the successful suppression of such firing rather than actual eradication of a SOO that yields a "focal" ablative cure. Similarly, it may be our inability to understand repetitive firing circuits that leads to focal ablation failure despite the impression of precise targeting of the SOO.

Type II macroentrant atrial flutter is a rhythm that is commonly observed early after APD initiation of AF. We have encountered six patients in whom this rhythm served reproducibly in transition between the initiating APD and AF (Fig. 9). AFL was mapped to the right atrium in two patients and to the left atrium in four patients. Successful catheter ablation of the atrial flutter was achieved in each case. With more than 6 mo of follow-up in all patients, three have been free of atrial tachyarrhythmias. The remaining patients continue to have episodic AF.

The notion of a "transitional anatomical region" is substantially more theoretical than that of a transitional rhythm. Conceptually, there is an atrial region(s), and it is necessarily engaged early in the evolution beetween APD and AF that is critical to the transition process. Although this region may vary among individuals, this is not always the case. For example, previous reports have suggested that interatrial conduction can be an important factor in the sustenance of AF (56-59). In a canine model, Sparks et al. demonstrated a reduction in AF inducibility associated with attenuation of interatrial conduction by septal catheter ablation (60). Anecdotal experience has suggested that this concept may be useful in man. A second region of potential interest in this regard is the posterior left atrium. It is conceivable that inadvertent ablative eradication of such regions is partly responsible for successful outcomes utilizing focal/linear ablative techniques.

In summary, a reliable ablative cure for AF by targeting transition currently has little precedent. With the possible exception of WPW, there is little data confirming the utility of ablating paroxysmal SVT. Type I atrial flutter does not appear to be a

Rspv Tachycardia Intracardiac

Fig. 7. Initiation of atrial flutter by a transient burst of tachycardia emanating from the right inferior pulmonary vein. Intracardiac electrodes are positioned in the high septal right atrium (His), coronary sinus proximal (CSp) and distal (CSd), right inferior pulmonary vein (RIPV), right superior pulmonary vein (RSPV), left inferior pulmonary vein (LIPV), left superior pulmonary vein (LSPV), and right atrium adjacent and parallel to the tricuspid annulus (Halo: distal = cavotricuspid isthmus; proximal = right atrial roof). The initial and subsequent tachycardia beats emanate from the right inferior pulmonary vein (hollow arrow). Initially, activation of the Halo catheter is early in both proximal and distal electrodes and later in the middle electrodes, suggesting the collision of separate right atrial wavefronts. After several beats, the Halo activation suddenly becomes counterclockwise without a significant change in activation cycle length (solid arrow). A few seconds later, the initiating pulmonary vein abruptly ceased to activate, right atrial counterclockwise activation continued, and subsequent cycle lengths were significantly longer. The rhythm at that point was atrial flutter. Note that because of the short cycle length of the atrial tachycardia burst, the surface ECG may give the impression of AF. These observations demonstrate a mechanism for spontaneous initiation of counterclockwise atrial flutter in which it is actually a "second" rhythm. In this patient, on many occasions the same initiation sequence would lead to AF (multiple wavelet reentry) rather than atrial flutter.

viable target. Although we have observed several cases in which macroreentrant type II atrial flutter appeared to be an obligatory transitional rhythm, despite consistent atrial flutter ablation success, recurrence of AF has been common, with follow-up short. The concept of transitional anatomical regions requires further evaluation.

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  • JONNE
    What is transitional afib?
    8 years ago

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