An unusual response to entrainment mapping in a patient with congenital heart disease

A 37-year-old woman with biventricular repair for pulmonary atresia and an intact ventricular septum was referred for an electrophysiological study in the context of recurrent atrial arrhythmias with multiple electrical cardioversions. Her clinical tachycardia was easily inducible and had a cycle length (TCL) of 340 ms. A 4-mm irrigated-tip ablation catheter was placed adjacent to a lateral right atrial scar identified by three-dimensional electroanatomic mapping (Figure 1A). From there, entrainment was performed at a cycle length of 320 ms (Figure 1B). How could the unusual response to entrainment be explained? In Figure 1A the ablation catheter is positioned at the superior border of an atrial scar (grey area), presumed to be the site of a lateral atriotomy along which separated double potentials were observed. Considering the low voltage area, entrainment was performed with higher output pacing (5 mAmp; 2 ms) at a cycle length 20 ms shorter than the TCL (Figure 1B). The first notable finding is that during the drive train, atrial activation is advanced (to ∼300 ms) with every second paced beat and precedes the next pacing stimulus on the mapping catheter. Secondly, when atrial activation is advanced, the P-wave morphology and intracardiac atrial activation sequence remain unchanged. Finally, at the end of the drive train, following an advanced atrial beat, the post-pacing interval (PPI) is 292 ms, which is 48 ms shorter than the TCL. Tachycardia is reset and continues at 340 ms. This response is consistent with 2:1 entrainment with concealed fusion. As schematically depicted in Figure 2A, higher output pacing can create a larger virtual electrode1Lei S. Jia F.P. He Q. Gao L.Y. Luo S.C. Effect of stimulation parameters on entrainment.Heart Rhythm O2. 2022; 3: 715-717Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar (shaded white circle) that captures tissue more distally along the tachycardia circuit, exemplified by the site of the asterisk. The return cycle length represents the activation time from the more distal site of pace capture to the electrode-tip of the ablation catheter, thereby producing a PPI that is shorter than the TCL. The slower the conduction between the catheter tip and captured tissue and/or the further along the virtual electrode penetrates the circuit, the more negative the PPI – TCL will be. The strikingly negative PPI – TCL of -48 ms was, therefore, highly suggestive of entrainment from a critical slow-conducting isthmus. Considering that entrained beats produced a shorter return cycle length than the pacing drive train of 320 ms, every second paced beat encountered refractory atrial tissue, yielding a 2:1 response. At this protected narrow isthmus, the first radiofrequency application resulted in tachycardia slowing and termination within 6 seconds (Figure 2B). The differential diagnosis of a PPI shorter than TCL includes intermittent loss of capture, double-loop reentry or TCL variability, and incorrect annotation of the PPI because of measuring time to a far-field electrogram. Loss of capture of the last stimulus can easily be excluded in this case, as the last paced stimulus clearly advances atrial activation. Double-loop reentry is highly improbable. When a double loop exists, the tachycardia usually propagates along the circuit with the shorter cycle length or alternates between the two if the faster circuit cannot maintain 1:1 conduction. In this case, the clinical tachycardia had a stable TCL of 340 ms. Atrial advancement during entrainment consistently resulted in a single shortened cycle length with immediate resumption of the 340 ms TCL. The possibility most difficult to exclude with certainty is that the true local signal under the ablation catheter was too small to be appreciated and that the signal measured to determine the PPI was a nearby far-field signal. Counterarguments include the 2:1 response to entrainment and the clear and sharp signals recorded by the tip of the ablation catheter. Moreover, entrainment at a higher output (10 mAmp; 2 ms) performed to exclude diaphragmatic capture resulted in a more negative PPI – TCL (-55 ms) consistent with further downstream capture by a larger virtual electrode. High-output pacing during entrainment that produces a PPI shorter than TCL has been described as a predictor of tachycardia termination during ablation of typical and atypical atrial flutter.2Dall'Aglio P.B. Johner N. Namdar M. Shah D.C. Significance of post-pacing intervals shorter than tachycardia cycle length for successful catheter ablation of atypical flutter.Europace. 2021; 23: 624-633Crossref PubMed Scopus (1) Google Scholar,3Johner N. Shah D.C. Jousset F. Dall'Aglio P.B. Namdar M. Electrophysiological and anatomical correlates of sites with postpacing intervals shorter than tachycardia cycle length in atypical atrial flutter.Circ Arrhythm Electrophysiol. 2019; 12e006955Crossref PubMed Scopus (8) Google Scholar The probability of interrupting tachycardia in these locations appears to be even higher than when the standard criterion of PPI – TCL between 0 and 30 ms is obtained. It likely reflects anatomically narrower and slower conducting critical isthmuses, which require fewer ablation lesions to block.3Johner N. Shah D.C. Jousset F. Dall'Aglio P.B. Namdar M. Electrophysiological and anatomical correlates of sites with postpacing intervals shorter than tachycardia cycle length in atypical atrial flutter.Circ Arrhythm Electrophysiol. 2019; 12e006955Crossref PubMed Scopus (8) Google Scholar In conclusion, this case illustrates an unusual 2:1 entrainment response with concealed fusion resulting in a PPI much shorter than the TCL. The response was indicative of a slow and narrow critical isthmus that was an excellent target for ablation. The case highlights caveats associated with entrainment and the potential utility of higher-output pacing in identifying critical targets for the ablation of intra-atrial reentrant tachycardia.

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