Discussion:
The tracing in Figure 1 demonstrates a 1:1 narrow complex tachycardia with alternating atrial activation. The atrial activation appears to alternate between “concentric” activation and eccentric activation. However, a closer look demonstrates that the distal CS activates the earliest in both cases. A differential diagnosis of this tracing includes AVNRT with intermittent conduction via a bystander pathway, AVNRT with intermittent premature atrial contractions, junctional tachycardia with intermittent conduction via a bystander accessory pathway, and finally, AVRT with intermittent mitral isthmus block.
The tachycardia had a VAHV response with RVA overdrive pacing, ruling out atrial tachycardia as the mechanism. The VA time was 118msec, and a His-refractory PVC during tachycardia advanced the atrium and reset the tachycardia. The post pacing interval minus tachycardia cycle length was 23 msec. Hence, the most likely diagnosis is AVRT.
A closer look at the tachycardia itself demonstrated some interesting observations (Figure 2). The TCL, VA time (measured to CS 1,2) and H-H intervals were all fixed, irrespective of the pattern of retrograde atrial activation. In addition, there appears to be a double atrial potential on CS 3,4 on the alternate beats. Given the history of a previous attempt at ablation of the left lateral pathway, a conclusion can be made that lesions where also delivered to the mitral isthmus, creating a functional line of block. The latter allowed for intermittent conduction in a retrograde fashion into the AV node across the mitral isthmus with every other beat. The beats with the “Pattern A” retrograde atrial activation (Figure 3a) conducted retrogradely via the left lateral AP, across the mitral isthmus in a lateral to septal fashion and subsequently to the AV node. In contrast, the beats with “Pattern B” conduct to the AV node via the roof of the left atrium (Figure 3b). The likely explanation for this is that with Pattern B, block occurs across the mitral isthmus and retrograde activation must occur via the roof of the left atrium for the tachycardia to be sustained. Interestingly, the tachycardia circuit of AVRT is sustained, despite the difference in retrograde activation. The likely explanation for this is that the electrical activation fortuitously reaches the AV node at the same time with each pattern, despite the two different circuits. There is likely some delay in conduction across the mitral isthmus with Pattern A, allowing a similar VA time to Pattern B despite the longer anatomical distance via the roof.
Mitral isthmus conduction block can occur in patients redo procedures for left lateral pathway ablations and most likely occurs due to poor mapping or unstable catheter positioning. The operator can be fooled by visualizing “concentric” atrial activation given the mitral isthmus line of block [1]. Voltage mapping can mitigate this issue by demonstrating scarring, low voltage and fractionation over the mitral isthmus.
The left lateral pathway was successfully ablated with mapping on the mitral annulus during ventricular pacing, with demonstratable VA block during ventricular pacing and failure to reinduce tachycardia.