Double transition sign—a marker of left bundle branch capture during physiological pacing

Excerpt

A 69-year-old male with non-ischemic cardiomyopathy and left bundle branch block (LBBB; QRS duration-164 ms) was referred for cardiac resynchronization therapy (CRT). Left bundle branch pacing (LBBP) was done using C315 sheath and 3830 Selectsecure^tm lead (Medtronic, Minneapolis) [1, 2]. Unipolar pacing threshold was 0.4 V/0.5 ms with non-selective to selective capture transition at near-threshold output. Bipolar pacing showed three different QRS morphology as the output was reduced from 3 V to near threshold value. There was a transition from “qr” pattern in lead V1 to “qR” and finally to “rsR” before losing the capture. This “double transition sign” of bipolar pacing during LBBP can be explained by three different patterns of LB capture (Fig. 1A). As the output was reduced below 2 V, there was loss of anodal capture resulting in non-selective capture of LB by cathode. With further reduction in output, a second transition was noted from non-selective cathode to selective cathode capture of LB producing a change in QRS morphology from qR pattern to rsR in lead V1. LB-lead tip showed progressive change in the duration between pacing artifact and the local ventricular electrogram. The “double transition sign” due to transition from non-selective bipolar to non-selective cathodal to selective cathodal capture could be seen only if LB is captured as opposed to LV septal pacing where only a single transition would be possible. QRS duration was reduced from 164 to 108 ms after AV interval optimization (Fig. 1B&C). “Double transition sign” during bipolar pacing is a simple way of differentiating LB capture from LV septal capture as conduction system capture is essential to provide effective resynchronization.

…