Double Transition in Left Bundle Branch Area Pacing

Figures

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  FIGURE 1. During bipolar threshold testing, three distinct paced QRS morphologies were noted, indicating sequential changes in capture pattern as the pacing output was reduced.

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  FIGURE 2. (A, B) Transition (at 1.5 V) from a “QS” pattern to a “Qr” pattern in lead V1. This transition was attributed to the loss of anodal capture, resulting in a shift from non‐selective bipolar left bundle (LB) capture to non‐selective cathodal LB capture. The corresponding local electrogram (EGM) did not clearly display a distinct LB potential due to the simultaneous activation of the left bundle and the local septal myocardium; (B, C) Second Transition (at 0.5 V): Transition from the “Qr” pattern to a “rSR” pattern in lead V1, followed by complete loss of capture (D). This transition was marked by the presence of a deep S wave in the lateral leads and widening of QRS duration (delayed RV activation). This shift signifies a progression from non‐selective cathodal capture to selective cathodal LB capture, without concurrent local myocardial capture (arrow—triple spike, read text for explanation). EGM, electrogram; H, Bundle of His; LB, left bundle; RB, right bundle.

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  FIGURE 3. (A) Lead electrogram (EGM) demonstrating non‐selective cathodal capture to selective (B) cathodal left bundle capture. (B) The local EGM at this output demonstrated a consistent triple spikes (box). The is probably due to unmasking of LB potential (which is merged with the pacing pulse) due to the lack of local myocardial activation.

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  FIGURE 4. The Left Ventricular Activation Time (LVAT) was similar (within 10 ms of each other, averaged in multiple beats at paper speed of 200 mm/s). A careful assessment of ECG (boxes) showed a very short isoelectric line during selective LB capture as well. However, the precise mechanism underlying this very short (than expected) isoelectric line remains unclear, but it is hypothesized to be due to the activation of septal myocardial cells by local Purkinje fibers. However, given the borderline LVAT and the ambiguity surrounding the isoelectric baseline, LV septal capture must be considered a robust alternative mechanism for the observed transition.