What is a key indicator of successful LBB capture?

The key indicator of successful left bundle branch (LBB) capture lies in the timing of the electrical impulses as reflected in the electrocardiogram (ECG) leads. When LBB capture is successful, it demonstrates a specific pattern in the timing of the peaks of the QRS complexes in different lead placements. Particularly, the peak of the V6 lead occurs before that of V1. This is significant because it reflects the correct and synchronized activation of the left ventricle, which should happen earlier when impulse conduction travels through the left bundle branch. This is crucial for ensuring effective cardiac contraction and optimizing hemodynamic performance.

In contrast, other indicators listed—such as a visible pulse on the monitor, the height of the R wave, and low heart rate variability—do not provide reliable insights on the electrical conduction specifically related to LBB capture. The visible pulse may indicate circulation but is not a direct metric of electrical capture. The height of the R wave can vary for many reasons unrelated to LBB activity, and low heart rate variability does not correlate directly with the effectiveness of bundle branch pacing or electrical capture. Therefore, the timing relationship indicated in option B serves as the best measure of successful LBB capture.