In S-LBBB capture, when is the V6-V1 interval the longest?

In S-LBBB (selective left bundle branch block) capture, the V6-V1 interval refers to the duration from the peak of the R wave in lead V1 to the peak of the R wave in lead V6. When LBBAP (left bundle branch area pacing) placement is implemented, the pacing is delivered directly to the left bundle branch area. This method aims to restore more normal conduction through the left ventricular myocardium, resulting in a more synchronized activation of the ventricles.

When LBBAP is used, particularly in the context of S-LBBB, the pacing can effectively shorten the interval between the leads because it optimally positions the pacing lead to improve conduction throughout the left ventricle. However, if the system encounters any issues with lead placement or pacing thresholds, the V6-V1 interval can become prolonged, particularly under different physiological conditions, such as higher outputs affecting pacing efficacy.

In contrast, in scenarios where the patient is at rest, pacing at high outputs, or with no lead placement, these conditions do not specifically optimize conduction and would typically not be associated with the prolonged V6-V1 interval, making LBBAP placement the context where the interval would be the longest due to its specific impact on conduction dynamics