1) What ECG features support true LBB capture rather than LV septal myocardial capture?
A short V6 R-wave peak time (typically ≤80 ms in narrow-QRS patients), QRS narrowing versus baseline, and an rSR′ or qR in V1 with rapid intrinsicoid deflection in lateral leads favor conduction system capture over septal myocardial pacing.
2) How do output-dependent transitions help distinguish LBB capture?
By decrementing or incrementing pacing output, a sudden change from a narrow, LBB-like QRS to a wider septal pattern (or vice versa) suggests selective/nonselective LBB capture transitions, indicating engagement of the conduction system.
3) What is selective versus nonselective LBB capture?
Selective capture depolarizes the His–Purkinje fibers first (isoelectric interval then QRS), while nonselective capture stimulates both septal myocardium and LBB simultaneously (no isoelectric interval); both can provide physiologic activation if conduction fibers are engaged.
4) Which intracardiac signals suggest LBB capture?
A recorded LBB potential with consistent stimulus–to–QRS latency, and a shorter stimulus–to–V6 R-peak time compared with septal myocardial capture, support conduction system engagement.
5) How should V1–V2 morphology be interpreted?
An rSR′/qR pattern or terminal R in V1 with rapid left-to-right activation suggests left-sided conduction recruitment; however, lead position, baseline conduction disease, and bundle branch blocks require comprehensive interpretation.