His bundle has shorter chronaxie than adjacent ventricular myocardium: Implications for pacemaker programming

Background Strength-duration curves for permanent His bundle (HB) pacing are potentially important for pacemaker programming. Objective We aimed to calculate strength-duration curve and chronaxie of the His bundle (HB) and of the adjacent right ventricular (RV) working myocardium and to analyze zones of selective HB capture and battery current drain when pacing at different pulse durations (PDs). Methods Consecutive patients with permanent HB pacing were studied. The RV and HB capture thresholds were assessed at several PDs. Battery current drain and zones of selective HB capture at PDs of 0.1, 0.2, 0.4 and 1.0 ms were determined. Results In the whole group (n =127) the HB chronaxie was shorter than the RV chronaxie. This difference was driven by patients with selective HB pacing (HB chronaxie of 0.47 vs RV chronaxie of 0.79 ms). Strength-duration curve for HB had lower rheobase and its steep portion started at shorter PDs thus creating wider distance - zone of programmable selective HB pacing - between the HB and RV strength-duration curves at shorter PDs. The battery current drain was lower with pacing at PDs of 0.1 - 0.4 ms vs 1.0 ms. Chronaxie adjusted PDs offered lowest current drain. Conclusion For the first time the strength-duration curves for permanent selective and non-selective HB pacing were determined. Selective HB capture and battery longevity can be promoted by shorter PDs (0.2 ms). Longer PDs (1.0 ms) offer bigger safety margin for RV capture and may be preferable if simultaneous RV capture during HB pacing is desired.