Intermittent loss of capture in a His bundle pacemaker: What is the cause?

Case report A 34-year-old man with a history of congenital complete heart block who underwent dual-chamber pacemaker implant at age 6 and multiple generator changes was referred for device replacement and lead extraction owing to malfunctioning atrial and ventricular leads. He underwent lead extraction of a dual-chamber pacing system with subsequent reimplantation of a Medtronic Viva CRT pacemaker (CRTP) generator and bipolar active fixation leads in the right atrium, right ventricle, and His bundle (LV port). The sensing, pacing threshold, and impedance measurements for all 3 leads were stable. On postoperative day 1, to promote His bundle–only pacing without RV pacing, the device was reprogrammed to have a subthreshold RV output of 0.50 V at 0.03 ms. Device parameters are shown in Table 1. On postoperative day 2, rhythm strips were obtained (Figure 1) on telemetry. What is the cause of intermittent loss of capture, and what programming changes would be most appropriate to resolve the issue?