Fig. 1
ECG and marker channel illustrate the mechanism of atrial undersensing. Ventricular sensing (VS) indicates T‑wave oversensing not preceded by a P wave, which initiates a 400 ms postventricular atrial refractory period (PVARP). The following P wave falls in this PVARP (indicated by AR) and is consequently not followed by ventricular pacing. This recording shows a 1:1 loss of pacing similar to the first two undersensing events in the figure in the question. In the second part of the figure in the question, 1:2 loss of pacing was observed, most likely related to small variations in the intrinsic atrial rate or changes in T‑wave amplitude, which can only be proven by marker channel and intracardiac EGM recordings