Undersensing of P waves in the presence of an adequate P wave due to automatic postventricular atrial refractory period extension.

Two cases in which a DDD pacemaker failed to track P waves due to repetitive activation of the automatic postventricular atrial refractory period (PVARP) extension are reported. In one case, the cycle was initiated by a premature ventricular contraction (PVC); in the other, by touching the unipolar pacemaker with a needle while suturing. We demonstrated that chest wall stimulation can be used to induce cycles of repetitive automatic PVARP extension and failure to track P waves. We demonstrated in vitro that touching a unipolar pacemaker with a needle can generate a signal sufficient to mimic a QRS complex, and initiate a cycle of repetitive P wave undersensing.