Pacemaker ventricular block.

Pacemaker ventricular block is a rare and poorly recognized electrocardiographic abnormality usually identified in terminally ill pacemaker patients. Because the patient is frequently moribund and the phenomenon transient, it is probably overlooked and not well documented. It is characterized by an altered temporal relationship between the pacemaker stimulus artifact and the subsequent paced QRS. The most common presentation is a delay or latency between the stimulus artifact and the QRS called first-degree pacemaker ventricular block. This can then deteriorate to periodic episodes of failure to capture the myocardium referred to as second-degree pacemaker ventricular block that may manifest as a classical Wenckebach or higher levels of block. Any further deterioration results in a third-degree pacemaker ventricular block, which is failure to capture the ventricle and asystole. This article describes electrocardiographic examples of this phenomenon.