A nurse is caring for a client with a ventricular pacemaker who is on ECG monitoring. The nurse understands that the pacemaker is functioning properly when which of the following appears on the monitor strip?

A ventricular pacemaker functions by delivering an electrical impulse directly to the ventricles to trigger depolarization and mechanical contraction when the heart's intrinsic rate falls below the programmed rate. On an ECG, this stimulus appears as a sharp vertical line called a pacemaker spike. Because the spike represents the artificial stimulus delivered to the ventricle, it must appear immediately before the QRS complex — the waveform representing ventricular depolarization — to indicate proper pacemaker capture and function. The QRS complex that follows the spike is typically wide and abnormal in morphology because the impulse travels through the ventricular myocardium rather than the normal conduction system. A spike before each P wave would suggest an atrial pacemaker, not a ventricular one. Spikes with each T wave would be a dangerous scenario because stimulating the ventricle during the T wave coincides with the relative refractory period, risking the R-on-T phenomenon and potentially triggering ventricular fibrillation. Spikes after the QRS complex would indicate the pacemaker is firing after spontaneous depolarization has already occurred, which would represent a malfunction or a sensing error rather than appropriate function. Therefore, pacemaker spikes immediately preceding each QRS complex confirm that the ventricular pacemaker is capturing and functioning correctly.