Fig. 6. Asynchronous interference mode in a VVI pacemaker, with temporary VOO pacing at the programmed basic-rate interval. The ventricular refractory period (VRP) begins with a noise-sampling period (black rectangles), during which time the sense amplifier is off. During the rest of the VRP, repeated noise (N) above a minimal frequency (e.g. , 7 Hz = 420 events/min) is interpreted as interference and restarts the VRP. Preempted portions of the VRP are indicated by dashed rectangles. Thus, so long as the noise persists, the device remains refractory, with the escape timing determined solely by the programmed basic-rate interval (asynchronous pacing at A). Note that a spontaneous R wave and the second paced beat occur in the noise-sampling period. Neither are sensed, but the latter initiates a new VRP. Reprinted with permission from Bernstein AD: Pacemaker timing cycles, American College of Cardiology Learning Center Highlights. Bethesda, American College of Cardiology.