Variations in discharge rate of demand pacemakers not due to malfunction

Abstract

Demand pacemakers may have variations in discharge rate which are not due to malfunction. Physiologic changes of intraventricular conduction can cause implanted ventricular units to sense earlier or later during the QRS interval and therefore discharge so as to produce a different R-R interval. Furthermore, if the patient's own ventricular rate exceeds the pacer's escape rate, demand pacing will not occur. Because of such features, demand units should be regularly tested in a continuous (fixed-rate) mode produced by application of an external magnet. The rate of the induced continuous mode will always by design exceed the escape rate in the demand mode. External demand pacers discharge more slowly in the demand than in the fixed-rate mode as a function of the interval from the pacemaker spike to the peak of the corresponding R wave which is sensed by the demand pacemaker circuit. The decreased heart rate is particularly striking with temporary atrial pacing, where the beat to beat interval is lengthened by the interposition of the P-R segment. Electrocardiograms from illustrative cases are presented to demonstrate these phenomena. The physician should be aware that variations in pacer discharge rate in the demand mode and between demand and continuous pacing are not necessarily due to pacemaker malfunction, but are rather inherent features of normal demand pacemaker operation.