Undersensing in VVI-pacemakers detected by Holter monitoring

Episodic failure to sense cardiac activity, with emission of pacemaker stimuli, was unexpectedly found on Holter monitoring of two asymptomatic patients who had had normal function of a VVI pacemaker on direct ECG recording. Pectoral muscle movement was then demonstrated to cause pacemaker noise reversion. After decrease of input sensitivity the phenomenon stopped.