Inhibition of programmable demand pacemaker by pacemaker-induced myopotentials.

Pacemaker-induced myopotentials caused inhibition of a demand pacemaker in two cases in which the unipolar partially encapsulated pacemaker flipped over in its pocket and directly stimulated the muscles of the chest wall. This pacemaker-induced muscular stimulation, possibly combined with afterpotential sensing or T-wave sensing (or both), caused the rate of the demand pacemaker to fall when the output of the programmable pacemaker was set to deliver high energy charges. By programming the unit to deliver a lower charge (shorter width of pulse), the rate rose appropriately to the programmed rate. Likewise, manual rotation of the unipolar pacemaker into proper orientation with the active uninsulated surface directed towards the skin also solved the problem temporarily.