Pacemaker malfunction following transthoracic countershock.

Electrical cardioversion or defibrillation may be necessary in patients with implanted artificial cardiac pacemakers. Sudden discharge of high electrical energy employed in DC transthoracic countershock procedures may damage the sensitive pacemaker circuitry and result in malfunction. We report a case in which, following synchronized DC countershock, a non-programmable demand pacemaker functioned in the R-wave triggered mode rather than the R-wave inhibited mode as designed. Further, the pacemaker output voltage was reduced to about half its specified output. Such a malfunction may not be readily apparent, and requires careful analysis following electric shock.