TRANSTHORACIC DC SHOCK MAY REPRESENT A SERIOUS HAZARD IN PACEMAKER DEPENDENT PATIENTS

External defibrillation is widely used for the termination of various atrial and ventricular tachyarrhythmias, including pacemaker patients. Our study was intended to evaluate the effects of DC shocks in 36 pat ients with unipolar pacemakers implanted in the right pectoral region (25 DDD, 10 WI, 3 AAI). The shocks were delivered with paddles on the anterior surface of the thorax, as far as possible away from the pacem aker. The pacing output was programmed at 0.5 msec and 5 V (25 patient s), 4 V (1 patient), and 2.5 V (10 patients). Transient loss of captur e occurred in 18 patients (50%). These patients, compared with those w ithout capture failure, received higher peak and cumulative shock ener gies, respectively, 216 +/- 99 versus 123 +/- 50 joules (P < 0.002) an d 352 +/- 62 versus 147 +/- 98 joules (P < 0.004) and had a lower pace maker pulse amplitude (4.0 +/- 1.2 vs 4.6 +/- 1.0 V, P = 0.11). Failur e to capture lasted from 5 seconds to 30 minutes (mean 157 sec). In 15 patients the ventricular stimulation threshold was measured before an d serially after cardioversion. A six-fold threshold increase was obse rved 3 minutes after the shock (P (0.004) with gradual recovery to nea rly baseline values at 24 hours. Transient sensing failure occurred in 7 of the 17 patients in whom it could be evaluated (41%). Furthermore , three cases of shock induced pacemaker malfunctions were observed re quiring replacement of the stimulator in two patients. In conclusion, the incidence of loss of capture in pacemaker patients subjected to el ectrical cardioversion/defibrillation is high. The phenomenon is due t o an abrupt rise in stimulation threshold, caused by the electrical sh ock, and may represent a serious hazard in pacemaker dependent patient s. The risk of pacing failure could be reduced by utilizing low shock energies when possible, and by programming the pacemaker at its maxima l output before cardioversion.