A high energy shock delivered by an automatic defibrillator may interfere with pacemaker function. To provide insight into the changes that occur in the threshold for ventricular pacing after the shock from an automatic defibrillator, we measured the time to capture during asynchronous ventricular pacing in dogs from endocardial or epicardial sites, after a 30 joule shock was delivered via conventional automatic defibrillator (AICD) patch electrodes. After a 30 joule shock, there was a transient loss of ventricular capture. The duration of capture loss was related to current strength. During endocardial pacing at threshold current, the time to capture was 4.9 +/- 1.2 s, whereas at current values twice threshold the time to capture from endocardial pacing was 2.2 +/- 0.9 s. No difference was found between endocardial and epicardial pacing sites in the time to capture. To ascertain the mechanism of capture loss we: (1) examined the effects of converting the pacing catheter to a current sink (transiently shunting to ground); (2) altered excitability by an infusion of flecainide; (3) blocked sympathetic input (propranolol). No change in time to capture was noted by shunting the pacer to ground. After an infusion of flecainide the time to capture from endocardial pacing was significantly prolonged to 14.9 +/- 2.2 s at the threshold value (P less than .01) and 5.6 +/- 2.1 s at twice threshold (P less than .05). Conversely, intravenous propranolol had no effect on the time to capture after shock from endocardial pacing. These data indicate that there is a transient increase in pacing threshold after the shock from an automatic defibrillator.(ABSTRACT TRUNCATED AT 250 WORDS)