Abstract
Summary
Repetitive firing due to synchronized pacemaker stimuli falling in the vulnerable phase of the ventricles was observed in 4 patients with complete or advanced A-V block. In 1 case this phenomenon occurred exclusively during idioventricular beating. In another patient, multiple responses led to ventricular fibrillation, which required electrical countershock. The minimal intensity required for the occurrence of repetitive responses was in the range of four to four and one half times above threshold. Threshold values ranged between 0.6 and 1.5 volts. The duration of the vulnerable phase was 20 to 40 msec. in 2 cases in which it could be measured. It is suggested that the energy employed when performing paired or coupled electrical stimulation or when considering the adequate outputs for clinical pacing should not exceed twice threshold.
