Countershock Strength‐Duration Relationship for Myocardial Refractory Period Extension

OBJECTIVES: To determine the strength-duration relationship for refractory period extension (RPE) in order to understand better the influence of shock waveform on RPE. METHODS: In six open-chest pentobarbital-anesthetized dogs, the RPE was measured by rectangular transcardiac shocks that produced 2- to 32-V/cm local voltage gradients at the measurement site. At each intensity, measurements were made for shocks with 2- to 32-msec durations delivered 30 msec before the end of the tissue refractory period. RESULTS: These shocks produced up to 40% RPE. The RPE varied strongly with shock intensity and duration, with more RPE for stronger or longer shocks. At 32 V/cm, early portions of the shock waveform contributed most to RPE. At 8 and 16 V/cm, later portions made relatively larger contributions that were still smaller than those of the early portions. At 4 V/cm, the contributions to total RPE were spread over the entire waveform. At 2 V/cm, shocks failed to produce significant RPE. CONCLUSIONS: For rectangular shock waveforms, the relationship between RPE and duration is approximately linear at low intensity, but at higher intensity greater RPE is produced by earlier, rather than later, portions of the waveform. This may be because RPE by early portions of the waveform changes the effective timing in the refractory period for later portions of the same waveform. These results provide new insight into the possible role of waveform on defibrillation efficacy.