COUNTERSHOCK STRENGTH DURATION RELATIONSHIP FOR MYOCARDIAL REFRACTORYPERIOD EXTENSION

Objectives: To determine the strength-duration relationship for refrac tory period extension (RPE) in order to understand better the influenc e of shock waveform on RPE. Methods: In six open-chest pentobarbital-a nesthetized dogs, the RPE was measured by rectangular transcardiac sho cks that produced 2- to 32-V/cm local voltage gradients at the measure ment 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 R PE varied strongly with shock intensity and duration, with more RPE fo r stronger or longer shocks. AT 32 V/cm, early portions of the shock w aveform 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 sp read over the entire waveform. At 2 V/cm, shocks failed to produce sig nificant RPE. Conclusions: for rectangular shock waveforms, the relati onship between RPE and duration is approximately linear at low intensi ty, but at higher intensity greater RPE is produced by earlier, rather than later, portions of the waveform. This may be because RPE by earl y portions of the waveform changes the effective timing in the refract ory period for later portions of the same waveform. These results prov ide new insight into the possible role of waveform on defibrillation e fficacy.