Restitution properties during ventricular fibrillation in the in situ swine heart

Although restitution has been hypothesized to determine action potential duration (APD) during ventricular fibrillation (VF), cardiac memory may also be important.

Transmembrane recordings were made with a floating microelectrode from the anterior right ventricular wall in 6 pigs during up to 60 seconds of VF. The recordings were divided into 5-second intervals, and APD60 and the diastolic interval (DI) were calculated for each activation cycle throughout each interval. Stepwise linear regression was used to determine how well each APD60 [APD60(n)] was predicted by the 4 previous DIs (n-1, n-2, n-3, n-4) and the 3 previous APD60s (n-1, n-2, n-3). A mean+/-SD of 3+/-1.5 of the variables entered the regression equation. DI(n-1) (70% of intervals) and APD60(n-1) (71% of intervals) appeared most frequently in the regression equations and were the first or second variables entered during the stepwise regression in 87% and 76% of the intervals in which they were present, respectively. The coefficients of DI(n-1) and APD60(n-1) were positive 89% and 98% of the time, respectively. R2 of the regression for all entered variables during all intervals was 0.39+/-0.05.

The high incidence and positive coefficient of DI(n-1) indicate that restitution is important in determining APD during VF, whereas the similarly high incidence and positive coefficient of APD(n-1) indicate that cardiac memory is equally important. The finding that the regression equation accounts for only 39% of the variability of APD indicates that factors other than restitution and memory are also important in determining APD during VF.