Effects of temperature on cycle length dependent changes and restitution of action potential duration in guinea pig ventricular muscle.

Objective: The aim was to investigate the effects of temperature on cycle length dependent changes of action potential duration and on restitution of action potential duration. Methods: Guinea pig papillary muscle action potentials were recorded using conventional microelectrode techniques. Action potential duration was measured at cycle lengths ranging from 500 to 2000 ms at both 27-degrees-C and 37-degrees-C. Restitution of action potential duration was determined by introducing an extra stimulus at progressively longer diastolic intervals from 40 to 9000 ms at pacing cycle lengths of 500, 1000, and 2000 ms. Results: At 37-degrees-C, action potential duration measured at 90% of repolarisation (APD90) during continuous pacing and the maximum value of APD90 achieved during restitution (APD90pl) decreased by 18(SEM 6) ms (n=7) and 24(7) ms (n=6), respectively, when pacing cycle length was reduced from 2000 to 500 ms. At 27-degrees-C, the magnitude of the shortening of APD90 and APD90pl observed when pacing cycle length was similarly reduced was greater than at 37-degrees-C, ie, 143(21) ms (n=6) and 115(11) ms (n=6), respectively. Thus the relation for restitution of action potential duration shifted downwards with reduction in pacing cycle length, and the magnitude of this shift was greater at 27-degrees-C than at 37-degrees-C. The difference between APD90 at the shortest diastolic interval (40 ms) and at diastolic interval of 100 ms (range of premature action potential durations) was much greater at 27-degrees-C that at 37-degrees-C at all three pacing cycle lengths. Conclusions: Reduction in temperature magnifies the cycle length dependent changes in action potential duration both during abrupt changes in cycle length, as with an extra stimulus, and during changes of steady state cycle length. This may indicate a greater dispersion of premature action potential durations during hypothermia, and hence predispose to hypothermia induced arrhythmias.