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 cy cle length dependent changes of action potential duration and on resti tution of action potential duration. Methods: Guinea pig papillary mus cle action potentials were recorded using conventional microelectrode techniques. Action potential duration was measured at cycle lengths ra nging from 500 to 2000 ms at both 27-degrees-C and 37-degrees-C. Resti tution of action potential duration was determined by introducing an e xtra stimulus at progressively longer diastolic intervals from 40 to 9 000 ms at pacing cycle lengths of 500, 1000, and 2000 ms. Results: At 37-degrees-C, action potential duration measured at 90% of repolarisat ion (APD90) during continuous pacing and the maximum value of APD90 ac hieved during restitution (APD90pl) decreased by 18(SEM 6) ms (n=7) an d 24(7) ms (n=6), respectively, when pacing cycle length was reduced f rom 2000 to 500 ms. At 27-degrees-C, the magnitude of the shortening o f APD90 and APD90pl observed when pacing cycle length was similarly re duced was greater than at 37-degrees-C, ie, 143(21) ms (n=6) and 115(1 1) ms (n=6), respectively. Thus the relation for restitution of action potential duration shifted downwards with reduction in pacing cycle l ength, and the magnitude of this shift was greater at 27-degrees-C tha n at 37-degrees-C. The difference between APD90 at the shortest diasto lic interval (40 ms) and at diastolic interval of 100 ms (range of pre mature action potential durations) was much greater at 27-degrees-C th at at 37-degrees-C at all three pacing cycle lengths. Conclusions: Red uction 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 le ngth. This may indicate a greater dispersion of premature action poten tial durations during hypothermia, and hence predispose to hypothermia induced arrhythmias.