Sensitization by low temperature of the isolated perfused heart to induced ventricular fibrillation

Summary The ventricular fibrillation threshold (VFT) in the perfused isolated heart of rabbits and cats was compared at 37° C and during hypothermia (17° to 34° C). The method consisted in measuring the minimal current of a single square-wave 10 msec pulse which was required to induce fibrillation or tachycardia when applied to the left ventricle during the vulnerable period of late systole.Hypothermia significantly lowered the VFT in both species and reduced the incidence of induced persistent (>60 sec) fibrillation or tachycardia in rabbit hearts so that only a non-persistent arrhythmia could be induced in a high proportion of trials. The vulnerable time (VT), i. e. the minimal delay after the R wave of the e. c. g. at which the single current pulse had to be applied to induce the arrhythmia, was lengthened by the hypothermia; the duration of electrical systole was lengthened to a similar extent as indicated by the unaltered VT:R-T ratio. The total refractory period in rabbit hearts was prolonged more than the duration of electrical systole by cold.The results in this study indicate that in ventricular fibrillation one should attempt to differentiate those factors which initiate the arrhythmia from those which are concerned in its maintenance.