Ventricular Conduction and Refractoriness During Hypothermia

The effect of hypothermia on ventricular conduction was determined with a multi-terminal electrode sutured to the epicardial surface. Measurements of conduction intervals revealed average increases of 95% at 22°C in 13 dogs and 148% at 20°C in seven dogs that survived to this temperature. The mean conduction velocity estimated at 0.55 m/sec at 37°C diminished to 0.29 m/sec and 0.19 m/sec respectively at 22° and 20°C. An apparent overall exponential relationship between conduction velocity and temperature was observed, although below 30°C the rate of conduction decreased almost linearly with a Q 10 , of 1.6 to 1.8. Individual measurements along both rows of intersecting electrode terminals revealed slightly different rates of conduction. However, within the entire group the direction of relatively faster conduction was inconsistent and, furthermore, the changes of conduction velocity with hypothermia were practically identical for the two recording alignments in all instances. During cooling to 22°C the absolute and the relative phases of ventricular refractoriness both increased by more than 200% of the control values observed at 37°C. This threefold effect on recovery of excitability was in contrast to the twofold slowing of conduction at the same temperature. It is concluded that the occurrence of ventricular fibrillation during hypothermia cannot be explained adequately by assuming a greater slowing of conduction relative to prolongation of refractoriness.