Methods: Ten sheep underwent systemic hypothermia using a venous-venous extra-corporeal circuit whilst instrumented with a 12 lead ECG. An epicardial sock recorded potentials to 30°C (n = 10) or 26°C (n = 6). Activation times (AT) and Activation Recovery Intervals (ARI) were calculated using custom software.

Results: The AT and ARI were significantly prolonged with increased heterogeneity during hypothermia. This effect was most pronounced between normothermia and 34°C during sinus rhythm (SR). For ventricular pacing (VP) however heterogeneity continued to increase with progressive hypothermia.

Conclusions: Hypothermia causes a significant increase in the heterogeneity of depolarisation and repolarisation. There is evidence to suggest that SR is protective with most of the increase in heterogeneity occurring with cooling to 34°C. This raises the possibility that the current target temperatures for therapeutic hypothermia may be safely lowered to provide a gain in cerebral protection.