Cardiopulmonary Resuscitation and Rewarming from Accidental Hypothermia

Abstract

This thesis springs from 3 different papers: Paper 1 A randomised controlled study in rats to investigate whether an infusion of crystalloids or colloids during rewarming from hypothermia could alleviate the alterations in cardiovascular function seen in untreated control animals. Paper 2 A randomised controlled study in pigs, examining normothermic vs hypothermic cardiopulmonary resuscitation (CPR). Our central hypothesis was that the cardiac output (CO) generated by CPR would not be affected by hypothermia, thereby replacing a larger portion of native CO at a reduced temperature. This, in turn, would provide sufficient oxygen delivery to sustain aerobic metabolism in organs during 3 hours of CPR at 27°C. Paper 3 The effects of extracorporeal membrane oxygenation (ECMO) rewarming to restore oxygen delivery and organ blood flow were explored. Pigs were cooled to 27°C, underwent 3 hours of hypothermic CPR and subsequent rewarming by ECMO. Main results and conclusions Volume substitution improved hemodynamic function during rewarming from hypothermia and mitigated post-hypothermic cardiac dysfunction. We saw a sustained hemodynamic effect in rats treated with dextran vs crystalloid solution. The treated animals displayed reduced levels of intracellular calcium (Ca²⁺) in cardiac tissue compared to controls. Maintaining euvolemia with intravenous fluids during rewarming from hypothermia diminished the cardiac dysfunction seen in untreated controls. CO and mean arterial pressure (MAP) generated by CPR were not affected by hypothermia. Three hours of continuous resuscitation at 27°C provided limited but sufficient oxygen delivery to allow aerobic metabolism. In contrast, the normothermic animals displayed deteriorated hemodynamic function and significantly reduced organ blood flow after 45 minutes of CPR. These findings encourage early and continuous CPR in victims of accidental hypothermia. Rewarming pigs by ECMO after 3 hours of CPR at 27°C restored CO, MAP, the oxygen extraction ratios (O2ER), and blood flow to heart, left temporal lobe, cerebellum, stomach and small intestine. After ECMO rewarming, the favourable prognostic markers of survival after cardiac arrest were present in all animals: moderately increased lactate levels, return of pH to physiologic levels, and a shockable heart rhythm. These factors may be attributed to the protective effects of hypothermia during prolonged CPR, but they also support the critical role of ECMO in rewarming victims of accidental hypothermia.