Global cardiovascular and microvascular responses to mild hypothermia

INTRODUCTION. Hypothermia is regularly used for brain protection after resuscitation from cardiac arrest but its impact on cardiovascular function, however, is not well defined.OBJECTIVES. The aim of this study was to evaluate the cardiovascular response to mild therapeutic hypothermia and rewarming in a large animal model.METHODS. Seven anesthetized, mechanically ventilated and invasively monitored sheep were cooled with a cold intravenous saline infusion, ice packs and nasal cooling (RhinoChill System, Benechill, CA) to achieve a core temperature of 34-35°C (the basal temperature in sheep is around 40°C). After maintenance of this temperature for 6 hours, sheep were progressively rewarmed to baseline temperature. A positive fluid balance was maintained during the entire study period to avoid any hypovolemia. The sublingual microcirculation was observed using sidestream dark-field (SDF) videomicroscopy and the proportion of perfused vessels (PPV) and perfused vessel density (PVD) evaluated using a semi-quantitative method.RESULTS. During cooling, systemic and pulmonary artery pressures did not change, but cardiac output decreased significantly along with the increase in vascular resistance. Left and right ventricular stroke work index decreased reflecting altered ventricular function. Nevertheless, there was an increase in mixed venous oxygen saturation (SvO2), reflecting a decrease in oxygen extraction. Sublingual microcirculation analysis showed a significant decrease in PPV and PVD. All the variables returned gradually to baseline during the rewarming phase.CONCLUSIONS. In this intact healthy large animal model, the alteration in cardiac function during hypothermia was well tolerated because of the simultaneous decrease in oxygen requirements. Arterial pressure was maintained by an increase in systemic vascular resistance associated with a reduction in peripheral microcirculatory density.GRANT ACKNOWLEDGMENT. *RhinoChill System was supplied by Benechill, Inc.