Hypothermia Is Associated With Improved Outcomes in a Porcine Model of Hemorrhagic Shock

Background: Hypothermia after trauma is, in current medical practice, both avoided and aggressively treated. However, the effects of environmental hypothermia during early resuscitation after hemorrhagic shock have been only poorly characterized. Methods: The objective of our study was to compare normothermia versus mild and severe levels of hypothermia in a porcine model of hemorrhagic shock. In a prospective survival study, we anesthetized 19 juvenile male pigs (Yorkshire-Landrace, 15-25 kg) and caused them to hemorrhage until their systolic blood pressure was 45 mm Hg to 55 mm Hg for a duration of 45 minutes. Then, we randomized them into three groups ( all of which underwent an 8-hour limited resuscitation period) as follows: normothermic ( 39 degrees C), mildly hypothermic ( 36 degrees C), and severely hypothermic ( 33 degrees C). We used ice packs to achieve surface cooling that mimicked environmental hypothermia. After 8 hours, we rewarmed the pigs and fully resuscitated them for 16 hours. We extubated the survivors and observed them for an additional 24 hours, before killing them. Results: Surface cooling resulted in significant reduction in core body temperature. The mortality rate was significantly higher in the normothermic group (60%) than in the two hypothermic groups combined (7%) ( p = 0.015) or in the severely hypothermic group (0%) ( p = 0.023). Hypothermic animals had significantly lower levels of creatinine kinase, lactate dehydrogenase, and lactate in addition to a lower base deficit after shock. However, severely hypothermic animals required greater volumes of colloid infusion and whole blood transfusion to maintain our target systolic blood pressure and hemoglobin levels when compared with normothermic animals. We saw a strong trend toward decreased oxygen consumption with hypothermia. Conclusions: In our porcine model, we found that simulating mild and severe levels of environmental hypothermia during early resuscitation after hemorrhage was associated with a significantly decreased mortality rate. Furthermore, markers of cellular stress and organ dysfunction, including lactate levels and the base deficit, were lower in hypothermic animals. Decreasing oxygen consumption with hypothermia may, in part, explain the protective effects observed with hypothermia.