Mild hypothermia improves survival after prolonged, traumatic hemorrhagic shock in pigs

Clinical studies have demonstrated improved survival after cardiac arrest with induction of mild hypothermia (34 degrees C). Infusion of ice-cold saline seems beneficial. The American Heart Association recommends therapeutic hypothermia for comatose survivors of cardiac arrest. For hemorrhagic shock (HS), laboratory studies suggest that mild hypothermia prolongs the golden hour for resuscitation. Yet, the effects of hypothermia during HS are unclear since retrospective clinical studies suggest that hypothermia is associated with increased mortality. Using a clinically relevant, large animal model with trauma and intensive care, we tested the hypothesis that mild hypothermia, induced with intravenous cold saline (ice cold or room temperature) and surface cooling, would improve survival after HS in pigs.

Pigs were prepared under isoflurane anesthesia. After laparotomy, venous blood (75 mL/kg) was continuously withdrawn over 3 hours (no systemic heparin). At HS 35 minutes, the spleen was transected. At HS 40 minutes, pigs were divided into three groups (n = 8, each): 1) Normothermia (Norm)(38 degrees C), induced with warmed saline; 2) Mild hypothermia (34 degrees C) induced with i.v. infusion of 2 degrees C saline (Hypo-Ice) and surface cooling; and 3) Mild hypothermia (34 degrees C), induced with room temperature (24 degrees C) i.v. saline (Hypo-Rm) and surface cooling. Fluids were given when mean arterial pressure (MAP) was <30 mmHg. At HS 3 hours, shed blood was returned and splenectomy was performed. Intensive care was continued to 24 hours.

At 24 hours, there were two survivors in the Norm group, four in the Hypo-Ice group and seven in the Hypo-Rm group (p < 0.05 versus the Norm group, Log Rank). Time required to achieve 34 degrees C was 17 +/- 9 minutes in the Hypo-Ice group and 15 +/- 4 minutes in the Hypo-Rm group (NS). Compared with the Hypo-Rm group, the Hypo-Ice group required less saline during early HS (321 +/- 122 versus 571 +/- 184 mL, p < 0.05). The Hypo-Ice group also had higher lactate levels than the Hypo-Rm group (p < 0.05). Hypothermia did not cause any increase in bleeding compared with normothermia.

Mild hypothermia during HS, induced by infusion of room temperature saline and surface cooling, improves survival in a clinically relevant model of HS and trauma. However, the use of iced saline in this model had detrimental effects and did not cool the animal more quickly than room temperature fluids. These findings suggest that optimal methods for induction of hypothermia need to be addressed for each potential indication, e.g. cardiac arrest versus HS.