Mild or moderate hypothermia, but not increased oxygen breathing, increases long-term survival after uncontrolled hemorrhagic shock in rats

Objective: To test the hypotheses that, for uncontrolled hemorrhagic shock (UHS) in rats, mild hypothermia, compared with normothermia, would increase long-term survival as well as moderate hypothermia, oxygen breathing would increase survival further, and hypothermia and oxygen would mitigate visceral ischemia (dysoxia) during UHS, Design: Prospective, randomized study. Setting: Animal research laboratory. Subjects: A total of 54 male Sprague-Dawley rats. Interventions: Under light anesthesia and spontaneous breathing, rats underwent UHS phase 1 of 75 mins, with initial withdrawal of 3 mL/100 II of blood over 15 mins, followed by UHS via tail amputation and limited fluid resuscitation to maintain mean arterial pressure at greater than or equal to 40 mm Hg; resuscitation phase II of 60 mins (from 75 mins to 135 mins) with hemostasis and aggressive fluid resuscitation to normalize hemodynamics; and observation phase III to 72 hrs. Rats were randomly divided into nine groups (n = 6 each) with three rectal temperature levels (38 degrees C [normothermia] vs. 34 degrees C [mild hypothermia] vs, 30 degrees C [moderate hypothermia]) by surface cooling; each with 3 Fio(2) levels (0.25 vs. 0.5 vs, 1.0), Measurements and Main Results: Hypothermia increased blood pressure compared with normothermia. Increased Fio(2) had no effect on blood pressure. Additional blood loss from the tail cut was small, with no differences among groups. Hypothermia and Fio(2) of 0.5 decreased visceral dysoxia, as measured by the difference between visceral (liver and jejunum) surface Pco(2) and Paco(2) during UHS. Compared with normothermia, mild hypothermia increased the survival time and rate as well as moderate hypothermia (p < .01 by life table), without a significant difference between mild and moderate hypothermia. Increased Fio(2) had no effect on survival time or rate. Conclusions:After severe UHS and resuscitation in rats, mild hypothermia during UHS, compared with normothermia, increases blood pressure, survival time and 72-hr survival rate as well as moderate hypothermia. Mild hypothermia is clinically more feasible and safer than moderate hypothermia, Increased Fio(2) seems to have no significant effect on outcome.