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 visce ral 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 under went UHS phase 1 of 75 mins, with initial withdrawal of 3 mL/100 II of bloo d over 15 mins, followed by UHS via tail amputation and limited fluid resus citation 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 he mostasis and aggressive fluid resuscitation to normalize hemodynamics; and observation phase III to 72 hrs. Rats were randomly divided into nine group s (n = 6 each) with three rectal temperature levels (38 degrees C [normothe rmia] vs. 34 degrees C [mild hypothermia] vs, 30 degrees C [moderate hypoth ermia]) 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 compare d with normothermia. Increased Fio(2) had no effect on blood pressure. Addi tional blood loss from the tail cut was small, with no differences among gr oups. 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 hypothe rmia. Increased Fio(2) had no effect on survival time or rate. Conclusions:After severe UHS and resuscitation in rats, mild hypothermia du ring UHS, compared with normothermia, increases blood pressure, survival ti me and 72-hr survival rate as well as moderate hypothermia. Mild hypothermi a is clinically more feasible and safer than moderate hypothermia, Increase d Fio(2) seems to have no significant effect on outcome.