Hypothermia, but Not 100% Oxygen Breathing, Prolongs Survival Time during Lethal Uncontrolled Hemorrhagic Shock in Rats

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Abstract
To test the hypothesis that moderate hypothermia (Hth) (30[degree sign]C) or breathing 100% oxygen (best with both combined) would prolong survival during lethal uncontrolled hemorrhagic shock (UHS) compared with normothermia (38[degree sign]C) and breathing air. Forty Sprague-Dawley rats were anesthetized with halothane during spontaneous breathing of N2 O/O2 (50:50). UHS was induced by volume-controlled blood withdrawal of 3 mL/100 g over 15 minutes, followed by 75% tail amputation and randomization to one of four UHS treatment groups (10 rats each): group 1 (control) was maintained on room air and rectal temperature of 38[degree sign]C; group 2 (Hth) was maintained on air and 30[degree sign]C; group 3 (O2) was maintained on FiO sub 2 100% (starting immediately after tail cut) and 38[degree sign]C; and group 4 (O2-Hth) was maintained on FiO2 100% and 30[degree sign]C. Rats were observed otherwise untreated until death (apnea and pulselessness) or for a maximum of 5 hours. During the initial blood withdrawal, mean arterial pressure (MAP) decreased to an average of 24 mm Hg. Seventeen of 40 rats then showed an increase in MAP (attempted self-resuscitation). Induction of hypothermia increased MAP to around 35 mm Hg at 30 minutes but did not increase bleeding. Additional blood loss from the tail stump averaged 1.0, 2.3, 2.9, and 1.7 mL in groups 1, 2, 3, and 4, respectively (not significant). Breathing 100% oxygen did not affect MAP or blood loss. Survival time was a mean of 47 and 52 minutes in normothermic groups 1 and 3 versus 121 and 135 minutes in hypothermic groups 2 and 4, respectively (p < 0.001, Kaplan-Meier). Breathing FiO2 100% increased PaO2 but did not change MAP, blood loss, or survival time. Moderate hypothermia, but not increased FiO2, prolonged survival time during untreated UHS in rats. The effect of hypothermia on survival after resuscitation from UHS needs to be determined.