The effect of resuscitative mild hypothermia and oxygen concentration on the survival time during lethal uncontrolled hemorrhagic shock in mechanically ventilated rats

Objective

To test the hypothesis that resuscitative mild hypothermia (MH) (34°C) or breathing FiO₂ of 1.0 would prolong survival time during lethal uncontrolled hemorrhagic shock (UHS) in mechanically ventilated rats.

Methods

Forty Wistar rats were anaesthetised with halothane, N₂O and O₂ (70/30%), intubated and mechanically ventilated. UHS was induced by volume-controlled blood withdraw of 3 ml/100 g over 15 min, followed by 75% tail amputation of its length. The animals were randomised into four UHS treatment groups (10 rats in each group): group 1 was maintained on FiO₂ of 0.21 and rectal temperature of 37.5°C. Group 2 was maintained on FiO₂ of 0.21 and induced MH. Group 3 was maintained on FiO₂ of 1.0 and 37.5°C. Group 4 was maintained on FiO₂ of 1.0 and MH. Rats were observed otherwise untreated until death (pulseless and mean arterial blood pressure [MAP] less than 10 mmHg, and without fluctuation).

Results

During the initial blood withdraw, MAP decreased to an average of 40 mmHg, and the heart rate (HR) increased to an average of 400 beats/min. Induction of MH increased the MAP to 60 mmHg, and the survival time moreover, it reduced the HR to 300 beats/min but did not increase bleeding. Ventilation with FiO₂ of 1.0 did not influence the MAP, the blood loss or the survival time, but increased the PaO₂. The mean survival time was 62 min, 202 min, 68 min and 209 min in group 1, group 2, group 3 and group 4, respectively. Blood loss from the tail was 1.0 ml, 1.2 ml, 0.9 ml and 0.7 ml, respectively, in group 1, group 2, group 3 and group 4 (not significant).

Conclusion

MH prolonged the survival time during UHS in mechanically ventilated rats. However, a FiO₂ of 1.0 did not influence the survival time or the blood loss from the tail.