HS at MAP 40 mm Hg for 60 minutes or MAP 30 mm Hg for 45 minutes does not cause subtle functional or histologic brain damage in surviving rats. Controlling MAP at 30 mm Hg carries a risk of sudden cardiac arrest. These data suggest that limited fluid resuscitation, to maintain MAP at about 40 mm Hg, as recommended for victims of penetrating trauma with uncontrolled HS, is safe for the brain.
Both mild and moderate hypothermia prolonged survival time during untreated, lethal UHS in rats. Increased FIO2 had no effect on survival. The effects of hypothermia and increased FIO2 during UHS on viscera, the ability to be resuscitated, and outcome should be explored further.
Protecting vital organs during hemorrhagic shock (HS) may extend survival time during untreated lethal HS, and increase survival rate after all-out resuscitation from HS, by preventing delayed multiple organ failure. We have pursued these hypotheses in rat models since 1990. Using a volume controlled HS model with hemorrhage rate of 3.25 ml/lOOg, moderate hypothermia or 100% oxygen breathing extended survival time and rate. Changing from breathing of air to 100% oxygen increased meant arterial blood pressure (MAP).Using a new, three-phased outcome model of uncontrolled HS (UHS) (Phase 1) of 90 minutes (min.) (with tail amputation), hemostasis and all-out fluid resuscitation (FR) (Phase II), and observation to 72 h (Phase III), the survival rate was greater with minimal hypotensive FR (MAP 40 mmHg) by lactated Ringer's solution (LR) during UHS, as compared to no FR or normotensive FR with LR. Using the same resuscitation-outcome model, survival times and rates were greater when UHS was under moderate hypothermia (Hth) (30° C), and greatest with Hth plus minimal hypotensive FR.In a preliminary study using the above UHS model phase I only (lethal UHS), survival time was about 1 h under normothermia and 2 h under moderate Hth, with a slight increase in survival time by 100% O 2 breathing. Visceral ischemia, monitored in terms of tissue PCO 2 rise, seemed less under 100% O 2 breathing.In a definitive study of lethal UHS, in 9 groups X 6 (total 54) rats, mean survival time was 164 min. under moderate Hth plus 50% O 2 , 134 min. under mild Hth (34° C) and 50% O 2 (NS); as compared to 51 min. under normothermia and air breathing (p <0.05). Mild Hth is easier to induce and safer than is moderate Hth. Survival time compared to air breathing was slightly increased (NS) not only with 100% O 2 but also with 50% O 2 which is more readily available in the field than is 100% O 2 . Survival time was longest with 50% O 2 plus 30° C. Visceral ischemia during UHS caused intestinal and liver surface PCO 2 to increase from about 50 to 70 mmHg baseline to over 200 mmHg before death, with lower values for a longer time under 30° C or 34° C compared to 38° C (NS), and lesser increase during HS under 50% O 2 compared to air breathing (p <0.05).Protective mild Hth during prolonged HS (with avoidance of shivering by insult or sedation) should be considered for clinical feasibility trials. A possible benefit from oxygen breathing in the field, which presents logistic obstacles in mass disasters or for military combat casualties, needs outcome studies.
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