Hypothermia for Hemorrhagic Shock: It's Cool to be Cool

A much more controversial area of interest is the effect of mild hypothermia during hemorrhagic shock. There is an ever increasing body of laboratory research suggesting that mild (33-36°C.) to moderate (28-32°C.) hypothermia improves survival from hemorrhagic shock. In some preliminary studies, this does not seem to be related to changes in systemic cytokines or in free radical production. The benefits seem to be more related to cardiovascular support, i.e., maintaining blood pressure and preventing cardiac arrest. This is even truth if hypothermia is not initiated until the end of shock and start of full fluid resuscitation.

In contrast, retrospective clinical studies of trauma patients have correlated hypothermia with increased mortality. The studies are compounded by the fact that the most severely injured patients are the ones who become hypothermic as a result of exposure, shock, administration of cold fluids, intoxication and analgesia/anesthesia. One prospective study comparing a more rapid rewarming technique (continuous arteriovenous rewarming) with standard rewarming procedures showed some physiologic benefits of more rapid rewarming, but no overall survival benefit. Unfortunately, this study did not compare totally equivalent groups of patients. More importantly, no studies have explored the effects of controlled, resuscitative mild hypothermia in this patient population.

In trauma patients, one of the main concerns regarding hypothermia is coagulopathy. There is some data, mostly in vitro, suggesting that clinically important coagulation changes do not occur above 34°C. In a trauma patient with significant tissue trauma, massive blood loss, shock, and massive transfusions, it is difficult to determine the direct effects of temperature. Also, novel hemostatic agents may make these issues less important.

Only a prospective, randomized clinical trial, with precise control of temperature and prevention of shivering and sympathetic response, will be able to clearly answer these questions.