1. †Department of Anesthesiology and Critical Care Medicine, Safar Center for Resuscitation Research, and Brain Trauma Research Center, University of Pittsburgh, Pittsburgh, Pennsylvania, U.S.A.
2. *Department of Pediatrics, Safar Center for Resuscitation Research, and Brain Trauma Research Center, University of Pittsburgh, Pittsburgh, Pennsylvania, U.S.A.
3. ‡Department of Neurological Surgery, Safar Center for Resuscitation Research, and Brain Trauma Research Center, University of Pittsburgh, Pittsburgh, Pennsylvania, U.S.A.
4. §Department of Psychiatry, Safar Center for Resuscitation Research, and Brain Trauma Research Center, University of Pittsburgh, Pittsburgh, Pennsylvania, U.S.A.

Correspondence: Patrick M Kochanek, Safar Center for Resuscitation Research, 3434 Fifth Avenue, Pittsburgh, PA 15260, U.S.A.

Received 3 January 1995; Revised 12 June 1995; Accepted 14 June 1995.

Abstract

The effect of posttraumatic hypothermia (brain temperature controlled at 32°C for 4 h) on mortality after severe controlled cortical impact (CCI) was studied in rats. Four posttraumatic brain temperatures were compared: 37°C (n = 10), 36°C (n = 4), 32°C (n = 10), and uncontrolled (UC; n = 6). Rats were anesthetized and subjected to severe CCI (4.0-m/s velocity, 3.0-mm depth) to the exposed left parietal cortex. At 10 min posttrauma the rats were cooled or maintained at their target brain temperature, using external cooling or warming. Brain temperature in the UC group was recorded but not regulated, and rectal temperature was maintained at 37 0.5°C. After 4 h, rats were rewarmed over a 1-h period to 37°C, extubated, and observed for 24 h. In the 37 and 36°C groups, 24-h mortality was 50% (37°C = 5/10, 36°C = 2/4). In the 32°C group, 24-h mortality was 10% (1/10). In the UC group, brain temperature was 35.4 0.6°C during the 4-h treatment period and 24-h mortality was 0% (0/6). Mortality was higher in groups with brain temperatures 36°C versus those with brain temperatures <36°C (50 vs. 6%, respectively; p < 0.05). Additionally, electroencephalograms (EEG) were recorded in subsets of each temperature group and the percentage of time that the EEG was suppressed (isoelectric) was determined. Percentage of EEG suppression was greater in the hypothermic (32°C, n = 6; UC, n = 4) groups than in the normothermic (36°C, n = 3; 37°C, n = 6) groups (23.3 14.3 vs. 1.2 3.1%, respectively; p < 0.05). Posttraumatic hypothermia suppressed EEG during treatment and reduced mortality after severe CCI. The threshold for this protective effect appears to be a brain temperature <36°C. Thus, even mild hypothermia may be beneficial after severe brain trauma.