Influence of closed head injury on isoflurane MAC in the rat

We designed the present study to determine whether the minimum alveolar concentration (MAC) for isoflurane is decreased after closed head trauma (CHT) in rats and, if so, whether the decrease of MAC is related to the severity of neurological impairment following CHT. Isoflurane MAC was determined in 36 Sprague-Dawley rats. Then, at time = 0 h, animals were grouped. Group 1 (n = 8) received no CHT, group 2 (n = 14) received moderate CHT, and group 3 (n = 14) received severe CHT. Neurological severity score (NSS, 0 = no deficit and 25 = maximal impairment) and MAC were determined at 1, 4, 24, and 48 h. In groups 1 and 2, isoflurane MAC at 1, 2, 24, and 48 h (1.0-1.1 ± 0.8-1.2%, median ± range) was not significantly different from baseline (1.0-1.1 ± 1.0-1.1%). In group 3, isoflurane MAC at 1, 2, 24, and 48 h (0.4 ± 0.2-0.5%) was decreased as compared to baseline (1.1 ± 1.0- 1.1%). In group 2, NSS at 1 h was 18 ± 11-21 and improved by 48 h to 9 ± 4- 15. In group 3, NSS at 1 h was 24 ± 22-25 and was not significantly different from NSS at 48 h (24 ± 24-25). Thus, moderate CHT does not significantly alter isoflurane MAC, whereas severe CHT equivalent to a Glasgow Coma Scale score of 3 to 6 significantly decreases isoflurane MAC.