Anesthetic effect and distribution of infused pentobarbital in the rat at 71 ATA

The anesthetic dose requirement and distribution of 14C-labeled pentobarbital were studied in rats at 1 ATA air and at 71 ATA He-O2. Pentobarbital was infused intravenously at a rate of 5 mg X kg-1 X min-1. The depth of anesthesia was assessed by EEG using the burst suppression of 1-s duration (silent second) as the biological end point. The mean anesthetic dose was 45.6 and 68.0 mg X kg-1 at 1 and 71 ATA, respectively, representing a 49% increase at pressure (P less than 0.001). The corresponding concentrations in the brain were 81.6 and 92.3 micrograms X g-1 (not significant). The hepatic and renal pentobarbital concentrations increased by 44 and 41%, respectively, at pressure (P less than 0.001). Interindividual variations in required doses and infusion lengths made comparison between tissue pentobarbital values difficult to interpret. A second series of experiments was therefore undertaken using a subanesthetic dose of pentobarbital infused at the same constant rate for 7 min. No significant changes were found in the organ distribution of the drug at pressure. The results show that high pressure antagonizes the narcotic effect of pentobarbital and that the distribution of pentobarbital is not significantly altered in the rat at 71 ATA.