Does the immobilizing effect of thiopental in brain exceed that of halothane?

BACKGROUND Recent studies suggest that anesthetics such as isoflurane act in the spinal cord to suppress movement that occurs during noxious stimulation. The authors examined the effect of halothane and thiopental on suppression of noxious-evoked movement using a model of differential anesthetic delivery. They hypothesized that halothane and thiopental, similar to isoflurane, would suppress movement primarily via an action in spinal cord. METHODS Goats were anesthetized and prepared for differential anesthetic delivery. Anesthesia was maintained with halothane (n = 5) or thiopental (n = 5). Anesthetic requirements were determined (noxious clamp on a dewclaw for 1 min) during halothane or thiopental (via infusion) delivery to the whole body and delivery only to the head. RESULTS Control (whole body) halothane requirement was 0.9 +/- 0.2%; halothane requirement in the head during differential delivery was 3.4 +/- 1% (P < 0.01). During selective halothane delivery, the electroencephalogram was greatly depressed or was isoelectric even though the animals moved during noxious stimulation. Control (whole body) plasma thiopental requirement was 20 +/- 10 microg/ml. When thiopental was selectively delivered to the head, the electroencephalogram was active in all five animals, and cranial thiopental requirement was 42 +/- 6 microg/ml (P < 0.01). CONCLUSION These data suggest that halothane and thiopental, like isoflurane, act in spinal cord to suppress movement occurring with noxious stimulation. However, halothane appears to be less potent in the brain as evidenced by the electroencephalogram data, suggesting that action in spinal cord plays a more significant role for halothane than for thiopental.