The effect of halothane on the respiratory pattern was investigated in lightly anaesthetized cats. Following the administration of 2.0halothane there was a transient increase in respiratory frequency because of a decrease in the duration of expiration, followed by a progressive decrease in respiratory frequency as a result of prolongation of inspiration; both events could be demonstrated before and after vagotomy. During the steady state obtained following the administration of 0.5, 1.0 and 1.5halothane and after vagotomy there was a decrease in the duration of expiration and a progressive increase in the duration of inspiration. Tidal phrenic output decreased progressively as the concentration of halothane was increased. These results suggest that changes in respiratory pattern seen during halothane anaesthesia might involve some excitatory central mechanisms other than the bulbopontine system.
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