Effects of varying concentrations of halothane on the activity of the genioglossus, intercostals, and diaphragm in cats: an electromyographic study.

To determine the possible differential effects of depth of inhalation anesthetics on inspiratory muscle activity, the following were studied in seven adult cats: the phasic activity of the diaphragm, the external intercostals, and the genioglossus, by means of electromyography (EMG) and its moving time average (MTA). The animals spontaneously breathed 1.0-3.0% halothane in O2, while arterial PCO2 was maintained constant at approximately 60 mmHg by adjusting CO2 in the inspired gas mixture. Muscle activity was evaluated in terms of peak height of MTA, with measurements at 1% halothane used as control values. Halothane anesthesia attenuated inspiratory muscle activity significantly (P less than 0.05) in a dose-dependent fashion; muscle activity decreased most in the genioglossus, least in the diaphragm, and intermediately in the intercostals. Respiratory frequency, inspiratory time, and inspiratory duty cycle did not change significantly with increasing concentration of halothane.