Differential Sensitivity to Halothane Anesthesia of the Genioglossus, Intercostals, and Diaphragm in Kittens

Recent studies in humans and animals have indicated that different inspiratory muscles have different sensitivities to respiratory depressants. The sensitivity of inspiratory muscles during early growth and development relative to that in adults of the same species, however, has not been studied. We therefore studied the activity of the diaphragm, the external intercostals, and the genioglossus by means of electromyography and its moving time average with different concentrations of halothane in seven 2-mo-old kittens. The kittens spontaneously breathed 1.0%-2.0% halothane in oxygen while Paco2 was maintained at about 60 mm Hg by adding CO2 to the inspired gas as needed. Muscle activity was evaluated in terms of the peak height of the moving time average. Activity at 1% halothane was used as the control measurement because measurements at zero inspired concentrations of halothane could not be obtained without sedation, which is known to depress respiratory muscle activity. Halothane anesthesia significantly (P < 0.01) decreased phasic inspiratory activity of the inspiratory muscles in a dose-dependent fashion. Genioglossal activity was completely abolished at 1.5% and 2.0% halothane. By contrast, in our previous study in adult cats under nearly identical experimental conditions, the phasic genioglossal activity was depressed but present even at 3.0% halothane. The degree of depression at 1.5% and 2.0% halothane was least in the crural diaphragm (71.8% ± 5.8%, 66.6% ± 4.5% of control, respectively), intermediate in the intercostals (68.9% ± 9.6%, 35.4% ± 8.8%), and greatest in the genioglossus (0.0%, 0.0%). Respiratory frequency, inspiratory time, and inspiratory duty cycle did not change significantly with increasing concentrations of halothane. These findings suggest that among the inspiratory muscles in kittens the genioglossus, which maintains upper airway patency during inspiration, is most vulnerable to halothane anesthesia at or below the concentration of 1 MAC. As the MAC for halothane determined in kittens (1.39% ± 0.03%) was significantly larger than that in cats, it is probable that, at equipotent anesthetic concentrations, kittens experience greater upper airway obstruction and respiratory depression than do adult cats.