Dynamic properties of respiratory timing following cocaine administration.

We assessed the timing and amplitude characteristics of diaphragmatic muscle activity following administration of intravenous cocaine HCl (10 mg/kg) to awake, unrestrained cats. Cocaine produced a pronounced tachypnea which was interrupted by deep inspiratory efforts coincident with tonic-clonic movements over the first 10 min following cocaine administration. Following that period, diaphragmatic cycle rates slowly increased for up to 1 h and were interrupted occasionally by longer inspiratory efforts which were not associated with other overt motor activities. As respiratory rate increased, breath-to-breath variability decreased, and the incidence of deep inspiratory efforts decreased. As total cycle time decreased, the ratio of inspiratory time to expiratory time remained the same between precocaine and early, intermediate and late intoxication periods. The amplitude of diaphragmatic EMG activity increased with the extreme tachypnea. A number of neural mechanisms may mediate the changes in diaphragmatic muscle activity, including hyperthermia and alteration of rostral brain influences on brainstem timing mechanisms.