Respiratory responses to focal and generalized seizures in cats

We studied the effects on breathing of seizures induced by focal injection of penicillin G into the parietal cortex in 13 anesthetized cats. Electrocorticograms, ventilation, end-tidal PCO2, and intrapleural and arterial pressures were monitored; changes of these variables were related to the stages of motor seizure. The first respiratory responses, tachypnea and hyperpnea, usually occurred before any peripheral muscular contractions developed. Progression of the seizure was always accompanied by further tachypnea and hyperpnea. The hyperpnea associated with all stages of seizure activity resulted in hypocapnia, which was sustained even during prolonged tonic-clonic motor convulsions that caused a threefold increase of metabolic rate. The extreme tachypnea of tonic generalized convulsions led to increased end-expiratory lung volume because of dynamic hyperinflation associated with very short expiratory durations in the tonic phase. We suggest that the profound effects of seizures on respiration are by feedforward mechanisms from the cortical focus itself and from subcortical circuits, such as hypothalamus, that become involved during seizure propagation and generalization. Peripheral respiratory feedback mechanisms are not important for the genesis of seizure-induced hyperpnea.