Ventilatory effects produced by the i.v. administration of incremental doses of thiopentone in the dog.

Ventilation, ventilatory pattern and ventilatory drive were studied in five dogs during the administration of increasing blood concentrations of thiopentone. Ventilation (VE, RR, VT) and blood-gas tensions (pHa, PaCO2, PaO2,) were measured. Ventilatory pattern (VT, TI, TE, Ttot) and ventilatory drive (VT/TI) and occlusion pressure) were analysed. Occlusions of the airway were performed at end-inspiration and at end-expiration. Thiopentone produced a biphasic action on respiratory rate, ranging from tachypnoea during light anaesthesia to a progressive slowing of respiration with deepening anaesthesia. The Hering-Breuer reflex did not seem to be modified by the level of anaesthesia, whereas the central mechanisms which modulate the duration of inspiration and of expiration, were perturbed.