Effect of doxapram on control of breathing in cats

The effects of doxapram infusion (0.25 mg.kg-1. min-1) were studied in cats anaesthetized with pentobarbitone (35 mg . kg-1 intraperitoneally). Cats were studied breathing 50 per cent oxygen and the responses to two concentrations of inspired carbon dioxide were measured. Doxapram infusion increased pulmonary ventilation by increasing both tidal volume and respiratory frequency, and also caused increases in the volume inspired in the first 0.5 second after the onset of an inspiration (V0.5) and the pressure generated in the airway 0.5 second after the onset of an inspiration when the airway had been occluded (P degrees 0.5). V 0.5, P degrees 0.5 and the mean inspiratory flow rate (VT/VI) were essentially equivalent indices of inspiratory drive. Doxapram infusion did not alter the effective impedance of the respiratory system (P degrees 0.5/V 0.5). Doxapram infusion increased the ventilatory response to carbon dioxide. The slope of the ventilatory response to carbon dioxide was increased and the response line was shifted to the left. We conclude that the increase in pulmonary ventilation caused by doxapram infusion is due almost entirely to increased inspiratory neuromuscular drive (P degrees 0.5).