EFFECTS OF OXYGENATION AND HYPERCAPNIA ON DIAPHRAGMATIC FUNCTION AND CENTRAL DRIVE DURING RESPIRATORY-FAILURE

The effects of oxygenation and hypercapnia on diaphragmatic function a nd central drives were assessed during the development of respiratory failure in anesthetized unbound spontaneously breathing rabbits. Oxyge nation significantly altered endurance times, whereas hypercapnia had no effect. Isolated high-frequency contractile fatigue of the diaphrag m was found in hyperoxic animals; all other animals had no evidence of contractile fatigue. Oxygenation and hypercapnia did not significantl y alter the response of breathing frequency or duty cycle to loading. In all animals, there was a falloff in the intensity of central drive before apnea, with intensity of central drive remaining submaximal thr oughout loading. Oxygenation significantly altered the time and/or loa d at which drive intensity fell off, although critical blood gas level s were not associated with the fall off in intensity. We conclude that oxygenation influences the development of respiratory failure during inspiratory loading but does not directly explain the alterations in c entral drive. On the other hand, hypercapnia has no direct effect on r espiratory muscle function or central drives during loading to respira tory failure. When the effects of hypoxemia are obviated by hyperoxia, high-frequency contractile fatigue may occur.