Brain hypoxia and control of breathing: Neuromechanical control

The effects of graded brain hypoxia on respiratory cycle timing, the lung inflation reflex, and respiratory compensation for an inspiratory flow-resistive load were studied in unanesthetized goats. Two models, inhalation of CO and acute reduction of brain blood flow (BBF) were used to produce comparable levels of brain hypoxia. The lung inflation reflex was assessed as the ratio of inspiratory time of an occluded breath to that of the preceding spontaneous breath (TI(occl)/TI(spont). Compensation for flow-resistive loading was assessed as the effect of the load upon the airway occlusion pressure response to rebreathing CO₂ (ΔP(0.1)/ΔP(CO₂)). Major findings were 1) severe brain hypoxia (HbCO of 60% or BBF of 42%) caused tachypnea due to a 50% or more reduction of expiratory time but only a 20% or less reduction of inspiratory time; 2) moderate carboxyhemoglobinemia (HbCO of 25-30%) enhanced TI(occl)/TI(spont) from 1.5 ± 0.1 at control to 2.1 ± 0.1, while severe brain hypoxia (HbCO of 60% and BBF of 42%) reduced the ratio to 1.0 ± 0.2; and 3) compensation for a flow-resistive load, manifested by increases of ΔP(0.1)/ΔP(CO₂) of 75-300% in the control state, was abolished at HbCO of 45-50% and BBF of 60%. The data suggest that in unanesthetized animals brain hypoxia elicits tachypnea largely by an effect on the expiratory phase of the bulbopontine timing mechanism. The observed enhancement of the lung inflation reflex and abolition of flow-resistive load compensation are best explained by hypoxic depression of higher than brain stem neural function.