Ventilatory responses to chemosensory stimuli in quadriplegic subjects

Abstract

We tested the hypothesis that interruption of motor traffic running down the spinal cord to respiratory muscle motoneurons suppresses the ventilatory response to increased chemical drive. We compared the hypoxic (HVR) and hypercapnic (HCVR) ventilatory responses, based on the rebreathing technique before and during inspiratory flow-resistive loading in 17 quadriplegic patients with low cervical spinal cord transection and in 17 normal subjects. The ventilatory response was evaluated from minute ventilation (V̇(E)) and mouth occlusion pressure (P(0.2)) slopes on arterial oxygen saturation (Sao2) or on end-tidal Pco2 (Paco2), and from absolute V̇(E) values at Sao2 80% or a Paco2 55 mmHg. We found no difference in the unloaded HVR and HCVR between the quadriplegic and normal subjects. In the loaded HVR, the ΔV̇(E)/ΔSao2 slope tended to decrease similarly in both groups of subjects. The ΔP(0.2)/Sa02 slope was shifted upwards in normal subjects, yielding a significantly higher P(0.2) at a given Sao2. In contrast, this rise in the P(0.2) level during loaded HVR was absent in quadriplegics. Loaded HCVR yielded qualitatively similar results in both groups of subjects; ΔV̇(E)/ΔPaco2 decreased and ΔP(0.2)/ΔPaco2 increased significantly. The results show that the ventilatory chemosensory responses were unsuppressed in quadriplegics, although they displayed a disturbance in load-compensation, as reflected by occlusion pressure, in hypoxia. We conclude that the descending drive to respiratory muscle motoneurons is not germane to the operation of the chemosensory reflexes.