Abstract
Respiratory frequency and separate rib-cage and abdomen-diaphragm compartmental tidal volume (VT) responses to progressive isocapnic hypoxia to a level of 75% O2 saturation (PO2 approximately 40 torr) were measured in eight healthy subjects in both seated and supine postures. Hypoxia was induced by a rebreathing technique and compartmental contributions to VT were measured by inductive plethysmography. The ventilatory, frequency and spirometric VT responses to hypoxia did not differ significantly between sitting and supine trials. The range of VT responsiveness to hypoxia among sitting individuals (0.71 +/- S.D. = 0.56% VC X % fall in Sa,O2(-1), where VC is vital capacity and Sa,O2 is arterial O2 saturation) was determined by both rib-cage (0.507 +/- S.D. = 0.378% VC X % fall in Sa,O2(-1) ) and abdomen-diaphragm (0.194 +/- S.D. = 0.178% VC X % fall fall in Sa,O2(-1) ) compartments. Among supine individuals the range of VT responsiveness to hypoxia (0.49 +/- S.D. = 0.45% VC X % fall in Sa,O2(-1) was determined equally by both rib-cage (0.27 +/- S.D. = 0.31% VC X % fall in SA,O2(-1) ) and abdomen-diaphragm (0.23 +/- S.D. = 0.29% VC X % fall in Sa,O2(-1) ) compartments. Thus, the VT response to progressive isocapnic hypoxia in conscious man is determined by both rib-cage and abdomen-diaphragm compartments regardless of posture.