Effect of posture on ventilatory response to steady-state hypoxia and hypercapnia.

The ventilatory response to steady-state normocapnic hypoxia and hypercapnia was measured in eight normal subjects after 15 min inhalation of 10.5% oxygen (with added CO2) or 4.2% CO2 in air through a loose-fitting high-flow Venturi mask. The erect (sitting) and the supine postures were studied. Ventilation was measured with inductance coils around the chest and the abdomen (Respitrace). Oxygen saturation was measured with an ear oximeter and PCO2 was measured transcutaneously on forearm skin using a modified pH electrode (Radiometer). In the erect posture (without stimulation), compared to supine, VE(21%) and VT/TI(32%) were greater but TI(19%) and TE(8%), abdominal contribution to tidal volume (24%) and 'arterial' PCO2 (0.6 mm Hg) were less. The mean ventilatory response to hypoxia at an 'arterial' PCO2 of 41 +/- 4 mm Hg (SD) was 0.61 +/- 0.34 L X min-1 X Sa-1O2 erect and 0.84 +/- 0.58 supine and to hypercapnia 2.89 +/- 1.4 L X min-1 X mm Hg-1 erect and 3.73 +/- 2.35 supine. The postural differences did not reach statistical significance. The pattern of response to both stimuli was similar, with doubling of VT, constant TI and slight shortening of TE. The abdominal contribution to tidal volume decreased by 9% with both forms of stimulation. In the steady state, the response to peripheral and central chemoreceptor stimuli was identical and essentially independent of position.