Influence of body position on pressure and airflow generation during hypoxia and hypercapnia in man

1. Inspiratory oesophageal pressure and ventilatory responses to hyperoxic, progressive hypercapnic rebreathing (HCVR) and isocapnic, progressive hypoxic rebreathing (HVR) were studied in five normal males in both supine and upright seated positions. 2. No significant differences were found in the ventilatory response to hypercapnia between the supine and upright position. The slopes of the relationship between minute ventilation (VI) and the increase of end tidal PCO2 (delta P(ET), CO2) were 3.27 +/- 0.23 and 2.76 +/- 0.24 1 min-1 mmHg-1 supine and upright, respectively. However, the change in oesophageal pressure from the end expiratory level observed during quiet breathing to that at peak inspiration (delta P(oes), I) in relationship to delta P(ET),CO2 was greater supine than upright (1.23 +/- 0.07 versus 0.79 +/- 0.11 cmH2O mmHg-1, P < 0.01). 3. In contrast, during hypoxia-stimulated breathing the slope of the minute ventilation versus oxyhaemoglobin saturation curve (VI-Sa,O2) was flatter supine than upright (1.00 +/- 0.03 versus 1.75 +/- 0.05 l min-1 (percentage fall in Sa,O2)-1, P < 0.0001), but delta P(oes), I in relation to Sa,O2 during hypoxic rebreathing was similar supine and upright (0.38 +/- 0.03 versus 0.40 +/- 0.04 cmH2O (percentage fall in Sa,O2)-1, respectively. 4. It is concluded that body position does not affect the ventilatory response to progressive hyperoxic hypercapnia but does affect the relationship between delta P(oes), I and delta P(ET),CO2. In contrast, body position affects the ventilatory response to isocapnic progressive hypoxia, but does not affect the relationship between delta P(oes), I and Sa,O2.