EFFECTS OF POSTURE ON CARBON-DIOXIDE RESPONSIVENESS IN PATIENTS WITH OBSTRUCTIVE SLEEP-APNEA

Background-It is well known that upper airway resistance increases wit h postural change from a sitting to supine position in patients with o bstructive sleep apnoea (OSA). It is not known, however, how the postu ral change affects the ventilatory and occlusion pressure response to hypercapnia in patients with OSA when awake. Methods-The responses of minute ventilation (VE) and mouth pressure 0.1 seconds after the onset of occluded inspiration (P0.1) to progressive hypercapnia (DELTAVE/DE LTAPCO2, DELTAP0.1/DELTAPCO2) both in sitting and supine positions wer e measured in 20 patients with OSA. The ratio of the two (DELTAVE/DELT AP0.1) was obtained as an index of breathing efficiency. The postural changes in response to carbon dioxide (CO2) after uvulopalatopharyngop lasty (UPPP) were also compared in seven patients with OSA. Results-Th ere were no significant changes in the resting values of end tidal PCO 2, P0.1, or VE between the two positions. During CO2 rebreathing, DELT AVE/DELTAPCO2 did not differ between the two positions, but DELTAP0.1/ DELTAPCO2 was significantly higher in the supine than in the sitting p osition (supine, mean 0.67 (SE 0.09) cm H2O/mm Hg; sitting, mean 0.57 (SE 0.08) cm H2O/mm Hg), and DELTAVE/DELTAP0.1 decreased significantly from the sitting to the supine position (sitting, 4.6 (0-4) l/min/cm H2O; supine, 3-9 (0-4) l/min/cm H2O). In seven patients with OSA who u nderwent UPPP, DELTAVE/DELTAP0.1 improved significantly in the supine position and postural change in DELTAVE/DELTAP0.1 was eliminated. Conc lusions-These results suggest that in patients with OSA the inspirator y drive in the supine position increases to maintain the same level of ventilation as in the sitting position, and that the postural change from sitting to supine reduces breathing efficiency. Load compensation mechanisms of patients with OSA appear to be intact while awake in re sponse to the rise in upper airway resistance.