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The effects of posture on venous admixture and respiratory dead space in health.

Alveolar-arterial PO2 difference ([A-a]PO2), venous admixture, and physiologic dead space were measured in 24 healthy men and women 23 to 72 years of age in the sitting and supine positions, breathing air, breathing O2, and breathing O2 in deep breaths. In the supine (but not the sitting) position, (A-a)PO2 and venous admixture, breathing both air and O2, were more highly correlated with the difference between closing volume and expiratory reserve volume than with age. The change in (A-a)PO2 and venous admixture from sitting to lying was related to the change in closing volume minus expiratory reserve volume, during both air and O2 breathing. These results confirm previous work on the contribution of gravity-dependent airway closure to the air-breathing venous admixture. They further indicate that the same mechanism is important when O2 is breathed, and it may account for most of the alveoli that close during O2 breathing because of critically low ventilation perfusion ratios. Physiologic dead space in the supine position may be predicted by subtracting 12.5% from the normal sitting value for the same tidal volume and respiratory frequency.