Effects of Posture on Alveolar‐Arterial CO₂ and O₂ Differences and on Alveolar Dead Space in Man

Abstract

Abstract The increase in ventilation and lowering of the end‐tidal Pco 2 after changing from the supine to the standing position were observed to be associated with a significant rise in the arterial to end‐tidal CO 2 tension difference (average +2.1 mm Hg) in addition to an increased effective alveolar to arterial O 2 tension difference (+ 4.8 mm Hg). All dead spaces increased, the alveolar dead space (= the physiological minus the anatomical dead space) on an average by 28 ml. Assuming an unchanged distribution of ventilation this can be expressed as corresponding to 6 per cent of the alveoli being unperfused. A lowering of the alveolar Pco 2 during standing is thus in part due to an increased arterial to end‐tidal CO a tension difference. The end result is also influenced by a certain metabolic acidosis (average decrease in BHCO 3 s t = 0.6 mM/1), in addition to peripheral retention of CO 2 . The Eff. V A /V E ratio decreased in all subjects, indicating a decline in the respiratory gas exchange efficiency by an average of 6 per cent. The changes observed may be explained as a consequence of the influence of gravity on the blood flow and its distribution not only in the systemic but also in the pulmonary circulation.