Effects of Pulmonary Vascular Congestion on Postural Changes in the Perfusion and Filling of the Pulmonary Vascular Bed*

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Abstract
Postural changes in diffusing capacity (DL) and physiologic dead space (VD) were determined in normal subjects and in patients with chronic pulmonary congestion. Tilting (60[degree] head-up) decreased DL and VD in normal subjects but not in patients with chronic pulmonary congestion. The difference in response of the 2 groups may depend upon differences in systemic venomotor activity in addition to differences in supine pulmonary vascular pressures and regional vascular resistance. Acute central vascular engorgement (G suit inflation) decreased a-A CO2 gradient and alveolar dead space of upright normal men. These findings suggests that the upright normal lung is inadequately perfused and that increased pulmonary vascular pressures - either acute or chronic -improves perfusion and filling of previously underperfused regions.