Role of barometric pressure in pulmonary fluid balance and oxygen transport

To examine the role of barometric pressure in high-altitude pulmonary edema, we randomly exposed five unanesthetized chronically instrumented sheep with lung lymph fistulas in a decompression chamber to each of three separate conditions: hypobaric hypoxia, normobaric hypoxia, and normoxic hypobaria. A combination of slow decompression and/or simultaneous adjustment of inspired PO₂ provided three successive stages of simulated altitudes of 2,600, 4,600, and 6,600 m during which hemodynamics and lymph flow were monitored. Under both hypoxic conditions we noted significant and equivalent elevations in pulmonary arterial pressure (Ppa), cardiac output, and heart rate, with left atrial and systemic pressures remaining fairly constant. Normoxic hypobaria was also accompanied by a smaller but significant rise in Ppa. Lymph flow increased to a highly significant maximum of 73% above base line, accompanied by a slight but significant decrease in lung lymph-to-plasma protein ratio, only under conditions of combined hypobaric hypoxia but not under equivalent degrees of alveolar hypoxia or hypobaria alone. Arterial hypoxemia was noted under all three conditions, with arterial PO₂ being uniformly lower under hypobaric conditions than when identical amounts of inspired PO₂ were delivered at normal atmospheric pressure. We therefore hypothesize that alveolar pressure significantly alters the Starling forces governing transcapillary fluid flux in the lung and may affect the alveolar-arterial gradient for O₂ as well.