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We studied the effect of 10 cm H2O of continuous positive airway pressure breathing (CPAPB) on steady state lung fluid balance. In 9 of 20 chronically instrumented, unanesthetized sheep, we measured lung lymph flow, pulmonary vascular pressure, cardiac output, pleura pressure, and lymph and plasma protein concentration during a 2-hour baseline period and 3-4 hours of CPAPB. In eight sheep, we measured the same variables after increasing average left atrial pressure by 18 cm H2O to cause mild interstitial edema. At the end of the final experiment, we anesthetized the sheep, removed the lungs, and measured their water content. During CPAPB, pleural and left atrial pressures increased by 5 cm H2O, whereas puhnonary artery pressure increased by 7-10 cm H2O. Lung lymph flow as well as lymph and plasma protein concentrations did not change significantly. In six sheep, postmortem lung water content was increased above that predicted but was within the predicted range for the group as a whole. We conclude that moderate CPAPB does not measurably affect the steady state lung fluid balance. More important, however, the rise in puhnonary vascular pressure must have been balanced by a rise in perimicrovascular interstitial fluid pressure since filtration did not change. It appears that the fraction of increased alveolar pressure transmitted to the microvessels was via the perimicrovascular fluid rather than through solid tissue contact.

Effect of positive pressure breathing on lung lymph flow and water content in sheep.