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

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Abstract
The effect of 10 cm H2O of continuous positive airway pressure breathing (CPAPB) on steady state lung fluid balance was studied. In 9 of 20 chronically instrumented, unanesthetized sheep, lung lymph flow, pulmonary vascular pressure, cardiac output, pleural pressure and lymph and plasma protein concentration were measured during a 2 h baseline period and 3-4 h of CPAPB. In 8 sheep, the same variables were measured after increasing average left atrial pressure by 18 cm H2O to cause mild interstitial edema. At the end of the final experiment, the sheep were anesthetized, the lungs removed, and their water content measured. During CPAPB, pleural and left atrial pressures increased by 5 cm H2O; pulmonary artery pressure increased by 7-10 cm H2O. Lung lymph flow as well as lymph and plasma protein concentrations did not change significantly. In 6 sheep, postmortem lung water content was increased above that predicted but was within the predicted range for the group as a whole. Moderate CPAPB does not measurably affect the steady state lung fluid balance. More important, the rise in pulmonary vascular pressure must have been balanced by a rise in perimicrovascular interstitial fluid pressure since filtration did not change. The fraction of increased alveolar pressure transmitted to the microvessels was apparently via the perimicrovascular fluid rather than through solid tissue contact.