Increased pulmonary vascular filtration pressure does not alter lung liquid secretion in fetal sheep

The purpose of this study was to determine whether an increase in pulmonary vascular filtration pressure affects net production of liquid within the lumen of the fetal lung. We studied 14 chronically catheterized fetal lambs [130 +/- 3 (SD) days gestation] before, during, and after a 4-h rapid (500 ml/h) intravenous infusion of isotonic saline. In seven fetuses we measured pulmonary arterial and left atrial pressures, lung lymph flow, and protein osmotic pressures in plasma and lymph. In eight lambs with a chronically implanted tracheal loop cannula, we measured the change in luminal lung liquid volume over time by progressive dilution of tracheally instilled 125I-albumin, which stays within the lung lumen. Saline infusion increased pulmonary vascular pressures by 2-3 mmHg and decreased the plasma-lymph difference in protein osmotic pressure by 1 mmHg. Lung lymph flow increased from 1.9 +/- 0.6 to 3.9 +/- 1.2 (SD) ml/h; net production of luminal lung liquid did not change (12 +/- 5 to 12 +/- 6 ml/h). Thus an increase in net fluid filtration pressure in the pulmonary circulation, which was sufficient to double lung lymph flow, had no significant effect on luminal lung liquid secretion in fetal sheep.