Effects of an aorticopulmonary shunt on lung fluid balance in the young lamb

We studied the effects of increased pulmonary blood flow on lung fluid balance in seven chronically instrumented lambs (18 ± 1 d) with surgically created aorticopulmonary shunts. We measured mean pulmonary arterial and left atrial pressure (LAP), pulmonary blood flow, lung lymph flow, and lymph (CL) and plasma (CP) protein concentration with the shunt closed and opened. With the shunt partially open, a 35% increase in pulmonary blood flow resulted in an increase in pulmonary arterial pressure (15.5 ± 1 to 19.5 ± 1 torr) and LAP (2.0 ± 0.5 to 3.5 ± 0.5 torr). Lung lymph flow nearly doubled (1.53 ± 0.28 to 2.83 ± 0.52 mL/h) whereas the C_L decreased (4.1 ± 0.1 to 3.4 ± 0.1 g/dL) resulting in a decrease in the Cl/Cp ratio (0.67 ± 0.01 to 0.58 ± 0.01). With the shunt fully open, pulmonary blood flow increased 65% over baseline, pulmonary arterial pressure increased from 16.5 ± 2.0 to 26.5 ± 5 torr, and LAP increased from 1.5 ± 0.5 to 6.5 ± 2.0 torr. Lung lymph flow increased (1.1 ± 0.2 to 3.1 ± 0.2 mL/h) whereas CL (4.1 ± 0.1 to 3.1 ± 0.3 g/dL) and Cl/Cp (0.66 ± 0.02 to 0.51 ± 0.05) decreased. All changes were statistically significant (p < 0.05). When the effects of increased LAP on lung fluid balance (partial mitral valve obstruction) were tested in five additional lambs, an identical change in LAP as seen in lambs with partially open shunts (2.5 ± 0.5 to 4.0 ± 0.5 torr) resulted in only a 20% increase in lung lymph flow and a small decrease in the C_L/C_P ratio (0.61 ± 0.02 to 0.57 ± 0.02), whereas pulmonary blood flow did not change. We conclude that in the newborn lamb increased pulmonary blood flow from a large central shunt increased the rate of transvas-cular fluid filtration in the lung by increasing filtration pressure. This increase could not be explained simply on the basis of a change in left atrial pressure and implies that resistance downstream from fluid filtering sites is relatively fixed.