ABSTRACT. We studied the effects of increased pulmo-in the newborn lamb, increases in pulmonary blood flow would nary blood flow on lung fluid balance in seven chronically increase lung fluid filtration by increasing filtration pressure instrumented lambs (18 f 1 d) with surgically created independent of changes in left atrial pressure. aorticopulmonary shunts. We measured mean pulmonary arterial and left atrial pressure (LAP), pulmonary blood flow, lung lymph flow, and lymph (CL) and plasma (CP) MATERIALS AND METHODS protein concentration with the shunt closed and opened.we operated on all lambs during the lst wk after birth using O.ol to Oe58 O-ol). With the shunt Next, through a left thoracotomy, we placed catheters directly fully open, pulmonary blood flow increased 65% over base-into the left atrium and pulmonary artery and isolated and line, pulmonary arterial pressure increased from l6.5 divided the ductal remnant. We tunneled all catheters to the left 2.0 to 26.5 f 5 torr, and LAP increased from 1.5 f 0.5 to chest and covered them with a vinyl pouch sewn to the skin. 6.5 2.0 Lung lymph increasedFor lambs in which pulmonary blood flow was to be increased, 3.1 f 0.2 mL/h) whereas CL (4.1 2 0.1 to 3.1 f 0.3 g/dL) we surgically implanted a 6 mm woven Dacron graft (Meadox and CLICP (0.66 * 0.02 O.51 0.05) decreased. Medicals Inc., Oakland, NJ) between the aorta and main pulchanges were statistically significant ( p e 0.05). When the monary artery at the time of the left thoracotomy. We placed a effects of increased LAP On lung fluid (partial balloon-tipped catheter directly into the graft and an external mitral valve obstruction) were tested in five additional occluder around its aortic end so that partial or even complete lambs, an identical change in LAP as seen in lambs with shunt occlusion could be achieved noninvasively. partially open shunts (2.5 2 0.5 to 4.0 f 0.5 torr) resulted For lambs in whom left atrial pressure was to be increased, we in only a 20% increase in lung lymph flow and a inserted an 8 Fr Foley catheter into the left atrium at the time of decrease in the CL/CP ratio (0.61 + 0.02 to 0.57 f 0.02), the left thoracotomy and tunneled its injection port out of the whereas pulmonary blood flow did not change. We conclude chest along with the catheters. Inflation of the balloon with saline that in the lamb increased pulmonary resulted in partial occlusion of the rnitral orifice and increased from a large central shunt increased the rate of transvas-left atrial pressure. cular fluid filtration in the lung by increasing filtration ~h~~ 3 days later we performed a right t~oracotomy to canpressure. his increase could not be explained simply on nulate the efferent duct of the caudal mediastinal lymph node. the basis of a change in left atrial Pressure and implies First, we resected the systemic contributions to the node by that resistance downstream from fluid filtering sites is dividing the lower of the node and cauterizing obvious relatively fixed. (Pediatr Res 26:94-97, 1989) lymphatic chann...