PATENT DUCTUS-ARTERIOSUS INCREASES LUNG FLUID FILTRATION IN PRETERM LAMBS

Previous studies showed that increased pulmonary blood flow from a patent ductus arteriosus had little or no effect on the amount of fluid in the lungs of mechanically ventilated preterm lambs. The purpose of this study was to examine the effect of a patent ductus arteriosus on lung vascular permeability and to see whether increased pulmonary lymph flow might compensate for the increased rate of fluid filtration. Using a model that allows mechanical control of ductus patency, we studied the effects of increased pulmonary blood flow on lung vascular pressures in six mechanically ventilated premature lambs at 136 +/- 2 d gestation (mean +/- SD) (term = 145 d). We measured lung flow and protein concentrations in lymph and plasma to assess pulmonary vascular fluid filtration and protein permeability. We studied each lamb during sequential steady state periods, first with the ductus open and then with it closed. When the ductus was open, pulmonary blood flow was twice what it was when the ductus was closed. Mean pulmonary artery pressure and left ventricular end-diastolic pressure were greater with the ductus open [40 +/- 5 torr (5.3 +/- 0.7 kPa) and 8 +/- 3 torr (1.1 +/- 0.4 kPa), respectively] than when it was closed [24 +/- 3 torr (3.2 +/- 0.4 kPa) and 4 +/- 2 torr (0.5 +/- 0.3 kPa), respectively]. When the ductus was open, lymph flow was 68% greater and lymph protein concentration was 17% lower than when the ductus was closed. Lymph protein clearance (lymph flow x lymph protein concentration/plasma protein concentration) was 39% greater when the ductus was open. Thus, patency of the ductus arteriosus in preterm lambs increases fluid filtration in the pulmonary circulation as a result of increased blood flow and vascular filtration pressure. The resultant increases in lung lymph flow and protein clearance may help to protect the lungs from edema.