LUNG LUMINAL LIQUID CLEARANCE IN NEWBORN LAMBS - EFFECT OF PULMONARY MICROVASCULAR PRESSURE ELEVATION

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Abstract
Postnatal clearance of fetal lung liquid is complete within 6 h of birth in normal lambs. Most of the liquid drains directly from the lung lumen through the interstitium into the bloodstream, as pulmonary lymphatics appear to play a small role in this process (J Appl Physiol 1982; 53:992). To test the possibility that increased pulmonary microvascular pressure might slow the rate of removal of luminal liquid and redirect the liquid into lung lymphatics, we studied 25 lambs, 16 of which had a balloon catheter inflated in the left atrium to maintain lung microvascular pressure 10 torr greater than normal throughout the experiments. We measured pulmonary arterial and left arterial pressures, lung lymph flow, and concentrations of protein in lymph and plasma of 6 anesthetized, mechanically ventilated lambs, 1 to 3 wk old, for 2 to 4 h before and for 6 h after intratracheal instillation of warm, isotonic saline, 6 ml/kg body weight. Extravascular water was measured gravimetrically in lungs of 19 lambs (10 with increased and 9 with normal pulmonary microvascular pressure) killed at 1, 2, and 6 h after saline instillation. The percent liquid cleared from the lungs at 1 and 2 h after saline was significantly less in lambs with increased lung microvascular pressure than it was in lambs with normal microvascular pressure (48 versus 68% at 1 h, 60 versus 77% at 2 h, respectively). Thus, increased lung microvascular pressure slows liquid clearance from the newborn lung. Almost all liquid (> 92%) disappeared from the lungs of lambs in both groups by 6 h. In lymph flow studies, lymph protein concentration decreased after saline instillation and then slowly returned to baseline by 6 h. A transient increase in lymph flow accounted for < 10% of the instilled saline, indicating that fluid did not preferentially enter lymphatics in the presence of increased lung microvascular pressure. These results suggest that dilution of interstitial protein by alveolar liquid may cause a sufficient increase in the protein pressure difference between plasma and lung interstitial fluid to offset the increased microvascular hydraulic pressure and thereby facilitate absorption of liquid into the pulmonary circulation, albeit at a slower rate. It is possible that some liquid drains by way of the pulmonary interstitium into the mediastinum, with resultant uptake into the systemic circulation.