Permeability of premature lamb lungs to protein and the effect of surfactant on that permeability

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Abstract
The permeability of the alveolar epithelium of prematurely delivered lambs supported on ventilators was studied by following the movement of radiolabeled albumin and protein into and out of the airways. Lambs (22, .apprx. 136 days gestational age) were killed at times ranging from birth to 3 h of age. As assessed by alveolar wash, 11.5%/h of 125I-albumin administered to these lambs in fetal lung fluid at birth left the airways, while 1.7%/h of the arterially injected dose of 131I-albumin entered the airways. Total protein increased in the alveolar washes by 60 mg .cntdot. kg-1 .cntdot. h-1, concurrently, with an increase in the quantity of inhibition by the soluble protiens of the surface tension-lowering properties of surfactant. This large bidirectional flux of protein indicates very abnormal permeability properties of the alveolar epithelium of prematurely delivered and ventilated lambs with normal blood gases. To assess the effect of surfactant on the permeability, 1 lamb of each of eight pairs of twin lambs was treated with 50 mg/kg natural sheep surfactant at birth, and all 16 lambs were supported on ventilators for 3 h. The surfactant treatment significantly decreased the radiolabeled albumin flux and the total protein present in the alveolar washes. This surfactant treatment at birth also increased partial pressure of O2 (PO2) values (P < 0.002), decreased the peak inspiratory pressure needed to support the lambs (P < 0.002), and decreased the left-to-right shunt across the patent ductus arteriosus (P < 0.01). The protein leak in the lambs increased as the peak inspiratory pressure needed to ventilate the lambs increased (P < 0.001). Thus surfactant may decrease the protein leak by changing cardiopulmonary mechanics or by a primary effect on the alveolar epithelium.