Transpulmonary water exchange in newborn lambs under artificial placentation

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Abstract
Newborn lambs were anesthetized and maintained in a stable state with an extracorporeal oxygenator. o. 85% NaCl was made radioactive by adding tracer amounts of I) polyvinylpyrrolidone I131, a relatively nondiffusible substance, and 2) tritiated water. Forty or 60 ml of this solution were instilled into the trachea, gently agitated, and aliquots withdrawn at 30-sec intervals. Brachial artery blood was sampled simultaneously at 1-sec intervals. Polyvinylpyrrolidone-I131 in the tracheal fluid remained at a constant concentration for 20 min. Therefore it was concluded that the net volume of fluid in the lung space remained constant. Tritiated water, in contrast, passed rapidly from lung space into blood. Since intrapulmonary fluid volume remained constant it was concluded that an exchange was taking place, tritiated water leaving the lung and being replaced at an equal rate with blood water. The minimal rate of this exchange was between 1 1/2 and 3 times the injected volume in each minute. Available evidence indicates that this rate was determined by the characteristics of the alveolocapillary membrane and was not limited by the pulmonary blood flow.