Assessment of fluid balance in isolated sheep lungs

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Abstract
The validity and utility of an isolated lung preparation developed for the study of pulmonary fluid balance is reported. Lungs of 2- to 3-mo.-old sheep were perfused in situ with autologous blood treated with indomethacin (20 .mu.g/ml). Lung lymph flow (.ovrhdot.QL), uncontaminated by systemic lymph, was measured from either the efferent duct of the caudomediastinal lymh node or the thoracic duct in the superior mediastinum. Lung weight change (.DELTA.W) was measured as the opposite of the change in weight of the extracorporeal blood reservior. A unique feature of this experimental model is the ability to assess lung fluid balance from simultaneous measurements of .DELTA.W and .ovrhdot.QL. Hemodynamic and blood gas variables can be tightly controlled. Changes in .ovrhdot.QL and the lymph-to-plasma oncotic pressure ratio caused by an increase in microvascular pressure were comparable with those seen previously in intact sheep. When microvascular pressure was returned to control levels, .ovrhdot.QL fell despite a sustained increase in the amount of extravascular lung water, suggesting compartmentalization of the filtrate and/or effects of intravascular volume on lymph-driving pressure or resistance. Lymph flow was directly proportional to respiratory frequency over the range of 0-30/min when the change in frequency was maintained for periods as long as 30 min. This preparation should prove useful in the study of lung fluid balance, particularly when it is desired to use interventions which are precluded or difficult in intact animals.