Abstract
Extract: An indicator-dilution method is described for measuring fetal pulmonary fluid, FPF, volume (Ve), and volume change with time (Vs) in the lamb fetus in utero. The indicator, albumin, which is the predominant protein of normal FPF, was administered directly to FPF in the form of radio-iodinated human serum albumin (RISA). We have shown that (1) RISA is disbributed homogeneously throughout the FPF compartment, (2) it is not altered within FPF, (3) it does not alter the functional characteristics of FPF, and (4) it remains within the FPF compartment throughout the period of measurement. Ve varies directly with the weight of the fetus so that Ve per kilogram was 31.6–35.6 ml/kg in four of five fetuses. The Ve per kilogram was lower in one fetus at the start of the experiment but increased rapidly to 29.0 ml/kg within 45 min. Thus the relaxation volume of the fetus in utero is in the range of functional residual capacity (FRC) and thoracic gas volume of the air-breathing neonate, which indicates that the pulmonary transformation at birth is, in essence, an isovolumic change in state wherein the fetal liquid-lung becomes the neonatal air-lung. The rate at which FPF is formed (V) is about 1.5 ml/hr-kg; however, the rate may change in so far as both rapid increases and rapid decreases of FPF volume were observed. When breathing movements were induced in the fetus by stimulation of the sciatic nerve, FPF volume decreased rapidly (Vs = −3.0 ml/hr-kg), which indicates that negative intrathoracic pressures promote its resorption. When FPF volume fell transiently or was low initially, there was subsequent rapid restoration of volume to the range of anticipated FRC.
Speculation: Validation of a reliable method for measuring Ve and Vs of FPF in utero provides a basis for study of molecular transformations within this compartment during gestation, e.g., our investigation of the fate of FPF phosphatidylcholine in the accompanying report. Since FPF is the analog of the alveolar lining layer of the air-breathing animal, studies of the former (FPF) may give insight into the fluid and molecular dynamics of the latter which, to date, have been quite elusive because the lining layer cannot be sampled directly. Our observation that negative intrathoracic pressure alone seems to promote resorption of FPF under the conditions of our experiments suggests that this may be an important mechanism for the formation of the air-lung at birth. Since relaxation volume of the fetus is practically the same as that of the neonate, we have proposed that alveolar surface tension of the neonate is close to 0.
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Scarpelli, E., Condorelli, S., Cosmi, E. et al. Lamb Fetal Pulmonary Fluid. I. Validation and Significance of Method for Determination of Volume and Volume Change. Pediatr Res 9, 190–195 (1975). https://doi.org/10.1203/00006450-197504000-00010
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DOI: https://doi.org/10.1203/00006450-197504000-00010
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