Lobar blood flow and maturation of the fetal lamb lung.

Abstract

In utero, hydrostatic gradients are expected to be absent and pulmonary perfusion to be uniformly distributed. However, the apical lobe of the fetal lamb lung is known to develop surfactant and functional stability before the basilar lobe. The present study was undertaken to determine whether lobar distribution of perfusion is uniform, as expected from hydrostatic considerations, or is nonuniform, reflecting the differences in the rate of maturation. The relationship between lobar growth and blood flow was studied in 15 fetal sheep ranging in gestational age from 115 days to term (147 days). Although lobe weight approximately doubled, the lobe weight/lung weight ratio remained uniform throughout the latter 20% of gestation. Relative lobar DNA concentration also remained uniform, suggesting that the rate of increase in cell number within each lobe was similar. The proportion of the superior vena caval return to the heart which perfused the fetal lung did not vary with gestation and averaged 7.8% ± 0.7. The distribution of pulmonary blood flow was intimately related to the relative distribution of cells (b = 1.02 ± 0.04; r = + 0.95; p < 0.001). Thus, the differential appearance of surfactant in fetal sheep cannot be explained by an increased substrate availability on the basis of nonuniform pulmonary perfusion. Pulmonary perfusion in utero is uniformly distributed, as would be expected from hydrostatic considerations.