The main pulmonary trunk was banded in four fetal sheep at 63-69 days of gestation. The fetuses were killed after they had developed progressive pulmonary stenosis at 98, 123, 134 and 135 days of gestation. The right lung of each animal was perfused with glutaraldehyde and serial sections followed microscopically. The medial width/external diameter ratios for fifth generation resistance vessels were significantly less (0.13) than those from six normal control lungs (0.16, p lt; 0.001). In addition, the number of resistance vessels per cm' lung tissue in the lungs of the animals with experimental pulmonic stenosis was less than in normal controls.
The altered in utero hemodynamics with severe pulmonic stenosis results in thin-walled pulmonary arterial vessels. This may be caused by an increased blood oxygen tension of the blood perfusing the pulmonary circulation via reversed flow through the ductus arteriosus, or altered pulmonary arterial pressure characteristics in the pulmonary vessels distal to the obstructed pulmonary trunk.