Experimental fetal tracheal ligation and congenital diaphragmatic hernia: A pulmonary vascular morphometric analysis

Abstract

The authors have previously shown that fetal tracheal ligation (TL) reverses the pulmonary hypoplasia in experimental diaphragmatic hernia (DH) by accelerating fetal alveolar growth. The purpose of this study was to determine if growth of the accompanying macroscopic and microscopic pulmonary vasculature is also accelerated. Eighteen fetal lambs were divided into three experimental groups: diaphragmatic hernia (DH), DH and simultaneous tracheal ligation ( DH TL ), and sham-operated controls (C). Animals were delivered near term, the lungs retrieved, and pulmonary capillary growth (5 to 50 μm in diameter) evaluated by standard morphometric techniques. Capillary ultrastructure was evaluated by electron microscopy. Nine additional fetal lambs of the same gestational age were equally divided into the same three groups and their lungs analyzed by pulmonary arteriography for evaluation of large vessel growth (<100-μm diameter). Computer digital analysis of angiogram lung slices showed that the total area of large vessels was increased in DH TL lungs when compared with DH lungs and decreased in DH lungs when compared with C lungs ( P = .003); however, the ratio of large vessel area per unit of lung area was similar in all groups. Microscopic morphometry of the capillary bed showed that the total number of capillaries was increased in DH TL lungs over both DH and C lungs ( P = .0001); however, the number of capillaries per alveolus ( cap alv ) was similar in all groups. In DH TL lungs, electron microscopy showed normal capillary wall structure and normal thickness of the capillary-alveolar interface, whereas in DH lungs, capillary structure was abnormal and the capillary-alveolar interface was thickened. DH animals had three times as many fully muscularized vessels as DH TL and twice as many fully muscularized vessels as C animals ( P = .0001). Furthermore, in DH lungs, 29% of vessels less than 100 μm in diameter were fully muscularized, whereas in DH TL and C lungs, there were no fully muscularized vessels less than 100 μm in diameter ( P = .0009). From these data the authors conclude the following: (1) Experimental fetal DH results in hypoplasia of both large pulmonary vessels and the capillary bed. (2) Fetal tracheal ligation is capable of reversing these effects by accelerating both large vessel and capillary growth based on total large vessel area and total capillary number. (3) With TL, large vessel growth remains proportional to overall lung growth based on normal values for large vessel area per unit of lung area, and capillary growth remains proportional to alveolar growth based on normal values for cap alv and cap/cm 2 lung tissue. (4) Experimental fetal DH results in increased muscularization of pulmonary arterial vessels as evidenced by an increased overall percentage of muscular arteries and by extension of muscle into normally nonmuscular vessels. (5) Fetal tracheal ligation reverses the increased muscularization of pulmonary arteries seen in experimental fetal DH. (6) By electron microscopy, capillary wall ultrastructure and the capillary-alveolar interface are abnormal in DH lungs and normal in DH TL lungs. The authors speculate that tracheal ligation exerts its effects by enhancing normal mechanisms of pulmonary development, thereby preserving the normal template for both pulmonary arterial and alveolar growth.