Alveolar Vascularization of the Lung in a Lamb Model of Congenital Diaphragmatic Hernia

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Abstract
Pulmonary hypoplasia and pulmonary hypertension are factors limiting the survival of infants with congenital diaphragmatic hernia (CDH). A reduction in the number of pre-acinar pulmonary vessels and increased muscularization are the structural lesions implicated as causes of irreversible hypoxemia. Whether there is a reduction in the number of air-blood barriers, which represent the capillary surface area of the lung involved in gas exchange, is unknown. We sought to determine if the lungs of CDH lambs have: (1) a reduction in total capillary surface area proportionate to the reduction in the total alveolar surface area of the lung; and/or (2) a disproportionate reduction in the number of capillaries (air blood barriers) within each acinus. The latter measurement was determined by calculating the capillary load which we defined as the number of air blood barriers/unit of surface density. At 80 d gestation (pseudoglandular period), a diaphragmatic hernia was created surgically in one lamb fetus of a twin gestation. At term, the fetuses were removed, the chests opened and the lungs fixed by a tracheal infusion of 1.5% glutaraldehyde at 25 cm of water pressure. Tissues from the lower lobes were examined by light and electron microscopy. Using computerized interactive morphometry, alveolar and capillary surface area, and capillary load were determined by intersection and point counting for the right and left lungs. The data show that the total alveolar surface area of the left CDH and control lungs were 1.8 +/- 0.8 m2 and 6.1 +/- 1.1 m2 (p < 0.01), respectively, and for the right CDH and control lungs 2.5 m2 +/- 0.1 and 11.2 +/- 1.9 m2 (p < 0.01), respectively. The total capillary surface area for the left CDH and control lungs were 0.7 +/- 0.3 m2 and 2.8 +/- 1.2 m2 (p < 0.05), respectively, and for the right CDH and control lungs 0.9 +/- 0.3 m2 and 3.8 +/- 1.5 m2 (p < 0.05), respectively. The capillary load was not statistically different. These findings demonstrate that the lungs in CDH are deficiently vascularized at the alveolar surface due to a reduction in the total alveolar surface area. Each acinus contains the same number of air blood barriers per unit of alveolar surface area indicating a normal acinar composition.