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Pulmonary vascular structure was analyzed in the lungs of 10 patients with a secundum atrial septal defect (ASD) in whom pulmonary hypertension had developed. Four patients were aged 6 months or less, 5 were aged 2 to 9 years, and 1 was 21 years old. Pulmonary vascular structure was analyzed using lung biopsy tissue in 5 and autopsy material in the other 5. All the infants presented with heart failure and all had a marked increase in pulmonary arterial smooth muscle; only 1 infant survived surgery. Of the 5 older children, 1 presented with cyanosis, but in the rest the ASD was incidental to the presentation. Three patients had severe pulmonary vascular disease, similar to that seen in adults with a hypertensive ASD. Only 2 older children underwent successful surgery. In 1 child and in the 1 adult, the severity of the pulmonary vascular disease precluded surgery. The ASD was closed in 8 patients, but only 3 survived. Pulmonary hypertension develops rarely in secundum ASD in childhood.

Pulmonary vascular structure was analyzed in lung tissue taken from 38 patients, aged 2 weeks to 9.1 years, with complete atrioventricular (AV) septal defect without important left AV valve regurgitation. Biopsy material was used in 27 patients (71%). Pulmonary artery (PA) pressure and resistance increased with age (r = 0.5, p less than 0.001 and r = 0.4, p less than 0.02, respectively). Intra-acinar mean PA arterial medial thickness decreased with age (r = -0.6, p less than 0.001), PA pressure (r = -0.6, p less than 0.001) and resistance (r = -0.4, p less than 0.05). Reduction in intra-acinar PA muscularity was associated with an increase in severity of obstructive intimal damage in pre-acinar PAs with age. A PA resistance of less than 6 units m2 was associated with age younger than 3 years (usually younger than 2 years), PA medial thickness of more than 19% (normal = 7.4%), and muscle extension with or without intimal proliferation. A higher resistance occurred in patients of all ages, but in those younger than 3 years structural abnormalities usually resembled those in patients with a lower resistance, whereas older patients had a normal or slightly increased PA medial thickness of 14% or less (normal = 7.4%), or classic grade III or IV disease. The 4 perioperative deaths attributed to pulmonary vascular disease occurred in patients who were similar in age and hemodynamic status to the survivors, but who had more severe pulmonary vascular abnormalities; either less muscularity with more severe intimal fibrosis or a much greater increase in muscularity, emphasizing that even potentially reversible abnormalities can prejudice intracardiac repair.

Twelve Large White pigs aged less than 1 min, and 3, 5.5, and 14 days were exposed to hypoxia (380 torr) for 2.5-3 days. The wall structure of terminal bronchiolar (resistance arteries) and elastic arteries was assessed by light and electron microscopy using quantitative morphometric techniques. In animals exposed from birth, mean terminal bronchiolar arterial medial thickness was increased (p less than 0.05) because endothelial and smooth muscle cells (SMCs) retained their fetal shape, position, overlap, interdigitation and the low surface/volume ratio characteristic of fetal life. In all older animals, the cells had a normal postnatal shape and surface/volume ratio. In the elastic vessels hypoxia did not prevent the normal postnatal reduction in mean SMC diameter of animals exposed from birth. SMC hypertrophy did not occur in any age group, but all animals save those first exposed at 14 days, showed an increase in SMC myofilament volume density (p less than 0.01). Connective tissue volume density also increased (p less than 0.01), mainly due to an increase in elastin and ground substance. Thus a short period of neonatal hypoxia impaired adaptation and appeared to potentiate contractile capacity in stiff-walled arteries but elicited a less marked response from animals first exposed at 14 days.