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Insights into primary and secondary pulmonary hypertension in childhood.

Recent ultrastructural studies have shown that in the normal lung the pulmonary arteries adapt to extra-uterine life not by reducing the amount of vascular smooth muscle, but by reorganizing the components of the arterial wall, thereby reducing wall thickness and increasing lumen diameter. In persistent pulmonary hypertension of the newborn, either primary or secondary to hypoxia at birth, arterial wall reorganization fails to occur. Smooth muscle cells increase their contractile myofilaments and deposit connective tissue around themselves while the artery remains in its undilated state. The newborn lung is more vulnerable to hypoxia than the lung which has adapted normally in room air for three days or more. In pulmonary hypertension secondary to congenital heart disease, the type of pathological abnormalities and their natural history is determined by the type of intracardiac abnormality. Recent studies on normal lung development and on primary and secondary pulmonary hypertension have enhanced our understanding of the relation between structure and function in the immature pulmonary circulation.