Constrictive and restrictive pulmonary hypertension in the newborn and infant.

The normal pulmonary circulation is constricted at birth and, as judged by its low arterial density, is relatively more restricted than in the older infant and child. During adaptation to air breathing, pulmonary arterial dilatation occurs rapidly, but also the compliance of the resistance arterial segment increases. In the fetus and newborn, the resistance segment is proximal to the respiratory or alveolar surface. Further expansion of the pulmonary vascular bed occurs by growth in size of lumen diameter of existing arteries and growth of new ones. Multiplication of alveoli and arteries is relatively dissociated--alveolar density can increase normally without normal vascular multiplication. Persistent pulmonary hypertension of the newborn occurs because of (1) lung hypoplasia associated with hypoplasia of the vascular bed, usually affecting both size and number of units, (2) abnormal muscularization of intraacinar arteries before birth, causing restriction of vascular volume, (3) failure of the adaptation programs, and (4) hyperreactivity. Immaturity of the circulation is apparent as hyperreactivity or "twitchiness": this can be superimposed on each of the other types. A hyperirritable vascular bed can cause a labile and then a fixed pulmonary hypertension that does not respond to dilators.