Distribution of pulmonary vascular pressure as a function of perinatal age in lambs

The distribution of pulmonary vascular resistance (PVR) with respect to compliance was determined using vascular occlusion in isolated lungs from lambs at five ages, from 2 wk before birth to 1 mo of age. The major change in PVR occurred in the pressure gradient across the middle compliant region (ΔPm), which dropped sharply at birth, remained low for 2 wk, and increased at 1 mo. Pulmonary vasoreactivity also varied with age. Lungs at 0-4 days did not respond to hypoxia and responded poorly to prostaglandin F(2α) (PGF(2α)). In contrast, lungs at 13-33 days had significant increases in ΔPm and the gradient across relatively indistensible arterial vessels during hypoxia and increases in all gradients with PGF(2α). Ventilation of fetal lungs reduced PVR, mainly because of a 50% reduction in ΔPm. Our results demonstrate that the magnitude and distribution of PVR relative to compliance varied as a function of perinatal age and that pulmonary vasoreactivity depended on postnatal age. The major effect of ventilating fetal lungs was on the middle region.