Ontogeny of neonatal pulmonary vascular pressure-flow relationships.

Previously we reported that pulmonary vascular pressure gradients determined by vascular occlusion varied as a function of neonatal age. The purpose of this study was to evaluate the effect of blood flow on pressure gradients during normoxia (inspired O2 28%) and hypoxia (inspired O2 4.2%) in isolated, indomethacin-treated lungs obtained from lambs at less than 1, 2-4, 12-14, and 30-32 days of age (n = 6 at each age). The total pulmonary pressure gradient was partitioned into pressure gradients across relatively indistensible arteries and veins and relatively compliant vessels in the middle region (delta Pm) by inflow and outflow occlusions at flows of 25, 50, 75, and 100 ml.kg-1.min-1. During normoxia, lungs from lambs less than 1 day of age had significantly greater pressures at each flow than lungs from older lambs, due primarily to a significantly greater delta Pm in this age group. During hypoxia, the greatest vasoconstriction occurred in the youngest group, also due mostly to an increased delta Pm. This enhanced response to hypoxia in the younger age groups was due to inhibition of prostaglandin synthesis; without indomethacin treatment, the older age groups had a greater hypoxic pressor response than the younger groups. The arterial vascular resistance at all ages was relatively constant as a function of blood flow during both normoxia and hypoxia, whereas the total and middle vascular resistances generally decreased as a function of flow. The venous resistance initially decreased and then was constant. These results suggest that the neonatal pulmonary circulation h as a fixed arterial resistance, a slightly distensible venous region at low flows, and a more distensible middle region.