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Previously, we report that the major site of pulmonary vascular resistance in fetal lambs occurred in the middle region defined by vascular occlusion, and that this region exhibited the greatest decrease upon ventilation with O2. To assess the relative individual contributions of ventilation and oxygenation to this decrease, we determined the distribution of pressures across the pulmonary circulation in isolated perfused lungs from 20 fetal lambs (131 - 137 d gestation) by inflow and outflow vascular occlusions. A membrane oxygenator was included in the extracorporeal circuit to control the PO2 at 4 kPa (30 torr) in the unventilated fetal lungs. Half of the fetal lungs were ventilated first without changing the initial gas tensions, and the others were oxygenated first by changing the initial gas tension to a hyperoxic mixture [PO2 = 26.6 kPa (200 torr)] without ventilation. Finally, both groups of lungs were ventilated and oxygenated. In addition, indomethacin was added to the perfusate (0.112 mM, or 40 micrograms/mL) in half of the preparations in each group to determine the effect of prostaglandins on the distribution of pressures during these conditions. The decrease in the total pulmonary vascular resistance with ventilation and/or oxygenation was primarily due to changes in the middle pressure gradient (delta Pm). In fetal lungs without indomethacin, ventilation without oxygenation reduced delta Pm from 6.1 +/- 0.8 to 2.5 +/- 1.0 kPa, or 74% of the total ventilation- and oxygenation-induced decrease in delta Pm (final value = 1.2 +/- 0.6 kPa).(ABSTRACT TRUNCATED AT 250 WORDS)

Indomethacin Prevents Ventilation-Induced Decreases in Pulmonary Vascular Resistance of the Middle Region in Fetal Lambs