Attenuation of hypoxic pulmonary vasoconstriction in acute oleic acid lung injury--significance of vasodilator prostanoids

Abstract

To assess a significant role of hypoxic pulmonary vasoconstriction, HPV, on maintaining the gas exchange efficiency in acute lung injury, 24 mongrel dogs were treated with intravenously injecting 0.07 ml/kg of oleic acid. Hemodynamic and gas-exchange parameters were investigated at varied inspired O2 concentration, FIO2. To know a possible contribution of vasoactive prostanoids in regulating vascular reactivity under these circumstances, observations were repeated after infusion of indomethacin. The impairment of gas exchange in injured lungs was examined by measuring the fractional retention, R, of the gas in arterial blood. For this evaluation, a normal saline containing five foreign inert gases such as sulfur hexafluoride, SF6, ethane, cyclopropane, halothane and diethyl ether was infused at a constant rate through a peripheral vein. After a steady state was established, the expired gas was collected and the samples of both arterial and mixed venous blood were simultaneously taken for the inert-gas analysis. The concentrations of the indicator gases in the samples were measured in terms of a gas chromatograph equipped with an electron capture detector for SF6 and a flame ionization detector for the other four gases. Although pulmonary vascular resistance, PVR, after injecting oleic acid at FIO2 0.60 was significantly smaller than that obtained at FIO2 0.21, cardiac output, QT as well as extravascular lung water were not different between the two conditions. R value for the indicator gas was consistently lower at FIO2 0.60 irrespective of the gas species. As increasing FIO2, R estimate concerning SF6, RSF6, rational index of the fractional blood flow perfusing shunt area, decreased significantly. Administration of indomethacin caused the rise in PVR without an appreciable change in either QT or extravascular lung water but a considerable diminution in R value for the inert gas. RSF6 after infusion of indomethacin decreased from 0.35 to 0.27, accompanied by a significant rise in arterial PO2 from 84 to 99 Torr. The findings are highly compatible with the idea that HPV is distinctly attenuated in diseases areas induced by oleic acid probably due to a local accumulation of vasodilator prostanoids. Inhibiting prostanoid biosynthesis may selectively enhance the vascular reactivity to O2 in shunt vessels and may redistribute the perfusion from shunt to relatively normal areas, thereby improving gas exchange at alveolar region without altering the total amount of extravascular lung water.