Pulmonary microvascular response to prostacyclin (PGI2) infusion in unanesthetized sheep

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Abstract
The effect of prostacyclin (PGI2) infusion and cessation of infusion on the pulmonary microcirculation was studied. Lung lymph flow (.ovrhdot.QL) and the lymph to plasma protein ratio were used as sensitive indices of net fluid (.ovrhdot.F) and protein flux (Cp). After a 4 h base line period, PGI2 was infused (0.2 .mu.g.cntdot.kg-1 per min) into 8 unanesthetized sheep for 2 h. Vascular pressures and lymph were monitored during infusion and for another 18 h after PGI2. During infusion .ovrhdot.QL and cardiac output increased by 75 and 50%, respectively, over base line, whereas the lymph-to-plasma ratio (L/P) remained constant for both albumin and globulin. This resulted in a significant increase in both fluid and protein flux. Pulmonary vascular pressure decreased. The increase in .ovrhdot.F and CP was felt to be due to an increase in the surface area of fluid exchange vessels rather than increased permeability. After infusion, cardiac output rapidly returned to base line, whereas mean .ovrhdot.QL remained increased by 70% over base line for 2-8 h. Mean L/P decreased from 0.65 to 0.53. Pulmonary arterial pressure and pulmonary vascular resistance increased. The increase in .ovrhdot.Q/L and decrease in L/P indicate a rebound increase ion pulmonary microvascular pressure in the postperfusion period.