BACKGROUND: During one lung ventilation(OLV), the nonventilated lung profounds ventilation/ perfusion mismatcb, so arterial oxygen tension decreases. Hypoxic pulmonary vasoconstriction(HPV) is an autoregulatory mechanism to redistribute pulmonary blood flow and it prevents severe decrease in arterial oxygen tension. Effects of isoflurane on HPV are controversial in human studies. In this study, we measured intrapulmonary shunt(Qs/Qt) and arterial oxygen tension(PaO2) at two lung ventilation (TLV) and OLV, with and without isoflurane to find whether isoflurane depresses HPV in human or not. METHODS: Twenty adult patients were allocated to control group or isoflurane group. After intubation with a double lumen endotracheal tube, TLV with 100% oxygen was done with continuous intravenous infusion of fentanyl(4 mcg/kg/hr) and midazolam(0.1 mgfkg/hr). Pulmonary arterial catheter was inserted via right internal jugular vein. Hemodynamic variables and arterial and mixed venous gas analysis were measured as control values during TLV at decubitus position. For separated ventilation, the dependent lung was ventilated with 100% oxygen, the nondependent lung with 100% nitrogen. Isoflurane(1.2 vo1.%) was administered in isoflurane group and no inhalational agent was administered in control group. RESULTS: In both groups, during OLV Qs/Qt increased and PaO2 decreased significantly compared to TLV. But there was no difference in degrees between two groups. CONCLUSION: Isoflurane of 1.2 vo1.% at end expiratory state does not inhibit HPV significantly in human.