The effect of thoracic epidural anesthesia on hypoxic pulmonary vasoconstriction in dogs: An analysis of the pressure flow curve

The aim of the present study was to examine whether hypoxic pulmonary vasoconstriction (HPV) is preserved during one-lung ventilation combined with thoracic epidural anesthesia (TEA) in dogs. Using a separately ventilated left lower lobe (LLL) in situ, the pressure-flow (P-Q) curve was obtained. The HPV response was assessed by the shift of the P-Q curve, changes in blood flow diversion rate (FDR) and decrease in PaO2 during hypoxic gas ventilation of LLL. in the control group (n = 7), the shift of P-Q curve, changes in FDR, and decrease in PaO2 remained constant during four consecutive hypoxic stimulations. In the TEA group (n = 6), the P-Q curve shifted to the left during hyperoxia, but the magnitude of the shift during hypoxia was unchanged. FDR and decrease in PaO2 were significantly reduced compared with baseline values (P < 0.05 with analysis of variance). TEA reduced heart rate, cardiac output, mean arterial pressure, mean pulmonary arterial pressure, and mixed venous oxygen tension. Our results suggest that TEA did not affect the primary pulmonary vascular tone at baseline or during lobar hypoxia, but enhanced the diversion of blood flow and arterial blood oxygenation during lobar hypoxia. This enhanced HPV response probably reflects hemodynamic changes, such as decreased cardiac output with resultant low mixed venous oxygen tension, due to sympathetic nerve activity blockade by TEA.