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 combin ed with thoracic epidural anesthesia (TEA) in dogs. Using a separately ventilated left lower lobe (LLL) in situ, the pressure-flow (P-Q) cur ve 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 PaO 2 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 gro up (n = 6), the P-Q curve shifted to the left during hyperoxia, but th e magnitude of the shift during hypoxia was unchanged. FDR and decreas e in PaO2 were significantly reduced compared with baseline values (P < 0.05 with analysis of variance). TEA reduced heart rate, cardiac out put, mean arterial pressure, mean pulmonary arterial pressure, and mix ed venous oxygen tension. Our results suggest that TEA did not affect the primary pulmonary vascular tone at baseline or during lobar hypoxi a, but enhanced the diversion of blood flow and arterial blood oxygena tion during lobar hypoxia. This enhanced HPV response probably reflect s hemodynamic changes, such as decreased cardiac output with resultant low mixed venous oxygen tension, due to sympathetic nerve activity bl ockade by TEA.