Effect of thoracic epidural anaesthesia on ventilation-perfusion distribution and intrathoracic blood volume before and after induction of general anaesthesia

BACKGROUND: Gas exchange is impaired during general anaesthesia due to development of shunt and ventilation-perfusion mismatching. Thoracic epidural anaesthesia (TEA) may affect the mechanics of the respiratory system, intrathoracic blood volume and possibly ventilation-perfusion (VA/Q) distribution during general anaesthesia. METHODS: VA/Q relationships were analyzed in 24 patients undergoing major abdominal surgery. Intrapulmonary shunt (Qs/QT), perfusion of "low" VA/Q areas, ventilation of "high" VA/Q regions, dead space ventilation and mean distribution of ventilation and perfusion were calculated from the retention/excretion data of six inert gases. Intrathoracic blood volume (ITBV) and pulmonary blood volume (PBV) were determined with a double indicator technique. Recordings were made before and after administration of 8.5 +/- 1.5 ml bupivacaine 0.5% (n = 12) or 8.3 +/- 1.8 ml placebo (n = 12) into a thoracic epidural catheter and after induction of general anaesthesia. RESULTS: Before TEA, Qs/QT was normal in the bupivacaine group (2 +/- 2%) and the placebo group (2 +/- 3%). TEA covering the dermatomal segments T 12 to T 4 had no effect on VA/Q relationships, ITBV and PBV. After induction of general anaesthesia Qs/QT increased to 8 +/- 4% (bupivacaine group, P < 0.05 and to 7 +/- 2% (placebo group, P < 0.05). ITBV and PBV decreased significantly to the same extent in the bupivacaine group and the placebo group. CONCLUSIONS: TEA has no effect on VA/Q distribution, gas exchange and intrathoracic blood volume in the awake state and does not influence development of Qs/QT and VA/Q inequality after induction of general anaesthesia.