EFFECT OF THORACIC EPIDURAL-ANESTHESIA ON VENTILATION-PERFUSION DISTRIBUTION AND INTRATHORACIC BLOOD-VOLUME BEFORE AND AFTER INDUCTION OF GENERAL-ANESTHESIA

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 ((V) over dot (A)/(Q) over dot) distribution during general anaesthes ia. Methods: (V) over dot (A)/(Q) over dot relationships were analyzed in 24 patients undergoing major abdominal surgery. Intrapulmonary shu nt ((Q) over dot (s)/(Q) over dot (T) perfusion of ''low'' (V) over do t (A)/(Q) over dot areas, ventilation of ''high'' (V) over dot (A)/(Q) over dot regions, dead space ventilation and mean distribution of ven tilation and perfusion were calculated from the retention/excretion da ta 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, (Q) over dot (s)/(Q) over dot (T) was normal in the bupi vacaine group (2+/-2%) and the placebo group (2+/-3%). TEA covering th e dermatomal segments T 12 to T 4 had no effect on (V) over dot (A)/(Q ) over dot relationships, ITBV and PBV. After induction of general ana esthesia (Q) over dot (S)/(Q) over dot (T) increased to 8+/-4% (bupiva caine group, P<0.05) and to 7+/-2% (placebo group, P<0.05). ITBV and P BV decreased significantly to the same extent in the bupivacaine group and the placebo group. Conclusions: TEA has no effect on (V) over dot (A)/(Q) over dot distribution, gas exchange and intrathoracic blood v olume in the awake state and does not influence development of (Q) ove r dot (s)/(Q) over dot (T) and (V) over dot (A)/(Q) over dot inequalit y after induction of general anaesthesia.