ARTERIAL OXYGENATION DURING ONE-LUNG VENTILATION - A COMPARISON OF ENFLURANE AND ISOFLURANE

Background: Because maintaining arterial oxygenation (Pa-O2 during one -lung ventilation (OLV) can be a clinical problem, it is useful to be aware of factors that influence Pao, in this situation and are under t he control of the anesthesiologist. It is unknown whether, among the c ommonly used volatile anesthetic agents, one is associated with higher Pa-O2 levels, Clinical studies suggest that isoflurane provides super ior Pa-O2 during OLV than does halothane, These have not been compared to enflurane. The authors studied Pa-O2 and hemodynamics during OLV w ith 1 MAC enflurane versus 1 MAC isoflurane. Methods: Twenty-eight adu lts who had prolonged periods of OLV anesthesia with minimal trauma to the nonventilated lung (thoracoscopic or esophageal surgery) were stu died in a crossover design. Patients were randomized to two groups: Gr oup 1 received 1 MAC enflurane in oxygen from induction until after th e first 30 min of OLV, then were switched to 1 MAC isoflurane. In grou p 2, the order of the anesthetics was reversed. Results: Isoflurane wa s associated with higher Pa-O2 values during OLV (P < 0.0001), Mean Pa -O2 (+/-SD) after 30 min OLV isoflurane was 231 (+/-125) mmHg versus 1 84 (+/-106) mmHg after 30 min OLV enflurane. The difference in Pa-O2 b etween the two anesthetics was most marked in the patients with the hi ghest Pa-O2 during OLV: Pa-O2 isoflurane - Pa-O2 enflurane proportiona l to Pa-O2 isoflurane (r = 0.65, p < 0.001). There were no other sig s ignificant differences between anesthetic gases in the measured hemody namic or respiratory variables. In the subgroup of patients with pulmo nary artery catheters (n = 7), Pa-O2 correlated with cardiac output du ring OLV for both anesthetics (r = 0.81, P < 0.001). Conclusions: Duri ng OLV, the Pa-O2 values with 1 MAC isoflurane were greater than those with enflurane. The dependence of Pa-O2 on cardiac output does not su pport the hypothesis that an increase in cardiac output will cause a d ecrease in hypoxic pulmonary vasoconstriction and a decrease in Pa-O2 during OLV.