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 the control of the anesthesiologist. It is unknown whether, among the commonly used volatile anesthetic agents, one is associated with higher Pa-O2 levels, Clinical studies suggest that isoflurane provides superior Pa-O2 during OLV than does halothane, These have not been compared to enflurane. The authors studied Pa-O2 and hemodynamics during OLV with 1 MAC enflurane versus 1 MAC isoflurane. Methods: Twenty-eight adults who had prolonged periods of OLV anesthesia with minimal trauma to the nonventilated lung (thoracoscopic or esophageal surgery) were studied in a crossover design. Patients were randomized to two groups: Group 1 received 1 MAC enflurane in oxygen from induction until after the first 30 min of OLV, then were switched to 1 MAC isoflurane. In group 2, the order of the anesthetics was reversed. Results: Isoflurane was 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 184 (+/-106) mmHg after 30 min OLV enflurane. The difference in Pa-O2 between the two anesthetics was most marked in the patients with the highest Pa-O2 during OLV: Pa-O2 isoflurane - Pa-O2 enflurane proportional to Pa-O2 isoflurane (r = 0.65, p < 0.001). There were no other sig significant differences between anesthetic gases in the measured hemodynamic or respiratory variables. In the subgroup of patients with pulmonary artery catheters (n = 7), Pa-O2 correlated with cardiac output during OLV for both anesthetics (r = 0.81, P < 0.001). Conclusions: During 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 support the hypothesis that an increase in cardiac output will cause a decrease in hypoxic pulmonary vasoconstriction and a decrease in Pa-O2 during OLV.