During anesthesia maintenance, the effect of oxygen concentration on arterial blood gas values in patients undergoing lower extremity surgery

Due to both hypoventilation caused by anesthetic agents and increase in pulmonary shunts and decrease in functional residual capacity, higher O2 concentration than atmospheric air (%20.8) is required during general anesthesia practice. Nowadays 50% O2, and 50% dry air or 70% N20 mixtures are used in the maintenance of anesthesia. The composition of inspired gas can change arterial oxygenation. In our study, we aimed to investigate the effect of oxygen concentration which was used for the maintenace of anesthesia on values of arterial blood gas in patients who will will be underwent lower extremity surgery under general anesthesia.40 patients between 20 and 65 years old with normal body weight from both sexes and ASA I-II risk groups were studied in this study. 30 min. before operations, all patients randomly divided into two groups and had midazolam (0.05 mg/kg, im) and atropine (0.01mg/kg, im) as a premedication. 5-7 mg/kg sodium tyopental, 2 mcg/kg fentanyl and 0.1 mg/kg vecuronium iv were applied for induction of anesthesia. 50% O2 and 50% dry air for patients in Group 50, and 30% O2 and desflurane with 5-6% concentration in 70% N2O for patients in Group 30 were given to maintain anesthesia. Arterial blood gas analyses, mean arterial pressure (MAP), heart rate (HR), nausea and vomiting, postoperative pain intensity (with VAS) were preoperatively, intraoperatively (after preoxygenation, 5 minutes after intubation, intraoperative every one hour, 5 minutes before and after extubation) and postoperatively (at 1, 6, 12, 24, 48, and 72 hours) observed; preoperative and postoperaitive PA chest radiographs were eveluated.Demographic data, MAP, HR, serum electrolytes, postoperative nausea and vomiting and pain intensity were not different between the groups. Similar changes of bicarbonate, base deficit, and PaCO2 parameters were detected during observation periods. In the intraoperative and the postoperative first hour of SaO2 values in maintenance period of PaO2 values were higher in Group 50 then in Group 30 (p<0.001); lower pH values was detected in the intraoperative period. However, the changes in these parameters were observed to be reached to the basal values in the postoperative at 6 hour.As a result, we detected that instead of 30% O2 + 70% N2O, giving 50%O2 + 50% air in the maintenance of anesthesia provided only better arterial oxygenation within the intraoperative period and the postoperative first six hour, also in terms of patients? comfort it did not effect both the postoperative pain intensity and incidence of nausea and vomiting.Key words: General anesthesia, oxygen concentration, arterial blood gases.