Abstract Background: High fraction of oxygen in the general anaesthesia and for the critical ill can provide increased oxygen reserves but can promote the formation of atelectasis.low fraction of oxygen won’t produce atelectasis.About how different fraction of inspired oxygen effected oxygen reserves and how different fraction of inspired oxygen effected respiratory function is unknown so far.We investigate how different inspired oxygen concentration affect oxygen reserves and how different fraction of inspired oxygen affected respiratory function in the anesthesia recovery period and evaluate whether 35% oxygen,50% oxygen and 75% oxygen can take place of 100% oxygen in the general anesthesia. Method:152 patients scheduled for elective surgery of extremities had been operated through general anaesthesia. After entering PACU,all the patients were mechanically ventilated with SIMV model.I:E ratio on 1:2, tidal volumes of 7-10 ml/kg body weight.PETCO2 was kept on 40±5mmHg by adjusting respiratory frequency. The oxygenation continued until a stable state of end-tidal oxygen concentration is achieved. patients were radomly assigned to(a) a fraction of inspired oxygen(FiO2=0.35)(n=38),(b)FiO2=0.5(n=38),(c)FiO2=0.75(n=38), (d)FiO2=1.0(n=38). To reexpand atelectasis, a recruitment maneuver was performed as follows.The ventilator was switched to pressure-controlled mode,using an inspiratory pressure of 20 cm H2O After two breaths,the end-expiratory hold button was pressed,inspiratory pressure was changed immediately to 40 cm H2O and the longest possible inspiratory time was selected by changing ventilatory frequency to 6 bpm and inspiratory time to 80% of the total respiratory cycle.This resulted in an active inspiratory phase of 8 s.Thereafter,the ventilator was switched back to volume-controlled mode,as used before the vc manoeuvre. The oxygenation continued until a stable state of end-tidal oxygen concentration is achieved. The end-tidal values of oxygen and carbon dioxide at steady state were noted before ventilation was stopped.Oxygen consumption and carbon dioxide production at rest were measured according to gasometric analysis.ventilation was then withheld until the oxygen saturation, assessed by pulse oximetry, decreased to 90%.The oxygen saturation was continuously registered.The patients were ventilated by adjusted breathing frequency to 12 bpm and vital volume to 10ml per kg . The vital volume was lowered to approximately 7ml/kg body weight and breathing frequency was lowered to approximately 10 bpm to avoid hyperventilation, as soon as Spo2 was raised to 96% and end-tidal carbon dioxide was normalized. Airway pressure did not exceed 30 cm H2O in any patient.Blood samples were taken from dorsal artery of foot for blood-gas analysis. Blood samples were also taken at approximately 13 min after apnea for the posterior half groups breathing 35% oxygen,50% oxygen,75% oxygen.After spontaneous breathing was formed and the patients were concious, they were tracheal extubated.Results: The corresponding times to reach 90% oxygen saturation were129±42,178±61,340±152,421±153(p<0.001) in the group breathing 35% oxygen,50% oxygen,75% oxygen,100% oxygen group.Apnoea tolerance in the 100% oxygen group was longer without a statistically significant difference when compared with the 75% oxygen group. Apnoea tolerance in the 100% oxygen group was significantly longer when compared with 50% oxygen and 35% oxygen.Apnoea tolerance in the 75% oxygen group was significantly longer when compared with the 50% oxygen and 35% oxygen group. Apnoea tolerance in the 50% oxygen group was longer without a statistically significant difference when compared with 35% oxygen group. Shunt after apnea was 4.07%±1.09%, 4.55%±1.30%, 5.09%±2.47%, 13.08% ±4.47% in the groups breathing 35% oxygen,50% oxygen,75% oxygen and 100% oxygen respectively. Shunt in the groups breathing 35% oxygen,50% oxygen,75% oxygen is significantly lower than that in the 100% oxygen group(p<0.001).Shunt wasn’t signifiantly different among the the groups breathing 35% oxygen,50% oxygen,75% oxygen.shunt wasn’t signifiantly different at different times in the groups breathing 35% oxygen,50% oxygen,75% oxygen respectively.PaO2/FiO2 after apnea was 494.43±74.91 , 523.00±69.9 , 536.33±80.21, 423.0±93.61(p<0.001).There was no difference regarding oxygenation among the four group at Baseline.In contrast,Pao2/FiO2 after apnea in the 35% oxygen,50% oxygen,75% oxygen was significant larger when compared with 100% oxygen group. Pao2/FiO2 after apnea wasn’t signifiantly different among the the groups breathing 35% oxygen,50% oxygen,75% oxygen.Pao2/FiO2 wasn’t signifiantly different at different times in the groups breathing 35% oxygen,50% oxygen,75% oxygen respectively.Conclusion: the tolerance time of 100% oxygen in postanaesthesia recovery is longest but 100% oxygen may promote atelectasis and deteriorate the postoperative respiratory function.the use of 75% oxygen can improve respirotory function but the time margin before unacceptable desaturation occurred was shortened by 81 seconds when compared with 100% oxygen.