P Pu ur rp po os se e: : One hundred percent O 2 is used routinely for preoxygenation and induction of anesthesia. The higher the O 2 concentration the faster is the development of atelectasis, an important cause of impaired pulmonary gas exchange during general anesthesia (GA). We evaluated the effect of ventilation with 0.4 FIO 2 in air, 0.4 FIO 2 in N 2 O and 100% O 2 following intubation on the development of impaired gas exchange.M Me et th ho od ds s: : Twenty-seven patients aged 18-40 yr, undergoing elective laparoscopic cholecystectomy were administered 100% O 2 for preoxygenation (three minutes) and ventilation by mask (two minutes). Following intubation these patients were randomly divided into three groups of nine each and ventilated either with 0.4 FIO 2 in air, 0.4 FIO 2 in N 2 O or 100% O 2 . Arterial blood gases were obtained before preoxygenation and 30 min following intubation for PaO 2 analysis. Subsequently PaO 2 /FIO 2 ratios were calculated. Results were analyzed with Student's t test and one-way ANOVA. P value of # 0.05 was considered significant. R Re es su ul lt ts s: : Ventilation of the lungs with O 2 in air (FIO 2 0.4) significantly improved the PaO 2 /FIO 2 ratio from baseline, while 0.4 FIO 2 in N 2 O or 100% O 2 worsened the ratio (558 ± 47 vs 472 ± 28, 365 ± 34 vs 472 ± 22 and 351 ± 23 vs 477 ± 28 respectively; P < 0.05).C Co on nc cl lu us si io on n: : Ventilation of lungs with O 2 in air (FIO 2 0.4) improves gas exchange in young healthy patients during GA. 'ont altéré (558 ± 47vs 472 ± 28, 365 ± 34 vs 472 ± 22 et 351 ± 23 vs 477 ± 28 respectivement; P < 0,05 ENERAL anesthesia (GA) per-se with or without paralysis causes impairment in gas exchange with decreased blood oxygenation. [1][2][3] Atelectasis is an important cause of this impairment in gas exchange during GA. Atelectasis is caused by two mechanisms and both must be present to produce collapse. The two mechanisms are: loss of respiratory muscle tone causing reduction in functional residual capacity (FRC) and resorption of gases. An alveolus or a lung region that is closed off from the ambient air slowly collapses because of absorption of oxygen (O 2 ) into the blood stream whereas carbon dioxide (CO 2 ) delivered to the closed unit will dissolve in the surrounding tissue. The 1040 GENERAL ANESTHESIA