The Journal of Tehran University Heart Center Maryam Davoudi et al Results The demographic data between the two groups were comparable (Table 1). There were no significant differences in age, sex, body mass index, history of cigarette smoking, preoperative FEV1 and FVC, pre-CPB PaO2, mean CPB time, mean aortic cross-clamp time, and mean operation time between the two groups (Table 2). Post-CPB PaO2 was higher significantly in group I (Figure 1). Postoperative FEV1 and FVC decreased in both groups, but the decrease in group I was smaller (p value = 0.04, p value = 0.03). Time of extubation (p value = 0.01) was shorter in group I (Table 3). There were no complications in the two groups. Table 1. Preoperative patients> characteristics* Age (y) Gender (M/F) Body mass index FEV1 (L) FVC (L) History of cigarette smoking (%) Ventilated group 59.8±12.2 38/12 26.2±5.4 46 2.6±0.7 3.3±0.9 Not ventilated group 57.4±14.4 36/14 25.3±7.3 42 2.4±0.8 2.9±0.8 *Data are presented as mean±SD FEV1, Forced expiratory volume in 1 second; FVC, Forced vital capacity Table 2. Intraoperative patients> characteristics* Ventilated group 47.83±15.58 Cardiopulmonary bypass time (min) Aortic cross-clamp time 28.50±10.87 (min) Operation time (min) 172.17±20.18 *Data are presented as mean±SD **Mann Whitney test Table 3. Postoperative patients> characteristics* Not ventilated group 52.00±10.56 29.17±5.52 168.05±19.48 p value 0.39 0.94 0.41 0.98 0.23 0.08 p value** 0.81 0.93 0.93 p value 0.01 0.04 0.03 Figure 1. Values for oxygenation (mean PaO2) as a function of time Discussion In order to evaluate the influence of continued low tidal volume ventilation during CPB on post-bypass lung function and oxygenation, we designed the present study. We evaluated changes in pre and postoperative FEV1 and FVC on the basis of the theory that continued ventilation on CPB could reduce the decrease in these parameters. Also, the measurement of PaO2 and extubation time was the simplest and least expensive predictor of postoperative lung function. In adults undergoing coronary bypass procedures, there is a reduction of at least 25-50% in FVC and FEV1 postoperatively.8 In our study, the decrease in FEV1 and FVC was 25% and 32% in the ventilated group and 30% and 35% in the non-ventilated group. Gagnon et al. argued that low tidal volume ventilation (3 ml/kg) without PEEP ventilation per CPB could not significantly change in pulmonary vascular resistance index (PVRI), PaO2/FIo2 ratio, mean pulmonary artery pressure (MPAP), pulmonary complications, and total length of stay when comparing to the non-ventilated group.9 We applied low tidal volume ventilation (3 ml/kg) plus PEEP = 5 cmH O in the ventilated group. Improvement in 2 oxygenation and lung volumes in the ventilated group in our study may be due to the application of PEEP. In both Magnusson’s and Berry’s studies, there was a trend toward improved post-bypass pulmonary function by using CPAP.10, 11 The provisional results of this study are suggestive of a benefit from continued low tidal volume ventilation during bypass. It appears that lung inflation decreases the bronchial blood flow and that this is probably due to purely mechanical effects on the bronchopulmonary arterial anastomoses with both compression and stretching of these vessels. This raises the possibility that the repetitive inflation and deflation of lungs at physiological intra alveolar pressure is necessary for normal bronchial arterial flow secondary to the cyclical compression and relaxation of the vessels. In that case, cessation of ventilation during bypass would reduce the bronchial flow and predispose the patient to ischemic lung Ventilated group Not ventilated group 5 25 32 5.5 30 35 Extubation time (hr) Decrease in FEV1 (%) Decrease in FVC (%) 130 *Data are presented as mean±SD FEV1, Forced expiratory volume in 1 second; FVC, Forced vital capacity