The Effect of Low Tidal Volume Ventilation during Cardiopulmonary Bypass ... endotracheal suctioning, extubation, and physiotherapy that includes deep breathing and coughing exercises, incentive spirometry, and the application of a range of measurement to achieve positive airway pressures and alveolar recruitment. More recently, the effects of postoperative position, pain management, and early ambulation on pulmonary complications have been evaluated.4 A wide range of ventilatory strategies while on CPB have been attempted; they include continuous positive airway pressure (CPAP) with pressures of 5-15 cmH o, 2 high frequency low tidal volume ventilation (100 breaths/ min), inspired oxygen concentrations from 21% to 100%, and bilateral extracorporeal circulation using the lungs to oxygenate the blood while on bypass. Although some small and transient benefits for CPAP with 10 cmH O have been 2 demonstrated, no convincing clinical benefits for any of the ventilatory strategies have thus far emerged.5 In a swine model, Lamarche et al. concluded that mechanical ventilation prevented the pulmonary endothelial dysfunction due to reperfusion after CPB.6 The effects of ventilation during CPB have been tested in a number of studies using vital capacity maneuvers, CPAP, and continuous ventilation over the period of cardiac arrest.7 Meanwhile, better postoperative gas exchange and less pulmonary shunting were observed in patients who received CPAP (10 cmH2O), although the beneficial effects of CPAP were not evident in an animal CPB model. To date, the evidence for the benefits of maintaining ventilation alone during CPB has been inconsistent, with most studies showing no significant preservation of the lung function.7 It seems that further studies are necessary to find out which ventilation strategy could improve post-bypass lung function. The goal of this study was to evaluate whether continuous low tidal volume ventilation on CPB would prevent atelectasis, preserve lung mechanics, and improve oxygenation after cardiac surgery. Methods The study was approved by the institutional Ethics Committee, and informed written consent was signed by all the patients. In total, 100 patients undergoing elective CABG were enrolled in this prospective, randomized clinical trial. On the day of surgery, the patients were randomized via the closed envelope method into either group I (n = 50) or group II (n = 50). Exclusion criteria were comprised of the American society of anesthesiologists (ASA) class > III, age > 70 years, poor left ventricular function (left ventricular ejection fraction < 40%), valvular heart disease (any disease process involving one or more of the valves of the heart), reoperation, renal impairment (elevated serum creatinine), and significant pulmonary disease as defined by preoperative FEV1 or forced vital capacity (FVC) values < 40% of the TEHRAN HEART CENTER predicted value. All the patients received education and underwent spirometry on the day before surgery. Arterial and central venous cannulation was conducted for all the patients, who also received a standard anesthetic technique, using premedication with intramuscular morphine (0.15 mg/kg) and promethazine (0.5 mg/kg) 2 hours before the induction of anesthesia. After preoxygenation for at least 3 minutes, anesthesia was induced by using sufentanil (25µg), propofol (1.5 mg/kg), and atracurium (0.5 mg/kg). This was followed by manual ventilation at a fraction of inspired O2 (FIO2) of 1.0 for 3 minutes and subsequent tracheal intubation. After intubation, mechanical ventilation was started with volume control ventilation at the following setting: tidal volume (TV) = 7 ml/kg (lean body mass), positive end expiratory pressure (PEEP) = 5 cmH2O, inspiratory/expiratory (I/E) ratio = 1:2, FIO2 = 1.0, and Respiratory Rate (RR) = 12/ min. Anesthesia was maintained by a constant infusion of sufentanil, propofol, and atracurium as required. CABG was performed via a median sternotomy. During CPB, the patients in group I (n = 50) underwent low tidal volume ventilation (TV = 3 ml/kg [lean body mass], RR = 12, FIO2 = 1.0, PEEP = 5 cmH2O, and I/E ratio = 1:2), and the patients in group II (n = 50) underwent typical (no ventilation) CPB. In both groups, a standard CABG procedure was performed using the roller pump (Affinity, USA) and cold crystalloid cardioplegia (Martindale, U.K) for myocardial protection. After surgery, all the patients were transferred to the ICU and were managed and monitored according to usual routine requirements. In the ICU, the lungs were ventilated using the synchronized intermittent mandatory ventilation (SIMV) ventilation mode. The initial setting was: FIO2 = 1.0, RR = 12, TV = 7 ml/kg (lean body mass), and PEEP = 5 cmH2O. Respiratory rate and FIO2 were then adapted to obtain an arterial PaCO2 = 35-40 and PaO2 > 90 mmHg. Four hours after the completion of CPB measurements, FIO2 was 0.4 using oxygen enriched air. The decision for extubation was made according to the protocol such as adequate gas exchange (PaO2 > 70 mmHg, FIO2 < 0.4, PaCO = 35-40 2 mmHg, and spontaneous RR < 25), hemodynamic stability (i.e. adequate cardiac output, controlled arrhythmias, and no pulmonary edema), and absence of bleeding.8 PaO2 was measured after the induction of anesthesia and endotracheal intubation, before CPB, just after CPB, 5 minutes, 2 and 4 hours after the completion of CPB, and after extubation in the ICU. Time of extubation, FEV1, and FVC measured on the 5th postoperative day were recorded. Pre and postoperative spirometries were done by someone blinded to the groups. The anesthesia protocol and surgery were done by one anesthesiologist and one surgeon, respectively. ICU management as well as data collection was performed by the nursing staff, who were blinded to the study groups. The values are expressed as mean and standard deviation. The data were analyzed with SPSS 10th version, and p values < 0.05 were considered statistically significant. The Journal of Tehran University Heart Center129