During general anaesthesia, pulmonary atelectasis has been shown to occur in 85 to 90% of patient. Pulmonary atelectasis increases occurrence of postoperative pulmonary complication including pneumoniae. Pulmonary atelectasis can be prevented or reversed by alveolar recruitment manoeuvres (ARM). Two methods for ARMs have been described. A sustained continuous positive airway pressure (CPAP) or a stepwise increase in PEEP.The transient increase in intrathoracic pressure during ARMs decreases venous return and increases pulmonary vascular resistance. This result in a decrease in right and left ventricular stroke volume (SV). Finally, the deleterious hemodynamic effects of ARMs may be exacerbated by hypovolemia, heart failure, and in patients with chronic treatment wich impedes cardiovascular responses to hypovolemia. At our best knowledge, there is no study which compared the hemodynamic effects of ARM using sustained CPAP or stepwise increase in PEEP. Consequently, the present study was designed to examine the hemodynamic effects of 2 ARM methods in anesthetized patients.

This is a single-centre prospective observational study performed in operating rooms of the university hospital of Caen.

Inclusion criteria were adult patients aged 18 year and above, American Society of Anesthesiologists physical status II to IV, scheduled for intermediate and high risk abdominal and vascular surgery (as defined by the european guidelines on non-cardiac surgery: cardiovascular assessment and management), and equipped with a radial arterial catheter and transoesophageal doppler monitor Patients less than 18 year-old, adults under protection, pregnant women, patients with atrial fibrillation, history of right ventricular dysfunction, known left ventricular ejection fraction < 30%, or preoperative pulmonary disease were excluded.

After intravenous line placement and monitoring (IntelliVue MP70 Philips HealthCare, Amsterdam, The Netherlands) with continuous 5-lead electrocardiography, pulse oximetry, and bispectral index, after local anaesthesia a radial intra-arterial catheter was inserted and connected to a pressure transducer zeroed at the intersection of the mid axillary line and the fifth intercostal space. Arterial pressure and pulse pressure variation (PPV) were continuously displayed on the IntelliVue MP70 monitor. After a 3 to 5 min preoxygenation, anaesthesia was induced and maintained using target-controlled total intravenous anaesthesia with propofol and remifentanil. If a neuromuscular blocking agent was administered its effect was monitored by accelerometry at the thumb following Train-of-Four stimulations of the ulnar nerve repeated every 30 seconds. Following orotracheal intubation; patients were ventilated with controlled ventilation mode (inspired Oxygen fraction 40%, tidal volume: 8ml.kg-1, PEEP at +5 to +8 cmH2O, inspiratory to expiratory ratio of ½, respiratory rate between 10 and 15 min-1 to maintain an end-tidal carbon dioxide partial pressure of 30 to 35 mmHg). An oesophageal Doppler probe connected to its monitor was inserted after tracheal intubation (CardioQ-ODM, Deltex Medical, UK). Then, a fluid challenge was performed with 250 ml of colloid or crystalloid at the discretion of anaesthesiologist and repeated if the SV increased by more than 10%.

The first ARM was performed after orotracheal intubation and SV optimisation. Two preset ARM were available on the anaesthesia respirator: CPAP at 30 cmH2O for 30 seconds (CPAP30) and a stepwise increase and decrease in PEEP (PEEPsteps). The choice of the ARM was left at the discretion of the attending anaesthesiologist.

Heart rate, arterial pressure (systolic, diastolic, mean), PPV, pulse oximetry, cardiac index (CI), SV, peak velocity (PV), corrected flow time (FTc, averaged across 10 successive measurements), and bispectral index were recorded by an independent observer before the ARM, during the ARM at the nadir of SV variation, and 1 (after1) and 3 (after3) min after the end of the ARM.

Definitions of outcomes The primary outcome was the absolute variation of SV during ARM. The secondary outcomes were: variation of arterial blood pressure (mean, systolic and diastolic), PPV, CI, PV, FTc, and pulse oximetry

Inclusion Criteria:

• adult patients aged 18 year and above, American Society of Anesthesiologists physical status II to IV, scheduled for intermediate and high risk abdominal and vascular surgery (as defined by the guidelines on non-cardiac surgery: cardiovascular assessment and management), and equipped with a radial arterial catheter and transoesophageal doppler monitor

Exclusion Criteria:

• Patients less than 18 year-old, adults under protection, pregnant women, patients with atrial fibrillation, history of right ventricular dysfunction, known left ventricular ejection fraction < 30%, or preoperative pulmonary disease

adult patients aged 18 year and above, American Society of Anesthesiologists physical status II to IV, scheduled for intermediate and high risk abdominal and vascular surgery (as defined by the guidelines on non-cardiac surgery: cardiovascular assessment and management), and equipped with a radial arterial catheter and transoesophageal doppler monitor