The risk of complication associated with airway in obese patient is important. The result of pre-oxygenation gives the clinician a prolonged non-hypoxic apnea time. The relation between FRC and non-hypoxic apnea time has been correlated. However, the best condition to accomplish the pre-oxygenation in morbidly obese patient has yet to be described in the medical literature. A study previously done in our hospital (EPO2-PV) compared the effect of different positions and ventilation modes on the FRC in the laboratory. A significant difference has been established on the FRC between the inverse Trendelenburg position with positive pressure ventilation and the head up ("beach-chair") position without positive pressure. The current study, EPO2-A is designed to compared the two positions and ventilation modes during the induction of general anesthesia on morbidly obese and correlate the difference in FRC to difference in apnea time.

Obesity prevalence in the population is increasing. Thus a growing number of obese patient need surgical interventions. These patients have a four time higher risk of suffering of serious complication in relation with their airway management compare with non-obese patients. This is explained by an increased incidence of difficulty with the ventilation and intubation of the obese. The time available for the clinician to manage the airway is define by the non hypoxic apnea time. This laps of time is dependent of the oxygen stocks of the patient, which are dependent of the functional residual capacity (FRC) and his oxygen consumption. For a non-obese patient, a normal pre-oxygenation of three minutes at 100% of oxygen allows a non hypoxic apnea time (oxygen saturation > 90%) of 8,9 minutes. However, for the morbidly obese, this time is cut to less than three minutes.

The major goal of the pre-oxygenation is to increase the alveolar partial pressure of oxygen available in the end-expiratory pulmonary volume. This can be done by replacing the nitrogen in the alveolus by oxygen and by increasing the pulmonary stocks, the FRC. It has been demonstrated that the FRC after the induction of anesthesia is cut by half for the obese. This reduction is explained by a diminished thoracic compliance and an increase of the dependent lung regions' atelectasis because of a more cephalic position of the diaphragm.

Various pre-oxygenation methods have been described to prolong the non hypoxic apnea time in the obese population. Some proposed pre-oxygenation strategies with the patient in the head up position (beach chair). It is a position derived from the ramped position described as the best to visualized the obese patients' glottis. Others proposed pre-oxygenation strategies with positive pressure ventilation, but only the supine position has been studied concomitantly.

Individually, these techniques of pre-oxygenation are superior to the combination of supine position and no positive pressure. Indeed, studies demonstrated that the beach chair position (derived from the ramped position) or the positive pressure pre-oxygenation in supine position diminished the time needed to obtain a satisfactory pre-oxygenation (End-expiratory oxygen fraction >0,9) and a longer non hypoxic apnea time. Sill, these strategies have never been combined in the same protocol.

The beach chair position without positive pressure ventilation has become the standard of care because it is the position that allows the best glottis view. Though, it has been shown by Boyce and coll. that the reverse Trendelenburg position, and not the beach chair, increased the non hypoxic apnea time, the recuperation time and the minimal saturation obtained compared to the supine position. We think that there is an advantage to use the reverse Trendelenburg position to optimize the non hypoxic apnea time. Indeed, our hypothesis is that there will be less pressure on the diaphragm in comparison with the beach char position.

A studied realized by our group (EPO2-PV) evaluated the effect of three positions (Reverse Trendelenburg, beach chair and supine) and two ventilation strategies (spontaneous ventilation with or without positive pressure) on morbidly obese FRC in laboratory. The results showed a statistically significant difference on the FRC after a pre-oxygenation with positive pressure compared with the pre-oxygenation without positive pressure, and this regardless of the position. Moreover, for both ventilation strategies, results demonstrated a statistically significant superiority between the FRC obtained after pre-oxygenation in reverse Trendelenburg compared with the beach chair and the supine position. No improvement has been shown with the beach chair position.

Thereby, the current study will try to correlate the FRC results obtained in laboratory in actual non hypoxic apnea time in the operating room. This research design tries to compare, in patient receiving general anesthesia for bariatric surgeries, the effect of the pre-oxygenation with positive pressure and the reverse Trendelenburg position, on the non hypoxic apnea time in comparison with the actual standard of care, beach chair position without positive pressure ventilation.

Inclusion Criteria:

• BMI > 40
• Abdominal obesity : waist circumference: > 115 for the women waist circumference > 130 for the men

Exclusion Criteria:

• Facial hair
• Cranio-facial abnormality
• Asthma (continuous treatment)
• COPD (FEV1 < 80%)
• Severe cardiovascular disease (NYHA > 3)
• Pregnancy
• Tobacco use
• Know or suspected difficulty with intubation
• Severe GERD or risk of aspiration

15 minutes

the patients were not followed after the research protocol, after the patient oxygen saturation reached 97 %, the protocol ended and did not follow the patients afterwards

The main limitation of this study is that we did not compare the position and ventilation mode separately. The reason why we decided to do it as a bundle of interventions was that we wanted to minimize the number of patients exposed to the protocol.