• rate of increase of fractional expired oxygen level [ Time Frame: every 10 seconds during each 3 minute preoxygenation period ] [ Designated as safety issue: No ]
  

• Level of fractional expired oxygen. Patient tolerance. Ease of preoxygenation administration. Presence of secondary effects (gastric reflux, respiratory discomfort, burps) [ Time Frame: every minute during each 3 minute preoxygenation trial and at the end of each trial ] [ Designated as safety issue: Yes ]
  

Traditional pre-oxygenation in obese patients has been proven to allow a time of apnea far reduced when compared with patients closer to their ideal body weight. This effect is attributable to several factors :

• Reduced FRC
• Increased closing capacity
• Diminished tidal volume due to increased resistance of the airway, diminished compliance of the thoracic cage and diminished strength and endurance of respiratory muscles.
• Increase in atelectasis formation upon induction of general anaesthesia.
• Increased O2 consumption due to the increased workload of respiratory muscles and to the general increase in metabolism.
• Increase in true shunt. Studies have demonstrated a reduction in atelectasis formation by application of CPAP during pre-oxygenation and PEEP during anaesthesia. Also, 25º inclination of the patient in the supine position, on the operating table, has demonstrated prolongation of time of apnea in a significant amount. Outside the operating room, PEEP of 5 cmH2O has been demonstrated useful in prolonging time of apnea when used during pre-oxygenation in hypoxic patients requiring tracheal intubation in the intensive care.

Yet, few studies have evaluated BiPAP in the operating room setting as to the effect on FeO2 and to the tolerance by the patients.

Thus, it is our intent to evaluate the efficiency of application of an inspiratory assistance pressure of 4cmH2O and PEEP 4cmH2O during a 3 minutes pre-oxygenation trial as to the level of FeO2 attained, the rate of attainment of FeO2 > 90% and the tolerance by the patient of the procedure, when compared with a 3 minute trial of the traditional method.

For this purpose, consenting, non pre-medicated obese patients scheduled for surgery in the operating ward of our institution will be brought earlier to the operating room, the day of their surgery. In an idle operating room, they will go through two trials of 100% O2 pre-oxygenation lasting 3 minutes each, at an interval of 20 minutes during which the patients will rest in a semi-sitting position, breathing spontaneously at ambient air. The order of the two trials will be randomly assigned between standard pre-oxygenation vs BiPAP pre-oxygenation. Both patients and anesthesia assistants providing the pre-oxygenation will be blinded as to the ongoing trial. Data from the anesthesia machine will be recorded visually to be analysed subsequently by a blinded third party. During each trial, the comfort of the patient on a local 4 point scale will be graded every minute.

At the end of the study period, patients will be returned to the waiting area of the operating ward before the scheduled time of their surgery. The protocol is intended to not interfere with the planned anesthesia technique or the surgery in any way.