Assessment of Oxygenation Values Before General Anesthesia in Elective Surgery

Background of the study:

Preoxygenation is a widely used technique that improves the safety of endotracheal intubation. The procedure is carried out by supplying 100% oxygen (FiO2 of 1.0) before the induction of general anesthesia until both end-tidal oxygen (EtO2)>90% and end-tidal N2 (EtN2)<5% are reached. Both these markers define the efficacy of the procedure. As a result, the lung oxygen content is increased far beyond normal oxygen consumption by saturating the functional residual capacity with 100% oxygen. This allows for a longer safe apnea time (i.e. the time required for oxyhemoglobin saturation to drop below 90%). The rate at which oxyhemoglobin saturation drops during apnea indicates the efficiency of the maneuver.

This procedure is strongly recommended for all patients undergoing general anesthesia since it lengthens safe laryngoscopy time and grants a wider timeframe to respond to a "cannot intubate/cannot oxygenate" (CICO) scenario, a rare yet life threatening situation. It remains unclear whether this should be considered mandatory for non-critically ill and non-obese patients since their oxygen reserves should suffice for the time required to perform endotracheal intubation or regain spontaneous breathing in the event of a CICO scenario. Nonetheless, the guidelines for the management of endotracheal intubation, proposed by the Difficult Airway Society in 2015 United Kingdom state how it is pivotal to preoxygenate every patient before attempting to intubate. Several methods of preoxygenation have been validated and compared according to duration of safe apnea time, duration of the procedure, success rate (defined as "avoiding manual re-ventilation"), and patient tolerance. The choice between these techniques is based on patient characteristics (age, sex, Body Mass Index, American Society of Anesthesiologist score, Cormack-Lehane grade and Glasgow Coma Scale), settings (e.g., operating room, Intensive Care Unit, emergency situations), equipment, and anesthesiologist's preferences. The two standard approaches are six deep breaths in 1 min and tidal volume breathing for three to 5 min, both at 100% inspired oxygen via a face mask.

The main side effect of preoxygenation is absorption atelectasis that occurs when delivering 100% inspired oxygen. This can be avoided using a lower inspired oxygen concentration (90%), positive pressure techniques, and/or recruitment maneuvers post-endotracheal intubation. Due to the short duration of the procedure, the production of reactive oxygen species and cardiovascular responses are minimal and should not prevent routine preoxygenation.