This project will investigate the duration of apnea without desaturation when using transnasal humidified rapid insufflation ventilatory exchange (THRIVE) vs. low flow nasal oxygen at anesthesia induction in obese patients.

Background Providing safe anesthesia for surgery in obese patients requires specific consideration of the pathophysiological changes and respiratory complications are frequent. Anesthesia induction needs special attention in this patient population. A decrease in functional residual capacity (FRC) after induction of anesthesia dramatically shortens the duration of apnea without desaturation (DAWD) during which the airway can be secured.

Several measures such as positioning in a semi sitting position, preoxygenation with PEEP and passive insufflation of oxygen during laryngoscopy have been shown to prolong the DAWD. None of these measures is able to produce positive endexpiratory pressure (PEEP) during laryngoscopy which would counteract the loss of FRC and therefore prolong the DAWD further.

Transnasal humidified rapid insufflation ventilator exchange (THRIVE, also knowns as "high flow nasal oxygen therapy" and "heated and humidified nasal oxygen") devices have been used for several years in intensive care and are available from numerous vendors. With THRIVE, heated and humidified oxygen is insufflated via nasal prongs at a rate up to 120 l/minute and it has been shown that this creates levels of PEEP, proportional to the amount of gas flow. Using THRIVE during the apneic phase of anesthesia induction in obese patients might be superior compared to low flow oxygen techniques as THRIVE generates PEEP during laryngoscopy, which might counteract the loss of FRC and consequently might prolong the DAWD.

Aim of the project This project will investigate the DAWD when using THRIVE vs. low flow nasal oxygen at anesthesia induction in obese patients.

Hypothesis The duration of apnea without desaturation is longer in the "THRIVE" group compared to the "low flow oxygen" group.

Methods The study will be conducted as a single center, prospective, randomized, controlled trial at the Medical University of Vienna, Austria.

Forty adult patients of the ASA classification 2 and 3 with a BMI >40 undergoing elective surgery under general anesthesia will be included. Patients with expected difficult airway, patients with an oxygen saturation <98% despite preoxygenation, patients with nasal obstruction, smokers (>10 cigarettes/day), patients with chronic respiratory disease, patients unable to give consent, and pregnant or breastfeeding patients will be excluded.

Patients will be randomly assigned to one of two groups ("low flow oxygen" or "THRIVE" group) and anesthesia will be induced in a standardized fashion in both groups. During laryngoscopy, patients in the low flow group will receive nasal oxygen insufflation at a rate of 10 l/min and patients in the THRIVE group will receive oxygen at a rate of 120 l/min via a THRIVE system. Difficult laryngoscopy will be simulated and intubation will be performed as soon as oxygen saturation drops to 95%. The time from anesthesia induction until oxygen saturation drops to 95% (DAWD) will be measured.

Inclusion Criteria:

• Adult patients
• American Society of Anesthesiologists physical status classification class 2-3
• BMI > 40
• elective surgery under general anesthesia

Exclusion Criteria:

• expected difficult airway (El Ganzouri Risk Index > 6)
• oxygen saturation of less than 98% despite adequate preoxygenation,
• nasal obstruction
• smoker, more than 10 cigarettes per day
• chronic respiratory disease
• FEV1 (forced expiratory volume in one second) < 70%
• elevated intracranial pressure
• unable to give informed consent
• Pregnant or breast feeding

not yet decided

The monitor measuring the main outcome parameter will be in a separate room