The purpose of the study is to compare effectiveness of different methods of achieving pre-oxygenation in bariatric patients. The investigators intend to compare facemask pre-oxygenation with a high-flow humidified nasal oxygen.

The primary outcome measure of interest is time to desaturation to 92% or 18 minutes of apnoea.

General anaesthesia involves giving patients anaesthetic and muscle relaxant medications to allow transition to sleep and airway relaxation to permit laryngoscopy, respectively. During laryngoscopy, a breathing tube is placed to 'secure' the airway. There is therefore a length of time from administration of these medications to when the breathing tube is placed, when the patient is not breathing and the airway is not secured. This time is known as apnoea time. In a patient who is not breathing, blood oxygen levels fall to dangerously-low levels, within 1-2 minutes. In order to prevent this, patients breathe 100% oxygen before induction of anaesthesia as part of a process known as pre-oxygenation. There are different ways of delivering pre-oxygenation, but the common aim is to increase the oxygen reservoir within the lungs to give the anaesthetist more time to place the breathing tube. With good pre-oxygenation, apnoea times of up to 7 minutes are possible.

Apnoea time is a potentially hazardous period during induction of anaesthesia and it is particularly so in patients with severe obesity. On the one hand, obesity makes the upper airway anatomy difficult, and on the other hand, lung collapse (atelectasis) caused by abdominal contents compressing the base of the lungs reduces the amount of oxygen available within the lungs. Apnoea time can be as short of 30 seconds. Identifying the most effective method of pre-oxygenating bariatric patients can therefore significantly improve the safety of delivering general anaesthesia to these patients.

Inclusion Criteria:

• Able to provide informed consent.
• Undergoing general anaesthesia .
• Between the ages of 18 and 80 years.
• Body mass index (BMI) greater than 40 Kgm-2.

Exclusion Criteria:

• Unable to give informed consent.
• Significant cardiac history which is defined as current angina, myocardial infarction <12 months before the date of the procedure.
• Significant peripheral vascular disease defined as claudication on minimal exertion or Ankle-Brachial index of <0.7, a stroke or a transient ischaemic attack <12 months before the procedure.
• Presence of significant heart-valve disease or congestive heart failure.
• Significant lower respiratory disease including brittle asthma; chronic obstructive pulmonary disease (GOLD stages 3 or 4).
• Strong predictors of both difficult intubation and difficult facemask ventilation on anaesthetic airway assessment.
• Patient with American Society of Anesthesiologists (ASA) grade of 4 for any reason.