Pre-oxygenation With Facemask Oxygen vs High-flow Nasal Oxygen vs High-flow Nasal Oxygen Plus Mouthpiece Oxygen

"Induction of anaesthesia after 2mins 45 seconds of pre-oxygenation with 1-1.5mcg/kg remifentanil plus 2-3mg/kg propofol. Propofol (10mg/kg/hr) and remifentanil (0.15mcg/kg/min) infusions commenced until study conclusion.~Positive pressure ventilation commenced at Sp02 92%. Failure to intubate the patient during the first minute of apnoea results in withdrawal from the study.~Blood samples obtained from an arterial catheter immediately prior to commencing pre-oxygenation, after 90 and 180 seconds of pre-oxygenation, after one minute of apnoea, and every two minutes during the apnoeic period."