High flow, humidified, temperature-regulated nasal insufflation is not a new concept. It is used widely in the treatment of hypoxaemic respiratory failure in critically ill patients, as an alternative to non-invasive positive ventilation via face or nasal mask. Recently, its use has been extended to the perioperative setting for pre-oxygenation and prolonging apnoeic time to desaturation. Both CPAP and high flow nasal cannula insufflation devices are licensed for use clinically in Singapore. We conduct this pilot study because its perioperative use in our adult OSA population is currently not well-defined.

Continuous positive airway pressure (CPAP) therapy is the only treatment for obstructive sleep apnoea (OSA) shown to reduce its long-term adverse effects on the cardiovascular system. However, due to difficult patient-mask interface and discomfort, compliance rate amongst patients with OSA is low, and reported to be only between 20-50%. High flow, humidified, temperature-regulated nasal cannula insufflation (of air or oxygen) is a well-documented and safe treatment in intensive care, paediatrics and respiratory medicine for patients with hypoxaemic respiratory failure. Its use in adult patients with obstructive sleep apnoea, especially in the perioperative setting is not well studied. Our hospital has a ''fast-track'' protocol for OSA patients who require surgery. They are started on CPAP therapy 2 to 5 days prior to surgery and in the immediate post operative period (ie. perioperatively). If these patients are non-compliant to the CPAP mask device, high flow nasal insufflation is a possible alternative treatment. We aim to randomise these patients into 4 groups, receiving conventional CPAP face mask or high flow nasal cannula at 3 flow rates, and study them with respect to incidence and number of documented desaturations. In addition, patient satisfaction, compliance to device and any adverse events will be monitored.

Inclusion Criteria:

• Patients aged 21-85 years old presenting for elective surgery.
• Suspected/possible OSA patients who are enrolled in the "fast-track OSA protocol" in the pre-assessment clinic
• Patients started on CPAP therapy at least one day pre-operatively
• Patients who are admitted to the High Dependency unit for postoperative monitoring.

Exclusion Criteria:

• Patients with nasal obstruction.
• Patients undergoing nasal or facial surgery, or surgery to treat OSA (eg. uvulopalatopharyngoplasty).
• Patients who already require a higher oxygen concentration (FiO2 > 0.21) preoperatively.