In this study the investigators will determine whether using high flow oxygen in moderate-to-severe Obstructive Sleep Apnea patients following a major non cardiac surgery is more efficacious than usual care in preventing decrease in oxygen level in blood.

The POSA-II trial is a culmination of the investigator's program of research to prevent postoperative adverse outcomes in surgical patients with untreated Obstructive Sleep Apnea (OSA). It is a multi center open label, randomized clinical trial of nasal high-flow vs. usual care in patients with untreated moderate-to-severe OSA undergoing major non cardiac surgery. It is a proof of concept trial to show that nasal high-flow reduces severe desaturation, the causal mechanism for OSA-related adverse events, and to collect key feasibility data for a large international multi center trial with enough power to detect effects on the clinical outcomes. Eligible patients undergoing major non cardiac surgery with high-risk OSA will have a home sleep study. One hundred and ninety patients with moderate-to-severe Sleep Apnea will be randomized into either nasal high-flow or usual care group. Sleep studies, oximetry, electrocardiogram (ECG), and Troponin will be done preoperatively. Postoperatively, ECG and Troponin will be determined daily for the 1st three days, and nocturnal oximetry for the 1st 3 nights. Patients will be followed during their hospitalization and for 30 days postoperatively to ascertain any adverse outcomes. A blinded clinical events committee will adjudicate all components of the composite outcome.

Inclusion Criteria:

• Adult, age ≥45 years, undergoing major elective non cardiac surgery under general and/or regional anesthesia with anticipated overnight stay or longer in hospital
• Patients with untreated moderate-to-severe Obstructive Sleep Apnea.
• STOP-Bang score 5 or higher

Exclusion Criteria:

• Predetermined need for postoperative CPAP therapy or ventilation,
• Cheyne-Stokes respiration or Central Apnea,
• Oxygen dependent due to moderate to severe chronic obstructive lung disease available Forced Expiratory Volume at 1 second of < 50% predicted) or advanced interstitial lung disease,
• Pre-existing chronic hypercapnia or obesity hypoventilation syndrome, Intracranial and otolaryngological procedures, and
• Unable to use nasal high-flow (nasal/oral malformations, pre-existing tracheostomy, planned postoperative nasal packing).