This Randomized Control Trial will directly compare helmet non-invasive ventilation (NIV) combined with high flow nasal oxygen (HFNO) versus HFNO alone in patients with Acute Hypoxemic Respiratory Failure (AHRF).

This is a pilot multicentre, concealed, stratified, permuted block randomized controlled trial enrolling patients with AHRF who are admitted to the intensive care unit. Patients with imminent need for intubation, or a contraindication to study interventions will be excluded. Patients will be randomized to receive either (1) sessions of at least 12 hours per day of helmet NIV oxygen interspersed with HFNO or (2) HFNO alone over at least 2 calendar days. Participants will be followed to hospital discharge or 60 days, with an additional quality of life assessment via telephone interview 6 months after enrolment.

Inclusion Criteria:

1. Intensive care unit admission (orders written)

2. Age ≥18 years

3. Hypoxemic acute respiratory failure for at least 1 hour and still present at time of screening for eligibility (symptom onset during last 14 days) with the following:

   1. Respiratory rate >21bpm or clinical evidence of increased work of breathing and
   2. Documented Hypoxemia defined as any one of:

   i. PaO2:FiO2 < 300 ii. If no arterial blood gas available, then SpO2:FiO2 < 315 iii. Oxygen saturation <98% on FiO2 >= 0.4 or higher by Venturi mask, TAVISH mask 10 Litres / minute, or any non-invasive oxygenation strategy

4. Not already intubated or with tracheostomy

Exclusion Criteria:

1. Already on HFNO or other non-invasive ventilation strategy at FiO2>=0.4 for the last 24 hours in the ICU.
2. Immediate need for intubation (e.g., inadequate airway protection or refractory hypoxemia or cardiopulmonary arrest, massive hemoptysis, etc)
3. Extubated in the ICU within past 72 hours
4. Clinician deems that face mask NIV is indicated to treat a primary diagnosis of hypercapnic respiratory failure or acute congestive heart failure
5. Known neuromuscular disease
6. Patients being transitioned to Palliative care or unlikely to survive more than 24 hours
7. ICU discharge is planned or anticipated on the day of screening
8. Previously enrolled in this trial
9. Trauma patients who remain in a cervical spine collar at the time of recruitment or who have upper limb/torso fractures that would preclude them from safe application of the helmet
10. Clinician decision that positive pressure to face and/or nasal pharynx is a contraindication (eg. Transphenoidal surgery, basal skull fracture etc.)