Patients with severe hypoxemic respiratory failure due to Covid-19 are often treated with oxygen delivered through a high-flow nasal cannula (HFNC). This is according to guidelines. We have made the clinical observation that oxygenation sometimes improves when a mask (e.g. oxygen mask or inhalation mask) is applied on top of the HFNC. This has quickly become a clinical routine at intermediary care units at our hospital, where patients with HFNC are offered to test the intervention (mask + HFNC) as part of clinical routine.

This study aims to evaluate this new routine in a standardised way.

Patients with severe covid-19, treated with HFNC, will be approached for consent. If inclusion critera and no exclusion criteria are fulfilled, baseline measurements will be taken without mask, including arterial blood gas analysis (ABG)

Then a simple oxygen mask will be applied covering nose and mouth, without providing supplemental oxygen. After 30 minutes with mask + HFNC, new measurements will be taken, including ABG.

After 30 minutes, the mask is removed and when steady state occurs, new measurements will be taken, without ABG. Then, the patients could continue using the mask as part of clinical routine

Inclusion Criteria:

• Verified Covid19 infection
• treatment with HFNC
• Estimated P/F ratio of ≤ 13 KPa

Exclusion Criteria:

• language barrier
• unable to provide consent
• risk of deterioration due to study