High-flow nasal cannula has spread faster than the evidence for it in hypercapnia, and the reason is not clinical. It is comfortable, it needs no mask fitting, patients tolerate it, nurses like it, and a breathless patient visibly settles on it. All of that is true and none of it is a reason to use it in a patient with a respiratory acidosis.
The discipline is to get a blood gas before choosing the modality, not after. A patient who looks like type 1 failure and turns out to have a pH of 7.20 needs non-invasive ventilation, and every hour spent comfortable on high flow is an hour of unloaded work of breathing they are not getting.
The trap is the patient who improves symptomatically on high flow while their carbon dioxide climbs. Comfort is not a physiological endpoint. Repeat the gas within an hour of starting any non-invasive support and again after any change, and set in advance what number will trigger escalation — write it in the notes so the night team inherits a threshold rather than a judgement.
Where this bites hardest is in units where non-invasive ventilation is available but requires a different ward, a different nurse ratio, or a consultant decision at night. The delay is organisational, and the way to shorten it is to make the escalation criterion explicit at the point the high flow is started, not when the patient deteriorates.
One more: in a patient with COPD, do not treat a normal saturation on high flow as reassurance. Oxygenation and ventilation are different problems.
- Get an arterial or capillary blood gas before choosing between high flow and non-invasive ventilation
- A pH of 7.25 or below with hypercapnia calls for non-invasive ventilation, not more flow
- Repeat the gas within an hour of starting support and after every change in settings
- Write the escalation trigger in the notes at the point of starting, so the night team inherits a number
- Do not read a normal saturation on high flow as reassurance in COPD — oxygenation is not ventilation
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