Humidified nasal high-flow oxygen. I haven’t seen this system in any of the places I’ve worked in Scandinavia, but it seems widespread and in common use in Australia. It is used as an intermediate between a non-rebreather mask and a full CPAP system. It delivers very high flow oxygen through the nose without damaging the mucosa as the oxygen is heated and humidified. Not only does it deliver high concentration oxygen, but the high flow also generates PEEP. And all this while being less claustrophobic and more comfortable than a CPAP mask.
What is HFNC?
As many other medical inventions, it has come from the neonatal and paediatric area of medicine. Everyone’s seen small people with nasal CPAP. The small people also have the high flow nasal system. And this is the version for big people. This system is not to be confused with 15 L/min high flow “dry” oxygen on standard nasal cannulas for pre-oxygenation for anaesthesia.
Humidified Nasal High-Flow Oxygen is also referred to as high flow nasal cannula (HFNC) or heated, humidified, high-flow, nasal cannula (HHHFNC). This is a longer term therapy that requires warmed, humidified oxygen, And when they say high flow, they mean high flow – up to 40-50-60 L/min(!). This high flow permits constant oxygen delivery even with high inspiratory flows from intense respiration efforts by the patient.
How it helps the patient
Here’s a paper that evaluates humidified nasal high-flow oxygen by measuring pharyngeal FO2, FCO2 and airway pressure during different flow rates, and with the subjects breathing just through their nose and then with their mouth open. They also test the HFNC during forced respiration induced by exercise, to simulate the effects acute respiratory failure might have on the system.
Their findings are that the FiO2 set by operator is very close to what gets delivered in the pharyngeal space, and thus probably to the patients lungs. Depending on air flow rates, there can be a significant PEEP effect when breathing with the mouth closed, up to 7 cmH2O. With the mouth open, there’s still a small PEEP effect, but hardly significant. Around 2 cmH2O at high flows. The humidified air in itself also seems beneficial to the patients respiration.
So the patient can control the PEEP effect by closing or opening his mouth, making it easier to get a short break from the “CPAP” treatment if it’s starting to feel claustrophobic. Also, the nasal cannula system is not in the way when talking, drinking or feeding, making the patient’s daily life a lot easier.
Is there any evidence out there?
Not much. Some small studies have been made. More studies are on their way.
So very few studies on adults, some more on neonates, but still small sample sizes. They do show a positive trend for HFNC in various settings. In addition to being used for primary respiratory support, it also looks promising as a bridge after being extubated, resulting in better clinical- and lab parameters and avoiding re-intubation. This would probably also hold true for adults.
Here’s a small pilot study on adults that finds a favourable outcome for HFNC treatment for acute respiratory failure. They have some interesting observations that are worth reading.
Here’s an attempt to do a meta-analysis for the current evidence for adult HFNC treatment. To summarize: There isn’t much. Most research has been done on very small patient populations and some are just small poster presentations. More to come… But the current pool of experience is encouraging.
An introduction to HFNC
I also found an introduction to HFNC treatment. It’s a very good read (and a few inaccuracies in the some medical details doesn’t take away from the overall principles on HFNC treatment) with three typical case reports and great primer for understanding the basics of HFNC and its place in the treatment of respiratory failure:
All in all, Humidified High Flow Nasal Cannula treatment seems like a very good alternative to CPAP masks for many patients. I’ve certainly seen enough patients with low compliance for a tight fitting CPAP mask to want to check out this HFNC thingie.
Here’s some of the neonate studies on HFNC for those interested:
Comparing two methods of delivering high-flow gas therapy by nasal cannula following endotracheal extubation: a prospective, randomized, masked, crossover trial, J Perinatol, 2006