High flow nasal cannula - Cedical
Introduction to high flow nasal cannula
The use of heated and humidified high flow nasal cannula (HFNC) has become increasingly popular in the treatment of patients with acute respiratory failure through all age groups. I first started using it as a pediatric intensive care fellow, but had little knowledge of how it actually worked. I noticed a few years after using it successfully in children, mainly with severe bronchiolitis, that we began to use it in the adult intensive care unit as well. It seems over the past several years many studies have come out reviewing the mechanisms of action as well as its use in a variety of conditions. In this part we will summarize how it works and for part 2 we will discuss the main indications for its use in adult and pediatric patients.
High flow nasal cannula - temperatures
Heat and humidified high flow nasal cannula or as most call it, Hi Flow Nasal Cannula (HFNC), isn’t just a standard nasal cannula cranked up to very high flow rates. It actually takes gas and can heat it to 37 o C with a 100% relative humidity and can deliver 0.21 – 1.00% fi02 at flow rates of up to 60 liters/min. The flow rate and fi02 can be independently titrated based on your patient’s flow and fi02 requirements.
High flow nasal cannula - How it works?
Heated & Humidified:
Heated and Humidified oxygen has a number of benefits compared to standard oxygen therapy. Standard oxygen therapy delivered through a nasal cannula or another device such as a non-rebreather, is cold (not warmed) and dry (not humidified). This can lead to airway inflammation, which can increase airway resistance and impair mucociliary function possibly leading to decrease secretion clearance (1). Also, a significant amount of calories can also be expended in individuals to both warm and humidify gas during normal breathing (2).
HFNC can warm (to 37oC) and humidify gas, which can decrease airway inflammation, maintain mucociliary function, improve mucous clearance and reduce the caloric expenditure in acute respiratory failure (1-2).
Inspiratory Demands:
One obvious benefit, but worth mentioning is that high flow can give you a very high flow of gas. This is important as patients in acute respiratory failure can be extremely tachypneic, and therefore their peak inspiratory flows, which may normally be 30L/min – 60L/min, can reach up to 120 L/min (3). So if you place your tachypneic patient with PIF rate of 120L/min and minute volume >20L/min on a 15L/min NRB mask, you may not be helping them as much as you think. I am going to get into this point a bit later on in this review when we discuss the concept of oxygen dilution.
Functional Residual Capacity:
You may have been looking for the bullet point that says that Hi Flow provides PEEP and you can’t find it? There has been some debate about truly how much PEEP a high flow device can deliver. HFNC has been shown to deliver up to 1 mm of Hg of PEEP for every 10L/min of flow delivered with closed mouth breathing. (4-5)
