Humidified High Flow Nasal Cannula
Mechanisms of Action of Humidified High Flow Nasal Cannula (HHFNC)
Humidified high flow nasal cannula oxygen is increasingly used in hypoxic respiratory failure. It is felt to be less invasive than intubation and more comfortable than non-invasive positive pressure ventilation (NIPPV) modes such as continuous or bi-level positive airway pressure. But how does HHFNC work? There are 5 chief mechanisms of action for HHFNC.
Most intuitively, HHFNC increases the fraction of inspired oxygen (FiO2). First, it delivers an oxygen rich gas, but because of the larger nasal prongs less room air is entrained. Because less room air is inhaled along with the oxygen rich gas, the effective FiO2 is increased.
The gas mixture from HHFNC systems arrives to the patient pre-humidified and heated. This is less irritating to the nasal mucosae and allows the patient to tolerate the higher flows.
The dead space through the respiratory tract is washed out with oxygen rich gas and reduces the degree of carbon dioxide rich gas that is re-inhaled with each breath. While HHFNC doesn’t change the patient’s tidal volume or necessarily their respiratory rate it increases their effective minute ventilation due to this washout.
At high flows (usually in excess of 40LPM) there is a modest degree of PEEP which is applied to the patient. This has not been studied in the critically ill, but in healthy volunteers the PEEP measured has been reported as approximately 5cmH2O at 40 LPM.
Due to a synthesis of the above mechanisms, there is an overall reduction in respiratory drive and work of breathing. Also, the increased flow reduces the degree of resistance from the nasal turbinates and offloads some of the work of breathing.
Be on the lookout for upcoming posts about non-invasive ventilation support!
Mauri T, Wang Y-M, Wang Y-M, et al. Nasal high flow: physiology, efficacy and safety in the acute care setting, a narrative review. Open Access Emerg Med. 2019;11:109-120. doi:10.2147/OAEM.S180197.
Helviz Y, Einav S. A Systematic Review of the High-flow Nasal Cannula for Adult Patients. Crit Care. 2018;22(1):71. doi:10.1186/s13054-018-1990-4.