October 2, 2018 by dailybolusoflr
By: Casey Carr
Mechanism of nasal oxygen delivery
During simple nasal prong oxygen delivery, unhumified 100% FiO2 oxygen is delivered at a set flow rate – typically 2-6 l/min. Given the ill-fitting nature and low flow rate, there is a significant amount of room air entrainment – hence the actual FiO2 delivery is much lower than 100%. A simplified example would be if a patient is breathing at a flow rate of 30 l/min (a typical rate while at rest), and 5 l/min of oxygen is applied, the patient is now breathing 25 l/min of room air and 5 l/min of 100% oxygen. High rate of oxygen delivery via simple nasal cannula is not ideal or possible for a variety of reasons – patient discomfort, lack of fitting, and inability of the system of tolerate high rates of flow
What is HFNC?
The high flow nasal cannula (HFNC) system consists of an air-oxygen blender, flow meter, heated humidifier, and large nasal prongs. This configuration allows a much higher flow rate and therefore much less air entrainment, which allows a specific titration of FiO2. This means that the clinician will both set a flow rate and FiO2. The high flow rate also impends expiratory flow, which can create positive end expiratory pressure (PEEP). As a general rule, every 10 l/min creates 1 cm H2o of pressure (50 l/min would create 5 cm H20 of PEEP). However this effect is lost when the patient breathes with their mouth open.
Initial settings for HFNC
Flow rates range from 30 l/min to 100 l/min. FiO2 ranges from 30% – 100%. An appropriate starting flow rate in a patient with respiratory distress would be 50 l/min, with the FiO2 titrated to the degree of on-going hypoxemia.
Who should receive HFNC?
Obvious candidates are patients with severe hypoxemia without hypercapnia that is not improved with conventional supplemental oxygen. In 2015, Frat and colleagues compared HFNC to standard oxygen therapy and non-invasive positive pressure ventilation (NIPPV) for acute, non-hypercapneic, hypoxemic respiratory failure. HFNC had lower rates of intubation (though not statistically significant) and better mortality outcomes (statistically significant) at 28 days.
Levy SD, Alladina JW, Hibbert KA, Harris RS, Bajwa EK, Hess DR. High-flow oxygen therapy and other inhaled therapies in intensive care units. Lancet (London, England). 2016; 387(10030):1867-78.
Frat JP, Thille AW, Mercat A, et al. High-flow oxygen through nasal cannula in acute hypoxemic respiratory failure. The New England journal of medicine. 2015; 372(23):2185-96.