High flow nasal cannula (HFNC) systems allow us to deliver warm, humidified oxygen at flow rates as high as 60 liters/min through small, pliable nasal prongs. The cannula itself is fit into the nares and stabilized with a head strap. Two parameters are set – the flow rate and FiO2. Due to the high flow rates, much of the anatomic dead space (especially in the upper airways) is “washed out” improving the efficiency of oxygenation/ventilation. With the mouth closed, HFNC at 60 L/m can generate ~7 cm H2O of positive end expiratory pressure (PEEP) which helps decrease the work of breathing and also improves oxygenation.
So when do we use HFNC? I think it’s better tolerated than oxygen masks (Venturi, nonrebreather, etc.) due to better comfort from the prongs, a reduction in the work of breathing, and a more reliable inspired oxygen content. We often also use HFNC to avoid intubation from impending hypoxemic respiratory failure; however, studies fail to show consistent data about HFNC’s impact on intubation rates and overall survival. I routinely use HFNC post-extubation to help deliver inhaled Flolan/Veletri in patients requiring extra cardiopulmonary support. Additionally, HFNC can provide a degree of “apneic oxygenation” when performing laryngoscopy in patients who may be difficult airways.