High-flow nasal cannula oxygen therapy during hypoxemic respiratory failure.

Oxygen therapy remains the first line intervention in acute hypoxemic respiratory failure. Several medical devices for oxygen therapy, which range from simple nasal cannula to non-rebreathing face masks, have been used in the management of acute hypoxemic respiratory failure.1 The choice of a specific oxygen delivery device is based on the patient’s oxygen requirements in terms of flow and desired oxygen concentration, as well as the type of device and its acceptance by the patient.1 The major challenge for clinicians at the bedside is to be able to provide the adequate, accurate, and stable oxygen requirements to the patients that will alleviate hypoxemia. The traditional oxygen therapy devices are constrained by flow limitation, with flows 15 L/min, by sub-optimal-humidity, by poor tolerance, and by inaccurate and inconsistent FIO2. 2 In patients with hypoxemic respiratory failure the patient’s inspiratory flow requirements are usually high and very often exceed the oxygen flow delivered by the traditional oxygen devices. This will lead to oxygen dilution and hamper the clinician’s capability for delivering consistent and accurate oxygen concentrations for patients in hypoxemic respiratory failure.2 High-flow nasal cannula (HFNC) oxygen therapy represents a new alternative to conventional oxygen therapy. In contrast to the traditional schemes for oxygen therapy, HFNC generates flows up to 60 L/min, yet using a nasal cannula as an interface to the patient. These high flows necessitate the optimal conditioning of the breathing gas in terms of humidification and heating to improve patient comfort and the patient’s adherence to the therapy. An active form of humidification is generally used during HFNC to condition the high flow gas to optimal heat and humidity (37°C and 44 mg H2O/L). Also, an incorporated air-oxygen blender allows the delivery of consistent and accurate oxygen concentrations in the range of 21% to 100% to ensure efficient initial management of hypoxemia in patients with hypoxemic respiratory failure.3 There are several key therapeutic advantages of HFNC that combine to provide comfortable and efficient respiratory support to patients with hypoxemic respiratory failure. First, HFNC oxygen therapy can meet or exceed the patient’s inspiratory flow demand and thus minimize or prevent air dilution, even when the patient is breathing orally.4 The accurate and consistent delivery of required oxygen concentrations during HFNC eliminates the need to switch among oxygen therapy delivery systems as patients wean off oxygen requirements or their condition becomes more acute. Second, HFNC while delivering high gas flows directly into the nasopharynx can induce a flushing of CO2 effect in the pharynx. As such, it will create a reservoir of fresh gas that will minimize CO2 rebreathing, reduce dead space, and increase the alveolar ventilation over minute ventilation ratio.5 Furthermore, the generated high flows that match or exceed the patients’ peak inspiratory flow demands are thought to decrease nasopharyngeal resistance, thereby decreasing resistive work of breathing.5 Third, recent data suggest that certain levels of positive airway pressure are generated during HFNC therapy (mean airway pressure in the range of 2.7–7.4 cm H2O). The degree of pressure generated is dependent on the flow, geometry of the upper airway, breathing through the nose or mouth, and the size of the cannula relative to the nares.6 Fourth, a recent study by Corley et al confirms that HFNC oxygen therapy generates substantial increases in end-expiratory lung volumes and tidal volumes, particularly in patients with higher body mass index.8 Finally, by conditioning the high flows delivered to patients with optimal active humidity, HFNC oxygen therapy emulates the balance of temperature and humidity that reduces airway dryness and maintains the function of the mucociliary transport system.9 Recent studies suggested that the remarkable tolerance of HFNC is attributable in part to the optimal heat and humidity provided during HFNC oxygen to patients with acute hypoxemic respiratory failure.10-12