For critically ill patients, is high-flow nasal cannula oxygen delivery a suitable alternative to mechanical ventilation?

The purpose of respiratory support is to maintain adequate ventilation and oxygenation. Consequently, ensuring adequate alveolar ventilation is essential for expelling carbon dioxide produced in the human body. Currently, during invasive or noninvasive ventilatory support, minute ventilation is manipulated to ensure adequate alveolar ventilation. For patients with COPD exacerbations, noninvasive ventilation (NIV) has become the preferred primary modality for respiratory support because it both enhances inspiratory tidal volume and maintains adequate alveolar ventilation. Because of poor mask tolerance, however, NIV is sometimes inapplicable. High-flow nasal cannula (HFNC) oxygen delivery has been gaining attention as an alternative means of respiratory support for critically ill patients. In the literature, this technique has also been called mini-CPAP, transnasal insufflation, nasal high flow, nasal high-flow ventilation, high-flow therapy, and high-flow nasal cannula oxygen therapy. Here, the term HFNC is used. The apparatus comprises an air-oxygen blender, an active heated humidifier, a single heated circuit, and a nasal cannula. The FIO2 is set in the air-oxygen blender from 0.21 to 1.0 with flows of up to 60 L/min. The gas is heated and humidified with the active humidifier and delivered through the heated circuit. Another big difference between NIV and HFNC is the interface. Although interfaces for NIV increase anatomical dead space, HFNC actually decreases dead space. Because neither inspiratory push nor expiratory pull is effective in such an open circuit, HFNC cannot actively enhance tidal volume. Even so, it helps patients with COPD mainly by decreasing anatomical dead space and improving alveolar ventilation. In clinical trials with subjects with COPD,1 response to HFNC has been varied. For some subjects, it has been shown to reduce breathing frequency and, in some cases, to decrease PaCO2. Tested using an unloaded bicycle ergometer, compared with spontaneous breathing,2 HFNC has also increased the exercise capacity of some subjects, who show improved oxygenation. HFNC has proved to be an innovative and unique therapy for some types of hypercapnic respiratory failure. Maintaining adequate oxygenation depends on properly managing FIO2 and PEEP. For hypoxemic patients, provision of supplemental oxygen has long been the frontline therapy. Oxygen is generally provided via a face mask or nasal cannula, with oxygen delivery limited to no more than 15 L/min. Using conventional methods, when there are large differences between patient inspiratory flow and delivered flow, FIO2 values are difficult to control and are usually lower than expected. However, HFNC literally delivers high flow, and actual FIO2 values are usually close to delivered FIO2. 3