A fresh look at the physiologic effects of high-flow nasal cannulae and the role they play in patient care.

Treatment strategies to address oxygenation problems in critically ill patients with hypoxemic respiratory failure commonly fall into 3 categories: use of simple oxygen equipment, use of noninvasive ventilation, or use of invasive ventilation. These categories typically represent a stepwise approach to the management of patients with increased levels of oxygen demands. High-flow nasal cannula (HFNC), nasal high-flow oxygen therapy, and high-flow therapy are terms used to describe flows of 6 L/min delivered through nasal cannula. Most often these high flows are delivered through an air/oxygen blender and heated humidifier. What makes this therapy unique is that it not only provides oxygen therapy similar to other simple oxygen equipment, but it also provides a low level of CPAP. This CPAP effect was first noted by Locke et al in 1993,1 and since has been well documented by a number of authors.2-9 The greatest fear clinicians have regarding high-flow therapy is the lack of knowledge of how much pressure is delivered to the airways. Although concern has been raised by some authors6,7 regarding the CPAP pressure in premature infants, particularly those weighing 1,000 g, similar concerns have not been documented in the adult population. Current evidence has demonstrated that airway pressures do not exceed 4–5 cm H2O at flows as high as 40 L/min.3,4,8,9 My colleagues and I, in an unpublished study of healthy adult volunteers using a HFNC system, also found that roughly 1 cm H2O of CPAP was associated with each 10 L/min increment in flow. The HFNC can then be considered a crossover strategy between basic oxygen equipment and noninvasive CPAP. In addition, it provides expanded capability while potentially maintaining some of the advantages of basic oxygen equipment. These potential advantages include increased patient adherence, increased patient comfort, and relatively low cost, compared to traditional noninvasive CPAP devices, and might free up ICU beds for patients requiring more invasive interventions. Peters et al investigated the use of HFNC in patients with do-not-intubate status and publish their findings in this issue of RESPIRATORY CARE.10 This was a small clinical study that enrolled 50 consecutive patients (25 men and 25 women) who met the inclusion criteria (do-not-intubate, dyspnea, tachypnea, hypoxemia, PaCO2 65 mm Hg, and pH 7.28). All 50 patients were placed on HFNC; the treatment was successful in maintaining 82% (41) of these patients. The remaining 18% (9) of patients required escalation to noninvasive ventilation. The authors’ findings suggest that HFNC may serve as a viable alternative to use of traditional noninvasive CPAP devices in this patient population.