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High-Flow Nasal Cannula Oxygen Therapy in Acute Respiratory Failure: What Do We Know?
By Eric C. Walter, MD, MSc
Pulmonary and Critical Care Medicine, Northwest Permanente and Kaiser Sunnyside Medical Center, Portland
Dr. Walter reports no financial relationships relevant to this field of study. This article originally appeared in the July 2012 issue of Critical Care Alert. It was edited by David J. Pierson, MD, and peer reviewed by William Thompson, MD. Dr. Pierson is Professor Emeritus, Pulmonary and Critical Care Medicine, University of Washington, Seattle, and Dr. Thompson is Associate Professor of Medicine, University of Washington, Seattle. Drs. Pierson and Thompson report no financial relationships relevant to this field of study.
Synopsis: Limited data on the use of high-flow nasal cannula oxygen therapy suggests it is associated with improvement in physiologic parameters.
Source: Sztrymf B, et al. Impact of high-flow nasal cannula oxygen therapy on intensive care unit patients with acute respiratory failure: A prospective observational study. J Crit Care 2012;27:324.e9-324.e13.
High-flow nasal cannula oxygen therapy (HFNC) is a generic name for oxygen delivery devices that provide oxygen nasally at higher flow rates than conventional nasal cannulae. Conventional ventilation usually can only provide flow rates up to 15 L/min, while HFNC can deliver flow rates up to 60 L/min. Both flow rate and fraction of inspired oxygen (FiO2) are titratable. HFNC has been successfully used in the neonatal intensive care unit (ICU) for neonates with acute respiratory failure (ARF). Although there are substantially fewer published studies outside the neonatal ICU, it is now being used with increasing frequency in adults with ARF.
This small, prospective study reported outcomes from 20 patients with ARF admitted to the ICU and placed on HFNC. ARF was diagnosed as pulse oximetry < 96% and/or a respiratory rate ≥ 25 while receiving oxygen through a facemask at an estimated FiO2 > 50%. The median age of enrolled patients was 59 years and the median SAPS2 score was 33 (estimating a 16% risk of hospital mortality). Pneumonia was the most common diagnosis (11/20). Following initiation of HFNC, median respiratory rate decreased from 28 to 24.5, with a significant increase in oxygen saturation (93.5% vs 98.5%). Both results were statistically significant. PaO2 increased from 65.5 to 114.5 mmHg with no significant change in pH. Six patients were ultimately intubated and two patients died.
This small study is one of a handful of studies beginning to critically evaluate HFNC. HFNC may represent another case of the adoption of technology in advance of the data. Small studies similar to that of Sztrymf et al have been published showing similar improvements in physiologic parameters and/or patient comfort. While many changes are statistically significant, one can argue the clinical significance of a respiratory rate improving from 28 to 24.5. Many of these studies lack a control arm so we do not know how patients would have done had HFNC not been started. However, taken together, the direction of clinical changes in this study, and others, does seem to suggest that HFNC can provide improvements in oxygenation and respiratory effort for patients.
There are a variety of mechanisms proposed to explain how HFNC may help to improve ARF. Patients with ARF often have very high inspiratory flow rates. Conventional oxygen delivery devices often only provide oxygen at flow rates up to 15 L/min, and thus a significant amount of room air may be entrained. HFNC is hypothesized to better match oxygen delivery with inspiratory flows. HFNC has been shown to provide low-level positive airway pressure, theoretically providing some alveolar recruitment as well. Furthermore, HFNC may wash out carbon dioxide from the nasopharynx, decreasing the rebreathing of carbon dioxide and increasing the amount of oxygen that reaches the lungs.
HFNC is relatively easy to use but there are some important considerations. Both FiO2 and flow rate are titratable. It is important that everyone on the medical team is aware of what changes are being made. For example, a patient who has gone from 70% to 50% oxygen may seem to be improving until it is discovered this occurred in the setting of increasing flow rates from 30 to 60 L/min. Arguments are being made that HFNC may prevent or delay intubation in patients with hypoxic ARF. It remains to be seen if this is a good or bad thing. Few would argue that preventing intubations is a bad thing but delaying a necessary intubation too long may actually be harmful.
In summary, HFNC appears to be here for good. It is relatively easy to use and early data suggest it improves physiologic parameters in patients with ARF. Data related to more clinically significant outcomes, such as length of stay, intubation rates, and mortality, are currently lacking. We should not let our excitement for new technology cause us to forget that only after critical evaluations can we be sure we are helping, and not hurting, our patients.