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.
The use of high-flow nasal cannula (HFNC) systems for oxygen delivery in critical care has exploded over the past decade. This has occurred despite relatively little scientific knowledge into the mechanisms of action and clinical efficacy. This review will describe the principles of HFNC and briefly review the current state of the evidence supporting its clinical use.
What is High Flow?
HFNC (often referred to as high-flow) systems are broadly defined as systems that provide an oxygen-gas mixture at flows that meet or exceed a patient’s spontaneous inspiratory effort. Traditionally, oxygen delivered via nasal cannula has required low flow rates, thus limiting the amount of supplemental oxygen that can be delivered. In adults, flow rates > 6 L/min are generally not recommended. This is primarily due to limitations in the ability to humidify oxygen at higher flows, leading to the drying of mucous membranes and general patient discomfort. It is generally taught that low-flow nasal cannulas can increase the fraction of inspired oxygen (FIO2) from 0.21 (room air) to perhaps 0.30-0.45 at most. However, subsequent research has shown that these estimates are overly optimistic, extremely variable, and widely fluctuate with increasing inspiratory flow rates.1 High inspiratory flow rates decrease inspired FIO2 because more room air is entrained, diluting the supplemental oxygen being provided via the nasal cannula.
Traditionally, this problem has been addressed by appending oxygen reservoirs to nasal cannulas or providing oxygen via face mask. HFNC addresses this problem by providing much higher flows (up to 30, 40, or 60 L/min) in an attempt to match the patient’s inspiratory flow. Heating and humidifying the oxygen allows high flows to be tolerated and is addressed via specialized equipment capable of mixing oxygen with heated water vapor. HFNC systems are more expensive than low-flow systems with an approximate cost of about $2600.1
How Does it Work?
There are several proposed mechanisms to explain the therapeutic effect of HFNC.1,2 These include matching delivered oxygen flow rates to inspiratory flow, creating a reservoir of oxygen within the nasopharynx, and providing continuous positive airway pressure (CPAP).
HFNC minimizes entrainment of room air and dilution of delivered oxygen by providing oxygen at flows high enough to match or exceed a patient’s inspiratory drive. Two abstracts have supported this proposed mechanism.3,4 In normal subjects breathing through an airway model, increasing nasal cannula flow rates produced increasing tracheal FIO2 levels. Furthermore, a higher FIO2 could be achieved via HFNC than via a non-rebreather mask.3 Even at high minute ventilations, HFNC can provide quite high levels of inspired oxygen. In an airway model with a minute ventilation of 20 L/min, an FIO2 of 0.83 was achieved with a flow of 25 L/min.4 In normal volunteers, FIO2 has been shown to decrease by 20% when a large differential between the HFNC rate and the subject’s inspiratory flow rate develops.5 Therefore, for HFNC to be effective, clinicians need to use higher flow rates for patients with very high inspiratory drives.
HFNC systems have been postulated to improve oxygenation and decrease work of breathing by creating a reservoir of oxygen within the nasopharynx and by washing out nasopharyngeal dead space.1,2 This effect is similar to what is observed with tracheal gas insufflation. However, there is only indirect evidence to support this theory. Finally, HFNC has been shown to provide a modest degree of CPAP.6,7 This is theorized to improve oxygenation by increasing mean airway pressure and recruitment of alveoli. CPAP may also decrease work of breathing by counteracting intrinsic PEEP and decreasing total airway resistance as well.1,2 However, there is no direct evidence supporting these theories in adults. The effect of positive pressure is debated as pressures decrease when the mouth is open, and in one study, pressures decreased to 0 during inspiration.7 It is likely that providing oxygen flow rates that match or exceed a patient’s inspiratory flow and creating an oxygen reservoir account for much of the effect seen with HFNC systems.1,2
Do We Know if it Works?
Despite widespread and increasing use, the clinical use of HFNC systems has not been critically evaluated. It has been best studied in neonates with three randomized, controlled trials and several observational studies. However, a Cochrane review on HFNC for respiratory support in premature infants concluded there was insufficient evidence to determine safety and effectiveness.8 In children, limited data suggest that HFNC may decrease intubation rates, primarily in children being treated for bronchiolitis.9
There are very few data on the use of HFNC in adults. Most studies have used measures of comfort, ease of use, or short-term changes in oxygenation as primary outcome measures. There have been very few studies evaluating key clinical outcomes such as duration of support, treatment failures, or length of stay.
The performance and acceptance of HFNC among patients and staff in a postoperative surgical “high-dependence” unit was described in 2008.10 HFNC was rated 90% positive by patients and well accepted by staff but there was no comparison to other oxygen delivery devices. In 2010, a randomized, crossover trial compared 30 minutes of HFNC to high-flow face mask in 50 patients requiring supplemental oxygen following extubation.11 Both systems were matched for flow and FIO2. There were no significant differences in gas exchange or vital signs, although there was a trend toward improved patient comfort with HFNC. Also in 2010, Roca et al performed a prospective sequential comparison of face mask oxygen (flow = 15 L/min) and HFNC.12 Patients (n = 20) were included if pulse oximetry was less than 96% or higher than 50% FIO2. Patients were placed on face mask oxygen for 30 minutes followed by HFNC for 30 minutes. After each 30-minute period, they were asked about dyspnea, mouth dryness, and overall comfort. Dyspnea, mouth dryness, comfort, respiratory rate, and measures of oxygenation were all significantly improved on HFNC compared to face mask.
Sztrymf and colleagues have prospectively evaluated HFNC in both the ICU13,14 and emergency department.15 Patients who remained hypoxic after oxygen treatment via non-rebreather were placed on HFNC. Oxygenation and work of breathing were improved after 1 hour on HFNC. A lack of improvement in oxygenation or respiratory rate, or a persistence of thoracoabdominal asynchrony, were suggested as early signs of high-flow failure.13 It should be noted that key limitations of all of the studies described above have been the lack of control groups.
In 2011, Parke et al reported, to my knowledge, the only prospective, randomized trial of HFNC.16 Sixty patients admitted to a cardiothoracic or vascular ICU with mild-to-moderate hypoxemic respiratory failure were randomized to HFNC or face mask oxygen. The primary outcomes were treatment failure within 24 hours (progression to noninvasive ventilation) and patient comfort. Patients managed on HFNC were less likely to require noninvasive ventilation, although this difference was not statistically significant (3/29 vs 8/27, P = 0.10).
HFNC is relatively easy to use and appears to fill a niche between low-flow nasal cannula/face mask systems and noninvasive ventilation. HFNC has the advantage of allowing patients to talk, eat, drink, and more easily clear secretions compared to noninvasive ventilation. There are some key points to consider. Current evidence suggests that HFNC improves oxygenation, improves work of breathing, and may be more comfortable. I think most clinicians’ bedside experience would agree. However, HFNC has not been shown to improve ventilation; its use in patients with hypercarbia should be carefully considered. Noninvasive ventilation with CPAP is the standard of care in patients with heart failure and it is not known if HFNC is adequate for this condition.
Everyone on the medical team needs to have a good understanding of HFNC. Care providers with a limited understanding of this technology run the risk of assuming a patient must not be very sick since he or she is “only on nasal cannula.” In fact, the patient may be quite sick and on very high oxygen supplementation. Finally, both FIO2 and flow 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% FIO2 may seem to be improving until it is discovered this occurred in the setting of increasing flow rates from 30 to 60 L/min.
As ICU clinicians, we have a history of gravitating toward the newest and brightest — new and improved ventilator modes, fancy new ways to monitor hemodynamics, HFNC, etc. Not unlike the case with most other new equipment, the use of HFNC in the ICU has advanced much faster than the evidence demonstrating its effectiveness. That is not to say that HFNC does not have a place in critical care. However, as with any new technology, we should proceed with a degree of caution and skepticism. Without good quality data to show that HFNC is an improvement from previous oxygen delivery systems, we are practicing on anecdotes and faith. Furthermore, we do not yet have a good sense of the potential harms. For example, arguments are being made that HFNC may prevent or delay intubation in patients with hypoxic respiratory failure. This is a good thing if it leads to better patient outcomes; however, if it leads to delaying a necessary intubation too long, it may lead to patient harm. Thus far, data related to clinically significant outcomes such as length of stay, intubation rates, and mortality are lacking. Fortunately, studies addressing many of these questions are currently underway.2 In the meantime, 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.
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