Abstract
Background
H
Equipment
The HFNC oxygen system consists of an air–oxygen blender, flow meter, heated humidifier, heated circuit, and nasal prongs, all of which are configured to deliver up to 60 L/minute of humidified air flow. 1 The air–oxygen blender allows precise delivery of fraction of inspired oxygen (FiO2) ranging from 0.21 to 1.
Physiology and Rationale for HFNC
Traditional nasal cannula oxygen administration is used at flows of 2–6 L/minute. At these low flows, there is significant mixing from room air limiting the maximal FiO2 delivery to 0.4. Conversely, with the use of HFNC, there is little entrainment of room air because of the high gas flow, allowing for more precise and higher FiO2 delivery. In addition, the high gas flow flushes carbon dioxide from the upper airway, reducing the anatomical dead space and impeding expiratory flow. Thus, HFNC can create continuous positive airway pressure, although less than NIV. These factors lead to a reduction in respiratory rate, hypoxemia, and the sensation of dyspnea compared with traditional oxygen administration through nasal cannula or face mask.3,4 Whereas NIV can be claustrophobic and impede eating and talking, HFNC use can allow select patients to talk and eat while receiving adequate oxygenation.
Clinical Evidence
HFNC can be used for severe hypoxic respiratory failure of any cause, including interstitial lung disease, cancer, and pneumonia. In a randomized controlled trial of eucapnic patients with severe hypoxemic respiratory failure, HFNC was comparable with NIV in reducing need for invasive ventilation but was superior in relief of dyspnea and reduction in respiratory rate. 5 Skin breakdown is less common with HFNC compared with NIV.6,7 In a small phase II randomized study involving advanced cancer patients with hypoxia and dyspnea, HFNC used in a non-ICU setting was able to improve dyspnea, oxygen saturation, and respiratory rate comparably with NIV. 8 There are limited comparative studies with other modalities of higher flow oxygen delivery such as a venturi mask. Compared with a humidified face mask, HFNC is more expensive, but it appears to be more comfortable and associated with less mucosal dryness.1,2 The use of HFNC in hypercapnic respiratory failure is not well studied and not recommended.
Limitations and Dilemmas
To the best of our knowledge, HFNC is available only in hospitals because of the need for an advanced air compression system and careful monitoring. Just as with NIV, its use is not restricted to an ICU. There are no studies evaluating whether HFNC is beneficial in reducing the dose of opioids or anxiolytic medications used to treat dyspnea. Palliative care clinicians may encounter patients receiving HFNC while dying. Like any technology employed at the end of life, its use has to be carefully considered in the context of the patient's care goals: it may prolong the dying process and be a barrier to hospital discharge, even as it improves dyspnea. Anecdotally, this can lead to clinical dilemmas whereby a dying patient with a do-not-intubate order, who has severe hypoxic respiratory failure without any realistic hope of improvement (e.g., a patient with terminal interstitial lung disease who cannot receive lung transplantation), is treated with HFNC as a temporizing measure. The HFNC can provide sufficient oxygenation to keep the patient awake and alive for weeks, however, without hope of improvement or even hospital discharge. Patients may not be agreeable with transitioning off HFNC, leading to communication and ethical dilemmas around care goals and hospital resource utilization. In this respect, those considering its use should think of it as being more like invasive ventilation (a high-resource intervention that is difficult to provide outside the hospital) than typical nasal cannula oxygen therapy.
Using HFNC
HFNC is often started at 50–60 L/minute of flow and FiO2 is adjusted to keep the saturation of O2 > 90%. Once the desired effects are achieved, flows are maintained and FiO2 is decreased while assuring adequate oxygenation. For patients who are recovering, HFNC can typically be transitioned to regular nasal cannula at an FiO2 of 0.4. Like with any weaning of oxygenation or ventilation interventions in dying patients for whom alleviating suffering is the primary care goal (as opposed to restoration or life prolongation), the potential benefits and harms of HFNC will need to be compared with use of regular nasal cannula oxygen therapy and medications. Similar to discontinuing invasive or NIV, standard medications to relieve dyspnea should be introduced to keep patients comfortable as HFNC is being weaned (see Fast Fact #27), even if hypoxemia results.
Summary
• Humidified HFNC can provide adequate oxygenation and relieve dyspnea in carefully selected patients with hypoxemic respiratory failure.
• Data are scant on its use in patients nearing the end of life. Clinicians should be cautious about nonevidence-based use of HFNC in patients without a realistic path to recovery.