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REVIEW ARTICLE |
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Year : 2020 | Volume
: 3
| Issue : 2 | Page : 66-75 |
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High-flow nasal cannula: A narrative review of current uses and evidence
Wan Jane Liew, Prit Anand Singh
Department of Anaesthesia and Surgical Intensive Care, Changi General Hospital, Singapore
Date of Submission | 26-May-2020 |
Date of Acceptance | 22-Jul-2020 |
Date of Web Publication | 30-Aug-2020 |
Correspondence Address: Dr. Prit Anand Singh Changi General Hospital, 2 Simei Street, 529889 Singapore
 Source of Support: None, Conflict of Interest: None
DOI: 10.4103/ARWY.ARWY_21_20
High-flow nasal cannula (HFNC) is a relatively new mode of oxygen supplementation. Warmed and humidified air/oxygen mixtures are delivered at high flows via a nasal cannula to patients. There are many observed physiological benefits for the use of HFNC. There is, therefore, increasing interest surrounding the use of HFNC in adult patients for improving oxygenation. This highly versatile device has been studied expansively in different clinical scenarios, from critical care, to the operating theatre, and even in palliative care. The usefulness of HFNC in management of the global pandemic of coronavirus disease 2019 further attests to its potential. However, evidence surrounding HFNC is still largely inconclusive. More high-quality randomised studies should be conducted to evaluate and justify the routine use of HFNC. Research efforts focused on developing clinical strategies on initiation, monitoring, escalation, de-escalation and titration will contribute to developing more precise guidelines for HFNC therapy.
Keywords: COVID-19, critical care, high-flow nasal cannula, high-flow nasal oxygen
How to cite this article: Liew WJ, Singh PA. High-flow nasal cannula: A narrative review of current uses and evidence. Airway 2020;3:66-75 |
Introduction | |  |
Conventional oxygen therapy (COT) devices commonly used in the clinical setting include fixed and variable flow devices such as nasal prongs, venturi masks and face masks. Other methods of delivering oxygen without mechanical ventilation include noninvasive ventilation (NIV) devices such as continuous positive airway pressure (CPAP) and bilevel positive airway pressure. However, COT and NIV can result in patient discomfort, interference of activities such as talking and eating, and pressure injuries of the skin.[1],[2],[3]
High-flow nasal cannula (HFNC) has been developed over the last 2 decades as a novel device for delivering oxygen at high flows. HFNC delivers heated humidified gas up to 37°C at high flows through nasal cannulae using an air/oxygen blender. Most commercial devices deliver air/oxygen mixtures up to a flow of 60 l/min at an adjustable fraction of inspired oxygen (FIO2) of 0.21 (room air) to 1.0 (100% oxygen).
HFNC was investigated in the paediatric population for treatment of respiratory distress.[4],[5],[6] In the last decade, there has been growing interest around the use of HFNC in adult patients. Increasingly, HFNC is utilised as an alternative mode of oxygen supplementation in different clinical scenarios. This review narrates the physiological benefits of HFNC, along with its potential applications and clinical evidence surrounding it. In light of the coronavirus disease 2019 (COVID-19) global pandemic, a brief discussion surrounding the use of HFNC in managing patients with COVID-19 has also been included.
Benefits of High-Flow Nasal Cannula | |  |
Improving mucociliary clearance and maintenance of immune function of mucosal surfaces
Cold air induces bronchoconstriction, dries up and causes desiccation of airway mucosal surfaces and impairs mucociliary clearance.[7] A major function of the nasopharynx is the humidification of inspired gas by warming it up and saturating it with water vapour. HFNC is unique compared to other devices as gas is humidified before it is delivered to the patient, replicating the normal physiology of breathing. Humidification reduces airway resistance, improves mucociliary clearance and protects the airway mucosa. Respiratory secretions loosen up and are easier to expel, reducing the incidence of mucus plugging and atelectasis.[8] There is also reduced inflammation of the airways, therefore protecting the integrity of the airway mucosa.[9]
Generation of positive end expiratory pressure and increasing end expiratory lung volume
Parke et al. demonstrated that HFNC generates positive pressure in the nasopharynx throughout the respiratory cycle, more so in the expiratory phase, hence generating a low-level positive end expiratory pressure (PEEP).[10],[11]
Similarly, Riera et al. found that HFNC increases end expiratory lung volume (EELV).[12] Increased EELV translates to alveolar recruitment and prevents atelectasis. It is believed that high flows delivered by HFNC create resistance to expiration, preventing alveolar collapse and recruitment of atelectatic areas.[11] These beneficial respiratory changes have also been observed in patients with respiratory failure treated with HFNC.[10]
Delivery of high flows and fixed FIO2
COT devices deliver oxygen flows of up to 15 l/min. However, the required inspiratory flow for patients in respiratory distress can significantly exceed 15 l/min. As a result, the entrainment of room air that occurs dilutes the delivered gas mixture, resulting in an FIO2 less than expected. In comparison, HFNC devices can deliver gas flows up to 60 l/min. Higher flows allow better matching of flow demand required by the patient and achieve delivery of a more accurate FIO2 by reducing the entrainment of room air.[7],[8],[13]
Washout of anatomical dead space
Anatomical dead space that does not participate in gas exchange is washed out by the constant high flow of oxygen via HFNC. This reduces the rebreathing of expired carbon dioxide. The rate of washout is linearly related to the flow rate delivered.[14],[15]
Reduced work of breathing
HFNC reduces the metabolic cost required to warm and humidify inspired gas.[8],[16] HFNC use is also associated with a lower respiratory rate.[12],[17],[18] Furthermore, HFNC was reported to improve thoracoabdominal synchrony.[18] Together, these factors contribute to reduced work of breathing.
Comfort and tolerance
Studies have described better tolerance with HFNC.[19] Since HFNC is applied only to the nostrils, patients can continue to eat and speak normally.[20] Nursing activities such as oral suctioning and serving medication are possible without the need to interrupt the process of oxygenation.
Nursing and monitoring
HFNC is easy to apply, use and easier to titrate compared to NIV and mechanical ventilation. As monitoring is less intensive, it frees up nursing resources to focus on other aspects of patient care. The simplicity of the system also means that non-clinicians can be empowered to initiate and titrate HFNC.
Clinical Scenarios for Using High-Flow Nasal Cannula: Operating Theatre | |  |
Preoxygenation, apnoeic oxygenation, difficult airway management
HFNC confers a few advantages over standard preoxygenation for intubation in an operating theatre setting. Firstly, patients tolerate HFNC better compared to tight face masks. Secondly, HFNC delivers gas at high flows of up to 60 l/min which helps denitrogenate the lungs faster than conventional flow rates of 10–15 l/min.[15] Finally, nasal cannulae can be kept in situ during laryngoscopy, enabling apnoeic oxygenation.
Apnoeic oxygenation describes the process whereby high flow rates allow continuous delivery of oxygen to the alveoli and participate in gaseous exchange despite the absence of active breathing. Patel and Nouraei have shown that HFNC increased apnoea time, therefore buying time to secure a definitive airway.[21] Ramachandran et al. also found that HFNC prolongs time before desaturation and reduces the incidence of significant life-threatening desaturation.[22] HFNC was also non-inferior to COT for preoxygenation in rapid sequence induction, with a lower incidence of significant desaturation.[23]
HFNC has been explored as an adjunct for difficult airway management. It has been described to optimise oxygenation in patients planned for awake fibreoptic intubation.[24] HFNC also reduced significant desaturation in obese patients during induction of anaesthesia.[22] Trials are being conducted to evaluate HFNC against standard preoxygenation in anticipated difficult airways.[25] However, HFNC should not be the sole device used for the management of a difficult airway as apnoea after induction can result in a slow desaturation.[26]
Transnasal humidified rapid-insufflation ventilatory exchange
Transnasal humidified rapid-insufflation ventilatory exchange (THRIVE) is a technique where HFNC is used to facilitate apnoeic oxygenation for patients undergoing laryngeal surgery. Booth et al. have described using HFNC for anaesthetised spontaneously breathing patients with previous difficult intubations. These patients tolerated microlaryngeal surgery with minimal desaturation.[27] THRIVE was also described in studies involving short laryngeal procedures, achieving an apnoea time of 17–22 min and eliminating the need for intubation.[28],[29]
Clinical Scenarios for Using High-Flow Nasal Cannula: Intensive Care Unit and Critical Care | |  |
Preoxygenation for emergency intubation
Emergency intubation is a potentially life-threatening procedure with severe morbidity and mortality.[30],[31] As such, it is crucial to optimise preoxygenation before intubating a patient in severe respiratory distress. HFNC is easier to apply and more comfortable compared to COT, which improves patient compliance during preoxygenation. Evidence has shown that HFNC is as effective as the standard face mask or bag-valve-mask for preoxygenation before emergency intubation, with no increased propensity for significant life-threatening desaturations.[32],[33] Conversely, other trials found that the use of HFNC before intubation of intensive care unit (ICU) patients did not improve the lowest recorded saturation levels and did not alter patient outcomes.[34],[35] There is room for further assessment of the use of HFNC in preoxgenation for emergent intubation.
Oxygenation after major surgery
HFNC has been studied for improving oxygenation in patients after major surgery. HFNC was beneficial in postcardiac surgical patients by improving respiratory indices and is not inferior to NIV.[36],[37] In another study, HFNC was incorporated in an enhanced recovery protocol for elective lung resection. There was no improvement in postoperative functional recovery, but there was a significant reduction in hospital length of stay.[38] Despite the benefits of HFNC in patients following cardiac and lung surgery, it does not benefit patients following abdominal surgery. The OPERA trial concluded that HFNC did not reduce significant hypoxaemia when compared to COT; hence routine use after abdominal surgery is not justified.[39] HFNC should not be indiscriminately applied to all postoperative patients, but should be considered on a case-by-case basis, especially those at risk of postoperative pulmonary complications.[40],[41]
Postextubation oxygenation and preventing reintubation
Extubation is a significant step towards recovery in the ICU. Extubation failure is associated with increased mortality.[42] Therefore, postextubation oxygenation should be optimised to reduce the risk of reintubation. In a meta-analysis, HFNC was found to reduce the rate of reintubation when compared to COT and was non-inferior to NIV. However, HFNC use did not reduce mortality or ICU length of stay.[43] Hernández et al. further subanalysed groups of patients by categorising them as either low or high risk for reintubation. They found that HFNC reduced the risk of reintubation for 72 h when compared to COT for patients at low risk of reintubation.[44] HFNC was non-inferior to NIV for patients who were at high risk of reintubation.[45] HFNC after extubation was observed to decrease respiratory rate, improve dyspnoea scores and improve PaO2:FIO2 ratios.[46],[47] Despite the benefits highlighted, these studies concluded that HFNC did not reduce mortality and ICU length of stay.
Preventing intubation: Treatment for acute respiratory failure
Developing strategies to prevent intubation allows clinicians to avoid the complications of intubation and mechanical ventilation. A meta-analysis concluded that the use of HFNC in adults with acute respiratory failure reduced intubation rates compared to COT, but was no different compared to NIV. Compared to NIV or COT, the use of HFNC does not reduce ICU length of stay or mortality.[48] Similarly, another meta-analysis found that HFNC reduced intubation rates without affecting mortality.[49] Contrary to the above, the landmark FLORALI trial found that HFNC or NIV did not reduce the rate of intubation. Nevertheless, the use of HFNC reduced ICU mortality compared to NIV and COT.[50] Patients consistently reported reduced dyspnoea scores and improved comfort across multiple studies, which may explain the benefits of HFNC for patients with acute hypoxaemic respiratory failure.[46],[47],[50],[51]
Intubation and mechanical ventilation are associated with significantly poorer outcomes in immunocompromised patients.[52] Therefore, effective strategies to manage immunocompromised patients in acute respiratory failure translate to better survival. A meta-analysis on the use of HFNC in immunocompromised patients has reported reduced intubation rates, but no effect on mortality or length of hospital stay.[53] However, studies from Coudroy et al. and Roca et al. have demonstrated reduced intubation rates and improved survival.[54],[55]
Other Clinical Scenarios for Using High-Flow Nasal Cannula | |  |
The use of HFNC in the following variety of clinical scenarios has been studied briefly. Evidence is sparse and hence its routine use is neither validated nor justified.
Palliative care
HFNC offers better comfort levels compared to face masks.[46],[50] It improves work of breathing and can palliate symptoms of breathlessness. As such, HFNC is a viable alternative for end-of-life care in maximising comfort care without compromising on medical management.
HFNC increased oxygen saturation and reduced the respiratory rate of patients with a 'Do-Not-Intubate' order. It was well-tolerated and patients stayed on HFNC much longer than NIV.[19] Survival rate for patients on HFNC was the same as NIV, but HFNC enabled patients to eat and talk just before demise, improving their quality of life (QoL).[20]
Chronic obstructive pulmonary disease
End-stage chronic obstructive pulmonary disease is characterised by worsening breathlessness at rest and hypercapnic respiratory failure. Supportive treatment includes oxygen supplementation via long term oxygen therapy (LTOT). HFNC with LTOT improved health-related QoL and decreased PaCO2 levels in hypercapnic patients. Moreover, HFNC was well-tolerated and improved compliance.[56] Long-term treatment with HFNC alone reduced exacerbation days, improved lung function and improved QoL scores.[57]
Obstructive sleep apnoea (OSA)
OSA is a sleep apnoea disorder characterised by repeated intermittent complete or partial obstruction of the upper airway during sleep. Classically, OSA is managed by CPAP devices. However, compliance to treatment is compromised as the CPAP face masks are tight-fitting and uncomfortable.[1] HFNC was explored as an alternative to improve the management of OSA as it generates PEEP and offers better comfort. McGinley et al. found that HFNC use reduced key indicators of OSA severity but did not eliminate OSA.[58]
Bronchoscopy
Bronchoscopy is a procedure that commonly causes hypoxaemia because of sedative medication and ventilation-perfusion mismatch from the procedure. HFNC was found to be similar to NIV[59] and superior to COT[60] in reducing hypoxaemia during bronchoscopy.
Emergency department
As HFNC can provide better oxygenation than COT, it may be of some value in the emergency department for the initial management of patients with respiratory complaints. A large randomised controlled trial by Jones et al. showed that HFNC did not confer benefit in terms of intubation and length of hospital stay.[61] However, a subsequent meta-analysis suggested that HFNC improved comfort and dyspnoea scores with reduced intubation rate.[62],[63]
High-Flow Nasal Cannula and COVID-19 | |  |
Severe acute respiratory syndrome Coronavirus 2 (SARS-CoV-2) is a novel Coronavirus that emerged late-2019 and resulted in a global pandemic affecting almost every country in the world. As of July 2020, an estimated 12 million recorded cases and more than 550,000 deaths have been reported worldwide. The disease from the infection is termed COVID-19. Patients with COVID-19 often present with acute respiratory symptoms of variable severity.[64] Some patients may be completely asymptomatic or have symptoms of minor upper respiratory tract infection. Others may develop viral pneumonia requiring oxygen supplementation. A proportion of patients may even progress to acute respiratory distress syndrome requiring ICU admission and trial of NIV, HFNC or mechanical ventilation.[65]
As COVID-19 mainly affects the respiratory system, oxygenation and ventilation strategies have become a topic of interest. Use of HFNC for management of COVID-19 has been explored and is increasingly important as a strategy for preventing intubation and mechanical ventilation.
The primary mode of viral transmission is via respiratory droplets; hence there is significant concern over using HFNC due to its potential for aerosol generation.[66] Similar to other respiratory-related equipment and manoeuvres (COT, NIV, intubation), HFNC use is considered an aerosol-generating procedure (AGP) which can put healthcare workers at risk of nosocomial infection. However, previous studies have demonstrated that HFNC did not increase the risk of bacterial or viral contamination.[67],[68] Other AGPs like intubation, NIV and bag-mask-ventilation were reported to increase the risk of transmission, whereas high-flow oxygen did not.[68] An earlier study also showed that air dispersion is limited if the nasal cannulae fit well.[69] Li et al. have conducted a bioaerosolisation study and concluded that the risk of generation and dispersion of aerosols is similar between HFNC and standard face masks.[70] On the other hand, Loh et al. have found that HFNC use increased aerosol dispersion, hence suggesting that HFNC use should be treated as an AGP requiring airborne precautions.[71] Although there have been no reports of transmission via HFNC to healthcare workers, understandably there remains a great amount of fear and uncertainty when it comes to HFNC as an AGP and potential exposure risk.[72]
Another criticism for the use of HFNC in patients with COVID-19 is its inability to address the progression of the disease process and may delay intubation.[73] There have been case reports detailing the success of HFNC therapy in preventing intubation.[74],[75] In fact, the Surviving Sepsis Campaign guidelines for COVID-19 have recommended HFNC over NIV for acute hypoxaemic respiratory failure after failed trial of COT.[76] However, the World Health Organisation interim guidelines acknowledge the lack of evidence-based guidelines for HFNC and its potential for aerosolisation.[77] It is therefore advised that healthcare workers take precautions against airborne particles during HFNC use, which can be logistically challenging in a climate of constrained healthcare resources.
A review by Agarwal et al. have summarised the dilemma of HFNC use well.[78] Weighing the risks (nosocomial airborne transmission) and benefits (reducing need for mechanical ventilation) requires careful consideration of logistical factors available to the medical team caring for the patient. This is reflected by the conflicting views expressed by different scientific societies throughout the world, which can further exacerbate the uncertainty of the pandemic situation.[72],[79] HFNC may benefit some patients in acute hypoxaemic respiratory failure with COVID-19, but its use should be monitored carefully to prevent undetected clinical deterioration. Caution should be exercised during HFNC use given its potential as an AGP and risk of nosocomial transmission to healthcare workers. With better understanding of the COVID-19 pathological process, targeted HFNC use may benefit patients and rationalise medical resources at the same time.[80]
Drawbacks of High-Flow Nasal Cannula | |  |
One of the most significant criticism of HFNC is the potential for delaying intubation when clinically indicated. A study of patients intubated after a trial of HFNC therapy found that delay of intubation led to poorer outcomes in the ICU.[81] It is hypothesised that the comfort provided by HFNC devices may delay the onset of respiratory distress, confounding the physician's assessment of the patient's respiratory status. Another issue with HFNC is the cost. HFNC is more expensive than COT devices and is therefore less accessible in resource-poor countries.
There has been a push to develop a scoring system to guide HFNC therapy. The ROX index was developed, which uses a simple formula to predict success in preventing intubation at 12 h after starting HFNC.[82] Subsequent validation studies show that there is some usefulness in predicting the success of HFNC therapy.[83]
At the moment, there is an abundant but inconclusive evidence base for the use of HFNC. This stems from fundamental difficulties in conducting statistically robust studies. Randomised trials cannot be truly blinded. HFNC settings vary between studies (FIO2, flow rate), and inclusion criteria vary widely across different studies.[84] Results are difficult to replicate and may not apply to a wider population. Assessment of HFNC success or failure depends on criteria applied by the individual workgroups. Comparison of HFNC to other devices may not be informative as there is no 'gold standard' to compare with. Clinical subjects, especially those in critical care, have significant comorbidities and ongoing organ dysfunction, which can confound outcomes such as mortality rates and length of hospital stay.
Overall, it appears at present that there are unidentified advantages of using HFNC in specific patient groups.[85] There is a lack of evidence-based indications for commencing and terminating HFNC. Hence, it is difficult for clinicians to make decisions on escalation and de-escalation of HFNC. It is common to detect discrepancies in HFNC strategies amongst different physicians.[86] Ischaki et al. have developed an algorithm detailing the indications, titration, escalation and weaning strategies for HFNC use.[87] However, this has not been validated in large scale studies.
The bottom line is that when HFNC fails, patients are left without respiratory support and can deteriorate quickly. Further studies should be aimed at studying clinical strategies for picking up HFNC failure early, the most appropriate timing to escalate therapy and the frequency of monitoring such that clinicians can tread the fine line between avoiding intubation and delaying intubation.
Conventional Oxygen Therapy, Noninvasive Ventilation or High-Flow Nasal Cannula? | |  |
Perhaps the most important question is where HFNC fits in the landscape of noninvasive respiratory therapies for oxygenation. Multiple meta-analyses and systematic reviews comparing COT, NIV and HFNC have been conducted.[43],[48],[49],[88],[89],[90] Evidence seems to suggest that HFNC reduced rate of mechanical ventilation and risk of intubation compared to COT, while HFNC is at best equal to NIV. Authors of the various reviews have acknowledged the fact that there is a lack of strong robust studies with large sample sizes forming the evidence base for these reviews and analyses, making it difficult to draw strong conclusions. As such, it would not be too outrageous to consider HFNC as an intermediate between COT and NIV, but it should be used with caution. Similar to COT and NIV, there should be appropriate monitoring for patients on HFNC. Clinicians should be vigilant to therapy failure and escalate support appropriately if needed.
Conclusion | |  |
HFNC is a relatively new device used for delivering oxygen at high flows. There is increasing interest in using HFNC for adult patients across different clinical settings. However, its practical application in clinical practice remains to be evaluated rigorously. With the current evidence, HFNC can be considered as an alternative mode of oxygen therapy between COT and NIV. Further work on developing strategies for initiation, monitoring, titration, escalation and weaning will help shape definitive guidelines for HFNC use to maximise its potential.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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