Dyspnea is one of the most common complaints in patients presenting to the emergency department (ED). Oxygen supply is the rst line therapy [1].
It can be delivered, according to current guidelines and depending on the severity of the patient’s respiratory distress by dierent devices. Oxygen therapy is used to correct hypoxemia,
to alleviate breathlessness and to avoid intubation (associated with high mortality). When assisted ventilation ( with non invasive or invasive mechanical ventilation) is not urgently required, conventional oxygen therapy is provided by nasal cannulas, face masks or nasal prongs. In fact, knowing which form of oxygen therapy to choose is still debated.
Several drawbacks are associated with conventional interfaces. First of all, oxygen ow through these devices generally do not exceed 15 L/min with a facemask. Secondly, using a mask is associated with patient discomfort. Given that noninvasive ventilation (NIV) for patients with acute hypoxemic respiratory failure is controversed, erapy with high-ow oxygen through nasal cannula (HFNC) may oer an alternative in patients with hypoxemia.
In this narrative review, we critically report the current literature focusing on the physiological eects, clinical benets and potential applications of HFNC in adults and highlight the many knowledge gaps requiring further studies.
How it works ?
(HFNC) is an evolving respiratory therapy where by warmed and humidied oxygen is delivered to the nose at high ow rates. The fraction of inspired oxygen (Fio2) can be adjusted by changing the fraction of oxygen in the driving gas with FIO2 of up to 1.0, a maximum ow of 60 L/min, and a temperature of 37°C (Figure 1).
Physiological effects of high !ow nasal cannula oxygen : Pharyngeal dead space washout : it’s one of the main effects of delivering high gas ows directly in the nasopharynx. Physiological extrathoracic dead space (oral, nasal, pharyngeal) decrease by ushing expired carbon dioxide from the upper airway, reducing CO2 rebreathing and so providing a reservoir of fresh gas[2]. We may reduce the minute ventilation (VE) and therefore reduce work of breathing by analogy with the interest of tracheal gaz insuflation (TGI) proved for intubated patients[3]. This would eliminate dead space attributable to the ushed-out volume, permitting a higher fraction of minute ventilation to participate in gas exchange [2,4]. This physiological eect was the corner stone of the elective indication of HFNC for neonatal and pediatric settings regarding the specicities of those patients which have a large extrathoracic dead space ( 3ml/kg) compromising an efficient minute ventilation.
Nasopharyngeal resistance : High ow nasal oxygen minimizes the nasopharyngeal resistance by counteracting the physiological phenomenon of local collapsus due to inspiration[5,6].
PEEP Effect : It is now admitted that the use of high ows generates a modest level of positive airway pressure( range 3–5 cm H2O) . is eect on oxygenation is not yet identified and did not replace CPAP for example in the treatment of cardiogenic oedema due to lack of evidence.
Increase in end inspiratory lung volume : this device permits to cover the end inspiratory lung volume ( up to 30l/mn) of patient (that substantially exceed the ow rates of standard oxygen delivery systems ) by a controlled oxygen fraction and so to reduce oxygen dilution with the entrained room air [7]. Humidication and tolerance : Using heated and humidified gas can decrease airway resistance. Some studies have found that HFNC oxygen therapy improves oxygenation, tolerance and comfort, and eases drainage of respiratory secretions [8-11]. Roca et al, found that compared to facemask oxygenation using a bubble humidier, HFNC was associated with greater overall comfort, lower dyspnea scores and reduced mouth dryness [10].
HFNC was rst used in pre-term neonates and pediatric care as a rst-line treatment in respiratory distress syndrom and prematurity apnea. It was better tolerated thanks to the heating and humidication [12]. But till now limited clinical datas are available for adult applications of HFNC. That said, we are currently noting an increasing use of HFNC in the intensive care units[13].
Hypoxemic respiratory failure : Based on the multiple physiological and subjective benets of HFNC compared to standard oxygen therapy, this new device has been tried in ICUs for adults with or at risk of acute respiratory failure. When standard modalities were insuficient, invasive ventilation could be delayed and because of the lack of evidence for NIV, HFNC seems to be best situated in the middle of the spectrum of therapies to treat hypoxemic respiratory distress. Conicting recommendations exist on whether high-ow nasal cannula should be administered. FLORALI study performed by Frat et Al was the rst large positive study. It was a multicenter, openlabel trial in which they randomly assigned patients without hypercapnia who had acute hypoxemic respiratory failure to HFNC, standard oxygen therapy (through a face mask), or NIV. There was a signicant dierence in favor of high-flow oxygen in 90-day mortality [9].
A recently published meta analysis including 6 randomized controlled trials has concluded that the intubation rate with HFNC oxygen therapy was lower than the rate with conventional oxygen therapy and similar to the rate with noninvasive ventilation among patients with acute hypoxemic respiratory failure [8]. But many clinicians found this device dangerous for emergency practice as it may be delaying imperative invasive ventilation and they noted the need for predictive factors of failure [14].
An Observational Pilot Study conducted by Frat and Al evaluating the clinical efficacy of HFNC alternating with noninvasive ventilation NIV in acute hypoxemic respiratory failure, has found that after HFNC initiation, a breathing frequency of > 30 breaths/min can be considered as an early factor associated with intubation[15]. Large randomized controlled trials were required to evaluate the real efficacy and safety of using HFNC because of controversed datas. In fact, Monro-Somerville and Al, through a systematic review and meta-analysis have not detected dierences in mortality or intubation rate in patients with acute respiratory failure treated with HFNC compared with usual care[16]. Moreover, the last systematic review from Cochrane database has concluded that there is insu#cient evidence to determine eects of HFNC in the delivery of respiratory support to adult intensive care patients [17]. Regarding those conicting results, we must note that there are many bias in the studies focus (with risk bias C). So, well conducted studies maybe needed to conclude the eect of HFNC in the hypoxemic respiratory failure.
Other indications : HFNC was rst used as a modality to facilitate secretion removal in patients with bronchiectasis [18]. Another interesting clinical indication was after extubation where Oxygen is commonly administered. The use of HFNC was associated with better comfort, fewer desaturations and interface displacements, and a lower reintubation rate[11].
Other indications for the use of HFNC include preoxygenation[19], maintenance of oxygenation during airway instrumentation( Fibroscopy, trans oesopharungeal echocardiography prevention and treatment of postoperative respiratory failure, and in palliative care [18].
HFNC is a new tested device for oxygen supply. It has very promising physiological eects but its use in clinical practice still controversed because of conicting datas. The majority of recently papers have negative results. More well conducted clinical trials are needed to evaluate its utility, efficacity and safety especially for emergency medical care.