The patient interface consists of special silicone nasal cannulas that are longer than the conventional cannulas. The silicone cannulas come in different sizes, and we must choose the size that best adjusts to the nostrils of each individual patient. Adaptors are also available for tracheotomized patients.

A system capable of administering a high gas flow (0–60l/min) and of adjusting the administered fraction of inspired oxygen (FiO2) is required. Two different systems are able to do this:

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  Mixing of oxygen and compressed air through two independent wall outlets connected by a Y-tube, or using commercial respirators (Dräger, Lübeck, Germany).

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  Mixing of oxygen with room air by means of a turbine (Airvo™ system, Fisher & Paykel Healthcare Ltd., Auckland, New Zealand).

The fundamental element for clinical use of the technique is effective humidification of the administered gas. The most widely used commercial systems are Optiflow™ (Fisher & Paykel Healthcare Ltd., Auckland, New Zealand) and Vapotherm™ Precision Flow (Vapotherm, Exeter, USA). These systems allow us to administer the gas warmed to body temperature (37°C) and with a relative humidity of 100%.

A number of commercial tubing systems are available. The important issue in deciding which system to use is that we need mechanisms for preventing or minimizing condensation in the tubes, since condensation has been shown to favor infections.23

High-flow oxygen therapy has a series of characteristics and mechanisms of action that may be particularly beneficial for patients with severe ARF. A review is provided below of the main mechanisms underlying the beneficial effects of HFOT.

One of the main benefits of HFOT is the improvement of hypoxemia in patients with ARF. The main mechanisms underlying this phenomenon are lesser dilution with room air of the gas administered during inspiration, and a certain pressurization of the airway, giving rise to a certain CPAP-like effect.

One of the benefits of HFOT is the possibility of providing sufficient flow to satisfy the peak inspiratory flow (PIF) demand of the patient, where the administered FiO2 comes close to the real value received by the patient. This is because the administered gas flow is not diluted with room air (Fig. 2). Under normal conditions, the PIF is about 30–40l/min24; consequently, this in principle could be regarded as the recommended flow range when starting to use HFOT. In patients with severe ARF, the PIF is probably higher, however. For this reason, these are precisely the patients in which the oxygen dilution phenomenon is particularly important. The above would at least partially account for the improvement in oxygenation of these patients with HFOT.25,26 On the other hand, thanks to minimization of the dilution of the administered oxygen with room air, we achieve better control of the administered oxygen concentration.27–29

Figure 2.

Dilution of the administered oxygen using a low- (a) and high-flow system (b). Dark gray: oxygen; light gray: room air. During normal inspiration, the peak inspiratory flow (PIF) demand is 30–40l/min. With the use of high-flow oxygen therapy, the administered FiO2 would equal the real FiO2, while in the case of a low-flow system the real FiO2 would be less than the administered FiO2.

(0.13MB).

Another potential mechanism of action underlying the effects of HFOT is the generation of a certain positive airway pressure. This pressure increment has been demonstrated at nasopharyngeal level and in the oral cavity, as well as in the end-expiratory esophageal and tracheal pressure values obtained.17–19

A number of studies have shown that the flows in the order of 35–60l/min result in mean expiratory pressures at pharyngeal level of 2–3cmH2O with the mouth open, and of 5–7cmH2O with the mouth closed.18,19 Furthermore, it has been seen that the use of HFOT allows an increase in lung impedance at the end of expiration15,16–this parameter in turn being correlated to lung volume. This is important, since it means that the improvement in oxygenation would be partially due to improved alveolar recruitment,5,26,30 attributable at least in part to the mentioned increase in airway pressure.

Another aspect to be taken into account is that the flow directly administered to the nasopharynx effectively washes out CO2 from the anatomical spaces, which in turn avoids re-inhalation and affords a reservoir of fresh gas.14,31 This phenomenon reduces the anatomical dead space and increases alveolar ventilation,12 giving rise to improved exercise tolerance,32 less dyspnea, and improved oxygenation.17

The use of fully conditioned gas (i.e., administered at ideal temperature and humidity) results in improved tolerance and comfort for patients with ARF,5,33 though it also allows a decrease in respiratory frequency5,16,29 and improves oxygenation. Conditioning of the administered gas with HFOT directly affects oxygenation. In effect, active humidification improves mucociliary function, facilitates the expulsion of secretions and reduces the formation of atelectasis–thereby improving the ventilation/perfusion ratio and oxygenation.5 Furthermore, the administration of fully conditioned gas allows a decrease in airway resistance, which in turn lessens the respiratory work of the patient.4,26

During inspiration, the negative pressure limits the inspiratory flow due to nasopharyngeal collapse. High-flow oxygen therapy, by generating a certain positive airway pressure, is able to counter this phenomenon.34 Since HFOT allows us to administer flows equal to or greater than the patient PIF, the technique could help minimize upper airway resistance, with a reduction in respiratory work.12 Improved thoraco-abdominal synchrony has moreover been demonstrated with HFOT versus conventional oxygen therapy.35

On the other hand, the use of HFOT increases lung impedance and thus raises the tidal volume.15,16 This situation is usually accompanied by a decrease in respiratory frequency, without changes in PaCO2.5,26,30 Both this change in ventilatory pattern–which could imply a decrease in respiratory work–and the generation of a certain positive intrathoracic pressure could lead to changes in the hemodynamic profile of patients with ARF.36 A recent study has analyzed the hemodynamic effect of HFOT in patients with stable heart failure.20 This effect would consist of a decrease in right ventricle preload (as measured by a decrease in inferior vena cava collapse), which could imply hemodynamic improvement in patients with ARF secondary to myocardial dysfunction.20

Failure of NIMV is often due to patient intolerance of the interfaces. In effect, patients may experience claustrophobia or anxiety, as well as erythema, pain or ulceration of the bridge of the nose, and nasal dryness and congestion, irritation of the eyes and gastric bloating. Furthermore, possible air leakage adds discomfort and can interfere with triggering and cycling of the respirator.

All the existing scientific evidence in this respect indicates that the tolerance of HFOT is as good if not better than that of other oxygen therapy techniques,30 since it affords superior patient wellbeing5,36 and generates less noise37 and less nasal dryness.5 In addition, HFOT facilitates verbal communication and allows food intake without requiring circuit disconnections.

Roca et al.5 were the first to demonstrate the benefits of the use of HFOT in patients with ARF. After only 30min of application of the technique, significant improvements were noted in both the clinical and physiological parameters. These results were subsequently confirmed by Sztrymf et al.,30–who corroborated a decrease in respiratory frequency and improved oxygenation in these patients.

Furthermore, the use of HFOT allows better handling of the respiratory secretions30,39–a fact which could prove particularly important in patients with ARF of infectious origin.

On the other hand, the use of HFOT could lessen the need for NIMV and even for invasive mechanical ventilation (IMV) in patients with ARF. This possible benefit of HFOT has been analyzed in four studies.26,29,30,40 Sztrymf et al.30 reported an HFOT failure rate of about 30%, which is comparable to that of NIMV.30 In turn, Parke et al.29 found that only 10% of the patients with moderate to severe ARF treated with HFOT required IMV, while this percentage was seen to reach 30% in those cases where an oronasal mask was used. More recently, in a population of lung transplant patients with ARF, HFOT was seen to significantly lessen the need for IMV, with a decrease in absolute mechanical ventilation risk of 30%. Only three patients were needed to avoid a case of intubation, with potential effects upon morbidity–mortality and the cost-effectiveness of the system.40

However, the main issue of whether the use of HFOT lessens the need for IMV remains to be settled. Many clinicians have the impression that HFOT effectively avoids intubation in many patients with ARF, though to date this has not been demonstrated by any controlled clinical trial. A recent study on the clinical impact of HFOT in patients with severe ARF reported a 68% success rate, with the need for mechanical ventilation (invasive or otherwise) in only 32% of the patients.26 Even more notorious are the findings of the FLORALI study,41 which reported the benefits of HFOT compared with conventional oxygen therapy and NIMV in terms of mortality and reduction of the intubation rates in patients with severe hypoxemia. This study analyzed patients with severe ARF treated with HFOT, conventional oxygen therapy or NIMV. The authors documented a lesser intubation rate, as well as a reduction in mortality in the subgroup of more seriously ill patients (with PaO2/FiO2<200mmHg) subjected to HFOT.

On the other hand, it is important to mention that there are a number of early predictors (within the first 12h) of HFOT failure, such as the persistence of tachypnea, thoraco-abdominal discoordination, and the absence of improvement in patient hypoxemia.26

In sum, the use of HFOT in patients with severe ARF could allow: (1) rapid improvement of dyspnea; (2) improvement of hypoxemia; (3) improved management of the respiratory secretions; and (4) a lesser need for mechanical ventilation.

In critical patients, orotracheal intubation maneuvering is associated to a 20% incidence of potentially serious complications basically related to the appearance of hypoxemia in the context of the actual procedure.42 Noninvasive mechanical ventilation could be used to pre-oxygenate the patient before intubation is carried out. However, it must be suspended during laryngoscopy, and therefore does not allow patient oxygenation during the procedure. In contrast, HFOT does not interfere with laryngoscopy and so could allow the administration of oxygen in the course of the technique.14 Miguel-Montanes et al. have demonstrated that the use of HFOT during preoxygenation lowers the prevalence of severe hypoxemia during intubation maneuvering compared with the use of a reservoir-equipped mask.43 Likewise, a recent experimental study has shown the administration of oxygen at a flow of 10l/min to delay the appearance of severe hypoxemia during apnea–thus evidencing that the use of HFOT during orotracheal intubation might prove useful particularly in hypoxemic patients.42

The use of HFOT after suspending IMV may be useful both for preventing and treating post-extubation respiratory failure.14,44 Four studies have been published in this regard, and all of them have reported improvement of all the respiratory parameters,45,46 as well as of patient wellbeing when compared with the use of an oronasal mask,28,46 with a decrease in the need for increased ventilatory support,44 and a drop in reintubation rate (from 21% to 4%).46 Recently, the group led by Bérnard47 has published a non-inferiority of HFOT versus NIMV (specifically BiPAP) study in post-heart surgery patients at risk of suffering respiratory failure. The mentioned study recorded no differences in terms of treatment failure or mortality in the Intensive Care Unit (ICU) between the two systems, though the use of HFOT was associated to a significant decrease in skin lesions within the first 24h of use. On the other hand, the use of HFOT in the decannulation of tracheotomized patients is another promising possibility, though no studies have been published in this concrete field to date.

During the conduction of invasive procedures such as fibrobronchoscopy, it is common to observe worsening of hypoxemia as a result of the hypoventilation produced and the use of sedatives. This worsening of hypoxemia can be even greater in the case of bronchoalveolar lavage, and can persist for several hours after completion of the exploration.48 The use of HFOT has demonstrated beneficial effects in the form of lesser worsening of hypoxemia and greater patient comfort compared with conventional oxygen therapy.48,49 Recently, Simon et al.50 reported lesser hypoxemia during the performance of fibrobronchoscopy in favor of NIMV in patients with severe hypoxemia; however, patient tolerance and wellbeing proved far superior with the use of HFOT.50 In the same way, the utilization of HFOT should also be considered in the context of other invasive procedures such as transesophageal echocardiography or digestive tract endoscopic explorations.14,31

It is not uncommon to find patients with acute heart failure who suffer persistent hypoxemia and dyspnea once stabilized, despite the use of conventional oxygen therapy. The results of a recent study show that in these patients HFOT improves dyspnea due to a decrease in respiratory frequency and the improvement of hypoxemia.36 On the other hand, it has also been seen that HFOT in patients with stable chronic heart failure gives rise to a decrease in inferior vena cava collapse without a drop in cardiac output–thus evidencing that HFOT reduces right ventricle preload probably through the generation of a certain positive intrathoracic pressure and an increase in lung volume.20 Hence, the improvement afforded by HFOT in patients of this kind fundamentally could be explained by improved oxygenation and improvement of the hemodynamic profile.11,20,36 However, no studies have compared NIMV versus HFOT in the acute phase of lung edema.

Hypoxemia and dyspnea are also among the most common symptoms found in patients admitted to the Emergency Care Department. Lenglet et al.37 examined the potential beneficial effects and feasibility of the use of HFOT in patients with ARF (in most cases secondary to respiratory infection or acute lung edema) admitted to the Emergency Care Department, in comparison with conventional oxygen therapy. The results showed HFOT to offer comparatively faster recovery of dyspnea, oxygenation and the rest of respiratory parameters, and the technique was better tolerated and more comfortable for the patients. In turn, HFOT did not involve greater difficulties of use than conventional oxygen therapy. The application of HFOT to the out-hospital transport setting remains to be evaluated.

In those patients in which IMV is not indicated, HFOT could be of benefit,15 since it reduces dyspnea and improves hypoxemia and cough. Furthermore, it is well tolerated by the patients, who can communicate and even be fed with this system.37,51–53 Indeed, a constant finding in many of the studies published to date is improved patient wellbeing with HFOT.

In the case of hypercapnic respiratory failure, HFOT could also offer benefit by reducing PaCO2 and improving the pH value, probably as a consequence of the pressure increase generated in the upper airway.54 However, it is important to mention that the administration of high gas flows could be useful in these patients subject to the condition that we use low FiO2 levels in order to lessen the risk of hypercapnia. Furthermore, these patients with chronic respiratory disease can especially benefit from the improved mucociliary function afforded by HFOT.54,55 Nevertheless, further studies are needed, comparing the use of HFOT versus NIMV/CPAP in this patient sub-group.