Respiratory failure (RF) is one of the most frequent causes of admission to intensive care departments, and oxygen administration remains a first-line therapy.1 The use in recent years of high flow nasal cannulae (HFNC) in patients with both acute and chronic RF appears to be a useful alternative to conventional oxygen therapy (COT) and other non-invasive systems of respiratory support (primarily non-invasive ventilation [NIV]). This technique helps optimize the control of the patient with RF and improves survival.1 HFNC can be used for the non-invasive administration of a fully conditioned flow of gas (37°C and 100% relative humidity) of 0–60L/min with the desired FIO2 (0.2–1L), offering rapid improvement in a significant number of patients.2

The most notable contribution of the HFNC active humidification system in mechanically ventilated patients, introduced about 10 years ago, is its excellent clinical tolerance and capacity to significantly reduce the breathing rate in patients with acute RF (ARF),3 compared to patients receiving COT or NIV.

In the early years, HFNC was indicated mainly on the basis of the benefit perceived from personal experience, and the notion that this was a potentially useful treatment had to be taken in good faith; now, however, there is sufficient evidence to support its use in patients with ARF (high flow with high FIO2),4 and some positive results are beginning to emerge on its use in chronic RF (CRF) (high flow without the need for high FIO2), in both the hospital and home care setting.5

It is clear that invasive ventilatory support should be avoided wherever possible, due to problems arising from either the artificial airway or the mechanical ventilation (MV) itself. When NIV was introduced, results were initially mixed, but it is now the reference treatment in COPD exacerbations, acute cardiogenic pulmonary edema, and ARF in immunosuppressed patients.6 However, its use remains controversial in patients with other causes of ARF,7 although HFNC may also play an important role in these patients, in addition to the standard NIV indications.

Four components are required for the administration of this high flow: first, silicone cannulas (adapters for tracheotomized patients are also available); second, a high-flow administration system to control the administered flow and FIO2 (with 2 wall outlets and a mixer flow meter, or a turbine that is separate from the wall outlet); third, a humidifying-warming system; and fourth, non-condensing tubing connecting the humidifier with the patient interface.2

Several pathophysiological mechanisms explain the effects of high flow, but the main results achieved with this system are the following1: the dilution of the administered O2 flow with ambient air can be reduced during peak inspiration (which in situations of RF can be >50 bpm8), thus ensuring that the real FIO2 administered is close to the desired level2; a CPAP effect is also achieved, with positive airway pressures ranging between 3 and up to 7–9cmH2O, depending on the flow and whether the mouth is open or closed3; the flow supplied directly to the nasopharynx flushes CO2 from the anatomical receptacle, preventing reinhalation and providing a reservoir of fresh gas, thus reducing the anatomical dead space and increasing alveolar ventilation, improving effort tolerance, and reducing dyspnea4; the effort required to metabolize the inhaled gas mixture is reduced5; active humidification improves mucociliary clearance, facilitating the expulsion of secretions and reducing the formation of atelectasis, which improves the ventilation-perfusion ratio6; in addition, the administration of conditioned gas helps reduce upper airway resistance, thus helping to reduce the patient's work of breathing7; the high flow also increases the circulating pulmonary volume, which is accompanied by a decrease in respiratory rate, without changes in PaCO28; both this change in the ventilatory pattern,9 which leads to a reduction of work of breathing,10 and the generation of intrathoracic positive pressure, with a reduced right ventricular preload, also explain the hemodynamic improvement in patients with heart failure.11 All these effects, then, together with the convenience of the technique, which facilitates oral communication and allows patients to eat without disconnecting the circuit, mean that HFNC achieves improved oxygenation in patients with ARF.12

Moreover, with high flows and FIO2, all the physiological effects listed above can benefit both patients with COPD exacerbation, without the deleterious effect of high concentrations of oxygen, as shown by some recent studies,5 and clinically stable patients on home oxygen therapy.

All this suggests that HFNC may well be the Holy Grail of treatment for both acute and chronic RF, and justifies the growing use of this system in intensive care departments, postsurgical resuscitation areas, and emergency and pulmonology departments. This is because it can used as first-line treatment for ARF, in weaning from MV in both acute and postoperative situations,13 during invasive procedures, and in palliative care. More evidence is still needed on its use in COPD exacerbations, heart failure, and preoxygenation during tracheal intubation. The initial flow and how to decrease it must be protocolized, with special attention given to FIO2, in order avoid entering into a hyperoxic spiral, while adjusting SpO2 according to the situation of each patient.14 The detection of patients at risk of failure15 and the effects of this procedure remain to be analyzed in greater depth.