HFO accomplishes washout of nasopharyngeal dead space. The extrathoracic dead space in children is proportionally two or three times greater than in adults. It may measure up to 3 ml/kg in newborns; after 6 years of age it becomes similar to the adult volume. Under normal conditions in children, during breathing, approximately 30 % of the tidal volume inhaled is anatomic dead space. At the beginning of the inhalation, this dead space is filled with gas from the previous breath remaining at the end of the exhalation. Therefore, HFO may improve breathing efficiency by flooding the nasopharyngeal dead space with clean gas, thereby contributing to improving the minute ventilation. As in the case of any reduction in anatomic or physiological dead space, this treatment contributes to establishing better alveolar gas fractions, facilitating oxygenation, and bringing a theoretical improvement in carbon dioxide (CO₂) elimination.

Because HFO provides a flow that is sufficient to equal or exceed the patient’s inhalation flow, it is likely to reduce the inhalation resistance related to the passage of air through the nasopharyngeal airway. This gives rise to a change in the work of breathing (WOB).

The appropriate heating and humidification of the airways is associated with improved pulmonary compliance and elasticity compared with dry, cold gas. In addition, the nasal mucosa receptors respond to cold, dry gas by provoking a protective bronchoconstriction in normal and asthmatic subjects. The heated, humidified air generates a beneficial effect on the ciliary movement, clearing of secretions, and prevention of atelectasis. HFO reduces the metabolic work required to heat and humidify external air, which is drier and colder with respect to body temperature and humidity.

Despite the different hypotheses postulated in the literature with respect to the mechanisms of action of HFO, there seems to be agreement about the fact that it originates a certain positive pressure on the airway. This pressure is variable (ranging from scant to excessive), is relatively unpredictable, cannot be regulated, and is related to the flow, size of the patient’s nasal cannulas, leaks, and whether the mouth is closed. One of the main differences between HFO and noninvasive ventilation (NIV) is that the former maintains a fixed flow and generates variable pressures, whereas NIV systems use variable flows to obtain a fixed pressure. HFO improves the ventilation pattern, reducing the respiratory frequency, heart rate, and oxygen needs, but does not normally affect alveolar partial pressure carbon dioxide (PaCO₂) or pH. The devices are easy to apply and allow the children to eat, talk, and move. The tendency to use HFO is partly the result of a perception of greater ease in using it and improved tolerance by the patient, thereby achieving further benefits.

There are few disadvantages because this system has good tolerance. In some cases, abdominal distension may be observed due to flatulence. Condensation may occur in the nasal cannula at low flows. There has been described air leak syndrome (pneumothorax, pneumomediastinum) [4]. In prolonged situations, nasal trauma may occur, and another disadvantage is the high level of noise correlated with the flow (Table 52.1).

No evidence can be found to allow determination of the safety or effectiveness of the high-flow nasal cannula (HFNC) therapy as a form of respiratory support in children [5]. Nor are there established guidelines or decision-making pathways to guide use of the HFNC therapy in adults. In summary, there are no clearly established indications for the use of HFO. They are extrapolated from observational or physiological studies, mainly in adults and premature infants without evidence (Table 52.2). It is used for the same indications as the traditional method of CPAP and may be more effective than standard oxygen supply devices for oxygenation in the post-extubation period.

For clinical practice, HFO seems feasible in mild to moderate forms of respiratory distress and hypoxemia, transcutaneous oxygen saturation (SpO₂) <90 %, despite standard flow oxygen. It is useful in hypoxemic, nonhypercapnic patients who require fraction of inspired oxygen (FiO₂) >0.3 using a face mask (type I respiratory failure). It is not considered useful in type II respiratory failure because it does not reduce PaCO₂ levels and is not indicated in CO₂ retainers because it reduces the respiratory stimulus triggered by hypoxia that is produced in hypoventilation.