Traditionally the oxygen therapy systems have been classified as low and high flow. Low-flow systems do not provide all the inspiratory flow demanded by the patient; do not ensure stable levels of FiO2 and it is not possible to control the temperature and moisture of inspired gas. The low flow oxygen system most widely used is the nasal prongs. By contrast, high flow systems are able to provide the entire atmosphere breathed by the patient and do ensure a stable FiO2 and it is possible to control the temperature and moisture of the inspired gas. The typical high flow oxygen system is the venturi mask.1 In recent years, a new way of supplying patients with oxygen has appeared which in our opinion has been incorrectly called high-flow nasal cannula oxygen therapy (HFNC).2 This therapeutic approach has a lot of physiological effects which makes it a really active treatment for patients with both acute and chronic respiratory failure; it is more than a simple oxygen delivery system. Several studies have shown that HFNC generates a low level of positive airway pressure, improves oxygenation, increases the end-expiratory lung volume, reduces airway resistance, increases functional residual capacity and alveolar recruitment and flushes nasopharyngeal dead space, thus helping to decrease the work of breathing. There is a better control of FiO2 and the gas humidification ensures better patient tolerance and comfort. Due to a better mucociliary clearance, pulmonary defense mechanisms are restored.3 Many of these effects are similar to those produced by non-invasive ventilation. Several studies have shown its utility in patients with acute hypoxemic respiratory failure, in the post-extubation period, in palliative care, in patients with acute heart failure, in chronic airway diseases and its indications are still rising. It is used in critical care areas, in the emergency department, in wards and it is being used at home in COPD patients.4,5
As we can see, oxygen plays a secondary role in this treatment. The name of high-flow oxygen therapy is confusing. Although the efficacy of improvement in respiratory gas exchange and effectiveness in outcomes has been demonstrated, we have not fully understood the main pathophysiological principles of this therapy. The role and importance of each of the mechanisms related to the high-flow therapy, the end expiratory positive pressure, the flushing of nasopharingeal dead space or the humidification and warming of the inspired air, has not been clarified yet. Up to now, we have not been able to identify the functional significance of each of the three mechanisms, maybe this is a question that will never be answered because in different clinical situations the relevance of these potencial mechanisms changes. What is clear is that high-flow is not an oxygen therapy device. Most international groups working in this field use the term “Nasal High Flow” avoiding the term “oxygen” in its definition.
We think such an active treatment should have a name that reflects the effects of the treatment better, in order to avoid confusion. And that deserves some careful reflection. High-flow can be applied through nasal cannula or tracheostomy, so the term “nasal” should not be part of the name. Perhaps the name “Active High Flow” would be more accurate so as to clearly distinguish it from the conventional or “passive” high flow like the venturi system. The Venturi system is usually administered by mask and high flow by cannula, which could be another distinguishing feature. Taking into account the different but also relevant effects of the therapy, we might ask why not include the term “humidification” or “heating” to the words “high flow”. The positive expiratory pressure effect could probably be more interesting to incorporate into the name. Although the pressure achieved is highly dependent on the individual patient and the interface used, is low in absolute value and, above all, not determined by the prescribed parameters of the equipment, we do have quite a lot about its relevance. Certainly, this therapy has demonstrated that can be useful in treating sleep apnea syndrome.6,7 Its effect on alveolar recruitment has been clearly shown by Corley et al.8 Roca et al.9 have also indirectly demonstrated the positive intrathoracic pressure reached with the HFNC, showing a reduction greater than 20% in the estimated inspiratory collapse of the inferior vena cava from baseline. Perhaps the provocative name of “High-flow positive pressure” would better define a treatment that gives the patient a high flow of heated and wet gas, with a level of positive pressure, and a controlled FiO2. Anyway, the name of high-flow oxygen therapy would be restricted to the classical high flow system such as the venturi system. We think that a consensus on a more precise name is required.Conflicts of interest
The authors have no conflicts of interest to declare.