Acute bronchiolitis is the most common respiratory illness and the main cause of respiratory failure in infants, often leading to hospitalization with high healthcare costs [1]. Bronchiolitis is caused by viral infections, particularly by respiratory syncytial virus (RSV), which lead to airway inflammation and obstruction of the lower respiratory tract. Most infants experience mild symptoms similar to those of a common cold; however, symptoms can get worse and become severe and they may develop breathlessness, tachypnea, retractions and hypoxemia with hypercapnia [2]. Effective therapy is not available; treatment is mainly supportive and focuses on management of respiratory distress and hypoxia.
A relatively new, safe and promising method of non-invasive respiratory support is oxygen delivery by high-flow nasal cannula (HFNC). HFNC provides a heated and fully humidified mixture of oxygen and air at a pressure slightly more elevated than the inspiratory peak pressure, via a nasal cannula interface. It delivers constant FiO2, improves washout of the nasopharyngeal dead space and mucociliary clearance and it decreases metabolic work related to gas conditioning. Moreover, HFNC creates a low level of positive pharyngeal pressure that might help to reduce inspiratory airway resistance [3]. Several papers demonstrated that infants supported by HFNC have a low rate of paediatric intensive care unit (PICU) admission and a reduced need for invasive ventilation. In spite of these beneficial effects, HFNC has not been demonstrated to reduce the length of hospital stay or the duration of oxygen therapy with respect to the oxygen supplementation by nasal prongs [4, 5, 6, 7]. In addition, several questions concerning HFNC clinical practice, such as the optimal timing to initiate HFNC and the features that can predict HFNC failure, which is defined as requirement for mechanical ventilation (MV), remain unanswered. Answering to these questions may help in preventing HFNC failure.
Over seven epidemic seasons, we retrospectively analysed the clinical course of infants hospitalized for bronchiolitis who underwent HFNC with the aim of identifying clinical, laboratory or radiological factors that can predict HFNC failure.  
MATERIALS AND METHODS