Bronchiolitis is the commonest respiratory infection in infancy leading to hospital admission. 46 per 1000 infants were admitted to hospital with bronchiolitis in England in 2011 and more recent studies suggest this number has remained static. This has a significant cost burden on the NHS. Respiratory support, mainly oxygenation, and keeping infants well hydrated are the mainstay of management in hospital.

Respiratory support has traditionally been the domain of intensive care settings. This has been provided through an escalation of therapy from simple oxygen delivery by nasal cannula, to non-invasive ventilation with Continuous Positive Airway Pressure (CPAP) and finally to intubation with mechanical ventilation. These latter two strategies require highly skilled staff, so are costly, and are associated with a greater incidence of adverse events including ventilator-induced lung injury, barotrauma, and potential neurotoxicity associated with sedation. Over the last decade High Flow Nasal Cannula (HFNC) therapy has emerged as a new method to provide respiratory support for bronchiolitis. HFNC therapy works by delivering an increased volume of air and oxygen into the nasal passages than standard sub-nasal oxygen therapy, using a higher flow of humidified and heated gas. These increased flow rates exceed peak inspiratory flow and thereby result in more efficient delivery of oxygen to the terminal airways. Physiological studies have demonstrated reduced work of breathing and improved gas exchange. The PARIS study has demonstrated that HFNC can be used in a ward setting to reduce admission rates to the Paediatric Intensive Care Unit (PICU). 12% of the study population receiving HFNC needed escalation to PICU compared to 23% receiving standard care (oxygen therapy). The safety data from the PARIS study shows no difference in adverse effects between HFNC and standard oxygen therapy. Numerous studies over the last three decades have investigated the role of various medications in managing infants with bronchiolitis including adrenaline, steroids, salbutamol, and hypertonic saline; none of these studies have definitively changed the outcome of the disease nor the length of stay in hospital. Although the PARIS study showed a reduction in number of patients requiring escalation of care, it did not demonstrate any difference to the total length of stay in hospital.

Aim Since HFNC is a relatively new method of providing respiratory support in bronchiolitis, there is lack of evidence on weaning strategy. The investigators primary aim is to examine the feasibility of different weaning strategies for infants, up to 12 months of age, supported on HFNC for bronchiolitis. The secondary outcomes include the time from decision to wean HFNC to the patient no longer requiring respiratory support and a safety assessment of the weaning strategies. This may lead to shorter total length of stay in hospital, without compromising their care. Infants with bronchiolitis who are on HFNC (the devices used will be Airvo 2, Fisher \& Paykel Healthcare) will be on a flow of 2L/kg/min (maximum 20L/min) and variable oxygen concentration to maintain target oxygen saturations \>90%. The participants will be randomised into one of two arms for weaning a) titrating oxygen to FiO2 21% and then stopping HFNC b) titrating oxygen to FiO2 30% and switching HFNC to low flow oxygen.