Flow veRsus OxygeNaTion In acutE ReSpiratory Failure

The study will recruit a total of between 180 and 270 patients across three hospital sites mainly from high dependence units. The patients recruited will be stratified into 3 groups according to their oxygen (O2) requirements at recruitment (low, medium and high). All recruited patients will then be given supplementary O2 using the Optiflow system. Patients from all three recruitment oxygen requirements (low, medium and high) will be randomised to 30 litres, 45 litres or 60 litres per minute with initial a fractional inspired O2 concentration(FiO2) of 0.90 (90% oxygen). The FiO2 is then decreased in 0.05 increments every 5 minutes and the minimum oxygen saturation recorded until it reaches 94%. End points: Minimal FiO2 required to maintain saturation above 94% for patients with low, medium and high recruitment oxygen requirements for 30 litres, 45 litres and 60 litres per minute. Use Kruskal Wallis to compare the median FiO2 requirements of those patients who recruited with low, medium and high O2 requirement. Obtain dose response curve 02 saturation against FiO2 for 3 different flow rates for each of the recruitment 02 requirements and use regression modelling to find best fit dose FiO2 response O2 saturation curves Patient satisfaction scores and number of patients unable to tolerate the three flow rates in each of the three starting groups and flow rates.