Objectives. Viral bronchiolitis represents one of the main cause of hospitalization for children in developed and incoming countries. Nasal High Flow (NHF) oxygen therapy improves oxygenation and reduces respiratory drive by enhancing carbon dioxide wash-out. However, little is known about physiological effects of noninvasive helmet-Continuous Positive Airway Pressure (h-CPAP) and NHF on respiratory Work Of Breathing (WOB) in bronchiolitis. Objective of the study is to measure Esophageal Pressure Time Product*minute (PTPes*min-1) as a surrogate for WOB in acute bronchiolitis during NH at different flow rates in comparison with conventional h-CPAP. Pressure Rate Product, physiological parameters and gas exchange were considered as secondary end-points. Methods. This is a physiological randomized crossover-study comparing four 20-minute trials: oxygen therapy delivered by non-fitting mask, NHF2l/Kg, NHF3l/Kg, h-CPAP 7cmH2O. Results. Were enrolled ten children with bronchiolitis needing noninvasive respiratory support. PTPes*min-1, Respiratory Rate and Pressure Rate Product decreased progressively from h-CPAP compared to NHF3l/kg, NHF2l/kg and oxygen mask (p<0.01 for all parameters). SpO2:FiO2 increased during h-CPAP versus NHF3l/kg, NHF2l/kg and oxygen mask (p<0.01). Transcutaneous carbon dioxide tension was affected by increasing flow rate showing a progressive reduction at NHF3l/kg, NHF2l/kg and during h-CPAP compared to oxygen mask (p<0.001). Conclusions. h-CPAP was associated with a reduction in WOB and with a better oxygenation compared to oxygen mask and NHF trials; 2) NHF trials improved oxygenation and reduced the carbon dioxide tension compared to oxygen mask 3) NHF3l/kg does not offer advantages compared to NHF 2 l/kg in improving oxygenation and carbon dioxide wash-out.