 Nasal continuous positive airway pressure nCPAP, widely used in neonatal intensive care is also more and more used in infants with severe acute bronchiolitis. There is no evidence that nCPAP improves outcome, but several studies showed that it reduces work of breathing WOB, improves gas exchange and decreases intubation rate. High flow nasal prong HFNP therapy, also used in neonatal units, has recently been suggested for bronchiolitis management. This technique allows warmed and moist gas administration protecting nasal mucosa, and preventing variations of inspired gas FiO2. HFNP also creates a continuous positive pressure, reducing WOB, and seems much more comfortable and better tolerated by babies than nCPAP. There are no studies comparing both techniques for bronchiolitis management. The main objective of this study is to compare WOB with nCPAP and HFNP therapy in infants with severe bronchiolitis in pediatric intensive care unit. Secondary purpose is to compare efficacy and tolerance of both techniques. We propose to lead an open, single center, crossover randomized study. Each patient will receive the two treatments. Three consecutive periods will be studied: first one minimum 2 hours, with the first technique CPAP or HFNP according to randomization treatment 1; second period 2 hours with the second technique treatment 2; Third period 6 hours with the second technique. The primary endpoint will be the comparison of WOB estimated by the calculation of the transdiaphragmatic pressure-time product at the end of the two firsts periods. Efficacy and tolerance of each technique will be evaluated and compared by compared measuring respiratory rate, heart rate, FiO2, SpO2, transcutaneous PCO2, modified Wood score, and EDIN score during the third period. Nine subjects by groups are required. 