For a study, researchers sought to conduct a prospective observational pilot study in newborns admitted to a paediatric intensive care unit. At admission, 24 hours, and 48 hours, diaphragmatic excursion (dExc), diaphragmatic inspiratory/expiratory time, and diaphragmatic thickening fraction (dTF) were measured in both hemidiaphragms. A total of 56 ultrasonographic assessments were performed on 26 individuals (14 on HFNC and 12 on NIV). Invasive ventilation was required in 3 of the individuals. About 64% of the HFNC participants needed NIV as rescue therapy, and 2/14 needed invasive ventilation (14.2%). There were no variations in dExc between individuals who needed NIV or invasive ventilation and those who didn’t in the HFNC group. Subjects on HFNC who needed invasive ventilation had a larger left dTF than those who needed NIV (left dTF 47% vs 22% [13–30]; P=.046, r=0.7). Diaphragmatic I: E ratios were larger, and diaphragmatic expiratory duration was shorter in infants on HFNC who required invasive ventilation (left P=.038; right P=.02). There were no variations in dExc, I: E ratios, or dTF between participants who needed invasive ventilation and those who didn’t in the NIV group. A clinical work of breathing score and echographic dTF had no association. The use of ultrasonographic left dTF in babies with moderate or severe bronchiolitis receiving HFNC could assist predict respiratory therapy failure and the requirement for invasive ventilation. The use of ultrasonographic dExc to forecast both was ineffective.