We hypothesized that the use of heliox in mechanically ventilated infants with RSV LRTD would result in decreased respiratory system resistance (R rs ). In addition, MV with heliox would result in less air-trapping defined by the relative change in end-expiratory lung volume (EELV), and improved CO 2 clearance. The objective of our study was to test this hypothesis in a prospective, double cross-over intervention trial comparing heliox 60/40 with conventional gas (nitrox) using lung function testing and electrical impedance tomography (EIT) measurements.