The decrease in R rs led not to an improved CO 2 clearance as defined by the VI or a reduction in PEFR. Some explanations for this may be proposed. First, it is uncertain how much of the observed reduction in R rs could be partitioned to the ventilator circuit or the endotracheal tube because no endotracheal suctioning was performed during the study. Increased mucus production during RSV LRTD is common, and may further obstruct the airways [26] . As the AVEA ventilator is able to calculate the R lung , we also studied if MV with heliox resulted in a reduction in R lung , but were unable to demonstrate this. This could mean that MV with heliox does not affect the resistance of the small airways of the infants; it cannot be ruled out, how-