The lack of improved CO 2 clearance in our study is compatible with the observations by Gross and colleagues [11] . They were unable to demonstrate a beneficial effect on PaCO 2 of various heliox mixtures (ranging from 50%/50% to 70%/30%) compared with T = 0 (PaCO 2 45 ± 10 mmHg) in 10 mechanically ventilated infants with moderate severe RSV LRTD. It should be mentioned, however, that our study population was probably more ill than theirs based on a higher T = 0 PaCO 2 and lower PaO 2 /FiO 2 ratio. Previously, we did observe a beneficial effect of heliox in a small infant with obstructive airway Effect of mechanical ventilation with heliox on relative change in end-expiratory lung volume Effect of mechanical ventilation with heliox on relative change in endexpiratory lung volume. Data are expressed as mean ± standard deviation. Negative values indicate a decrease in relative change in end-expiratory lung volume (relative Δ EELV ). EIT = electrical impedance tomography.