EIT is a non-invasive bedside technique to assess global and regional lung volumes that has primarily been used in acute lung injury or ARDS [35] . Hinz and colleagues have shown that compared with the validated nitrogen-washout method it is an appropriate tool to study EELV in critically ill patients [16] . To our knowledge, the use of EIT in the determination of the dynamic process of air-trapping in patients with small airway disease has not been used before, although its use in this disease condition can be rationalised. In our study, MV with heliox did not result in a universal reduction of air-trapping as defined by the relative Δ EELV . However, there were some patients who seemed to benefit from MV with heliox as they did show a reduction in relative Δ EELV that was reversed by MV with conventional gas. Several explanations for the non-universal reduction in relative Δ EELV may be proposed. First, not all alveoli have the same degree of hyperinflation due to the difference in time constants throughout the lung, indicating that hyperinflation is a regional phenomenon rather than a global problem [36] . This would implicate that the technique of EIT may be insufficient to detect regional differences in viralinduced small airway disease due to heterogeneity of the disease, a problem that can be overcome by increasing the reso-lution of the EIT signal. In favor of EIT, however, is the study by Adler and colleagues showing that with EIT dynamic hyperinflation could be adequately monitored [37] . Second, during the study no endotracheal suctioning was performed. Increased mucus production could obstruct the airways, resulting in the collapse of alveoli that is reflected by a decrease in EELV. As tidal volume remained constant throughout the experiment, we think that not performing endotracheal suctioning did not influence our results ( Figure 5 ). Third, if there is a difference in expression of clinical phenotype of RSV LRTD a universal response in relative Δ EELV would not be expected. Some patients responded with a decrease in relative Δ EELV whereas others did not in our study. Also, redistribution of ventilation within each lung or between the left and right lung was not significantly influenced by MV with heliox. This is in line with a heterogeneous clinical phenotype of RSV LRTD.