Effect of mechanical ventilation with heliox on static compliance Effect of mechanical ventilation with heliox on static compliance. Data are expressed as mean ± standard deviation. ever, that the resolution of the AVEA's signal of R lung (1 decimal) might not be sufficient enough to detect true differences in R lung in small children with little tidal volume. Second, the measured R rs in our patients is lower than previously reported in mechanically ventilated infants with RSV LRTD designated to have an obstructive disease phenotype [20, 22, 27] . This could indicate that our patients had mild-to-moderate airway obstruction, although hyperinflation suggesting airway obstruction on chest radiograph was present in all but one patient. Unfortunately, there is no gold standard for the radiological definition of hyperinflation especially in mechanically ventilated infants. Furthermore, the degree of air-trapping might vary between patients, indicating that severe RSV LRTD necessitating MV is a heterogeneous disease in which patients express to a varying degree both restrictive and obstructive disease characteristics explaining why some patients had a PaO 2 /FiO 2 ratio of less than 200 or a C stat less than 0.3 ml/cmH 2 O/kg in our study. This assumption opposes the previously proposed dichotomization of RSV LRTD by Hammer and colleagues, who have observed that mechanically ventilated infants with RSV LRTD showed either a disease pattern compatible with acute respiratory distress syndrome (ARDS) or a disease pattern characterized by increased airway resistance [27] . Although our study was not designed to investigate differences in clinical phenotype, we would dare to challenge this dichotomy in clinical phenotype for several reasons. Hammer and colleagues included prematurely born infants with chronic lung disease and infants with congenital heart disease [27] . C rs is significantly lower in these patients compared with healthy infants [28] [29] [30] . In addition, the term 'bronchiolitis' to describe RSV LRTD is strictly speaking a histopathologic diagnosis and hampered by universal differences in its clinical interpretation [31] . Controversy exists about whether differences in parameters for gas exchange correlate with clinical phenotype [32, 33] .