• Further studies are warranted to identify subsets of patients who are at high risk of developing CMV infection and to determine the role of antiviral agents on clinically important outcomes in critically ill patients. Heliox reduces respiratory system resistance in respiratory syncytial virus induced respiratory failure INTRODUCTION: Respiratory syncytial virus (RSV) lower respiratory tract disease is characterised by narrowing of the airways resulting in increased airway resistance, air-trapping and respiratory acidosis. These problems might be overcome using helium-oxygen gas mixture. However, the effect of mechanical ventilation with heliox in these patients is unclear. The objective of this prospective cross-over study was to determine the effects of mechanical ventilation with heliox 60/40 versus conventional gas on respiratory system resistance, air-trapping and CO2 removal. METHODS: Mechanically ventilated, sedated and paralyzed infants with proven RSV were enrolled within 24 hours after paediatric intensive care unit (PICU)admission. At T = 0, respiratory system mechanics including respiratory system compliance and resistance, and peak expiratory flow rate were measured with the AVEA ventilator. The measurements were repeated at each interval (after 30 minutes of ventilation with heliox, after 30 minutes of ventilation with nitrox and again after 30 minutes of ventilation with heliox). Indices of gas exchange (ventilation and oxygenation index) were calculated at each interval. Air-trapping (defined by relative change in end-expiratory lung volume) was determined by electrical impedance tomography (EIT) at each interval. RESULTS: Thirteen infants were enrolled. In nine, EIT measurements were performed. Mechanical ventilation with heliox significantly decreased respiratory system resistance. This was not accompanied by an improved CO2 elimination, decreased peak expiratory flow rate or decreased end-expiratory lung volume. Importantly, oxygenation remained unaltered throughout the experimental protocol. CONCLUSIONS: Respiratory system resistance is significantly decreased by mechanical ventilation with heliox (ISCRTN98152468). Respiratory syncytial virus (RSV) is the most important causative agent of lower respiratory tract disease (LRTD) in infancy [1] . Approximately 100,000 infants are annually admitted with RSV-induced bronchiolitis in the USA, and the number of hospitalizations is increasing [2] . Because of this, RSV-associated disease imposes a major burden on health care resources [3] .