The experimental protocol started within 24 hours of PICU admission and lasted for 90 minutes. At four intervals (T = 0 (baseline), T = 30, T = 60, and T = 90 minutes) data were collected and respiratory variables measured. At T = 0 and T = 60, patients were ventilated with nitrox. At T = 30 and T = 90, patients were ventilated with heliox (helium 60%, oxygen 40%). Ventilator settings were kept constant throughout the experimental protocol.