Mechanical ventilation with heliox had an overall significant effect on R rs (P < 0.001; Figure 2 ). R rs decreased from 69.1 ± 6.9 cmH 2 O/L/sec at T = 0 to 50.2 ± 6.0 cmH 2 O/L/sec (P = 0.020) after 30 minutes of ventilation with heliox. After reintroduction of nitrox, R rs increased significantly to 70.7 ± 7.2 cmH 2 O/L/sec (P = 0.016) but decreased again to 42.9 ± 3.3 cmH 2 O/L/sec (P = 0.001) when heliox was reintroduced.