The mean relative Δ EELV ± standard deviation at T = 0 was 76.6 ± 15.1 ml. With an estimated reduction of 25% with heliox, nine patients were needed to recruit in order to detect a statistically significant difference with α 0.05 and β 0.90. The degree of airtrapping as defined by the relative Δ EELV in ml was overall not significantly reduced by heliox (P = 0.493; Figure  6 ). This was due to differences in response to MV with heliox. Five patients showed a reduction in relative Δ EELV when heliox was introduced, and when conventional gas was reintroduced relative Δ EELV increased in only three patients (Table 2 ). There were also patients who had an increase of relative Δ EELV with heliox that was either reversed or increased when conventional gas was reintroduced.