However, the
CO2 effect in both studies (Hassanzadeh et al., 2002;
Everaert et al., 2008) might be confounded with EST
because the CO2 concentration of 0.4% (Hassanzadeh
et al., 2002) and the CO2 concentration of 4.0% (Everaert
et al., 2008) were both reached by decreasing the
ventilation rate