Study 1 : Under isohypercapnic conditions , a separation between GAL021 and placebo on minute ventilation was observed by 6.1 ( 3.6 to 8.6 ) l/min ( P < 0.01 ) and 3.6 ( 1.5 to 5.7 ) l/min ( P < 0.01 ) at low-dose alfentanil plus high-dose GAL021 and high-dose-alfentanil plus high-dose GAL021 , respectively .