Results of study 2: (A) Plasma alfentanil concentrations (CP), (B) poikilocapnia ventilation, (C) arterial pCO₂, (D) cardiac output, (E) sedation, and (F) antinociception. B represents baseline (no drug), P1 represents low-dose alfentanil (ALF) infusion before any GAL021 or placebo infusion (ALF-low), P2 the combination of low-dose alfentanil and low-dose GAL021 or placebo (ALF-low plus GAL021-low), P3 the combination of low-dose alfentanil with high-dose GAL021 or placebo (ALF-low plus GAL021-high), and P4 the combination of high-dose alfentanil with high-dose GAL021 or placebo (ALF-high plus GAL021-high). Values are mean ± 95% CI.