Effect of treatment on isohypercapnic ventilation at 7.3 kPa or 55 mmHg (VE55). Values are relative to the baseline value at t = 0 (100%, prior to any drug intake). Values are mean ± 95% confidence interval. Placebo (green symbols), tapentadol 100 mg (blue symbols), tapentadol 150 mg (grey symbols) and oxycodone 20 mg (orange symbols). Medication was taken at t = 0.