Examples of the effect of placebo (A), tapentadol 100 mg (B), tapentadol 150 mg (C) and oxycodone 20 mg (D) on the hypercapnic ventilatory response curve (data are from a single subject). Each circle is a 1-min average; the lines through the data points are the linear regression curves. The horizontal part of the curves denote data without any added inspired CO₂. The orange vertical lines (x = 7.3 kPa or 55 mmHg) intersect with the regression lines at VE55.