Fig. 2. Four ventilatory carbon dioxide response curves of one subject. The control response had a slope of 2.4 l/min per mmHg. While propofol decreased the slope to 0.8 l/min per mmHg, remifentanil caused a parallel shift to higher Pco²values of about 12 mmHg (slope = 2.2 l/min per mmHg). The combined administration yielded both a reduction in slope of the response curve (slope = 0.2 l/min per mmHg) and a right-ward shift of about 20 mmHg. These observations suggest synergy on the slope of hypercapnic response and ventilation at a fixed Petco².