Fig. 4. Pharmacodynamic curves from the models showing the effect of different combinations of remifentanil and propofol on blunting response to two different surrogate stimuli, laryngoscopy (top ) and algometry (bottom ). Each curve represents the concentration–response curve for remifentanil in combination with a fixed concentration of propofol. These curves represent the significant synergism, indicated by the leftward shift of the concentration–response curves, when the two agents are combined.