Figure 4. Assessment of alertness by subjects using a visual analog scale (top), sedation by an observer (middle), and performance on digit symbol substitution test (bottom). Mean values (SE) are plotted versus plasma propofol concentrations for propofol and propofol/alfentanil treatments. For the alfentanil treatment, values have been aligned with the results of measurements made at comparable times during the propofol/ alfentanil treatment. Circles = treatment with propofol; squares = treatment with alfentanil/propofol; triangies = treatment with alfentanil.