Figure 3-39 Influence of alfentanil on the concentration of propofol associated with a 50% probability of response to intubation and incision, as well as a 50% probability of awakening at the end of surgery. Moderate doses of alfentanil greatly reduce the dose of propofol required to ablate the response to noxious stimulation. However, below propofol concentrations of approximately 2 µg/mL, even large doses of alfentanil cannot reliably ablate the response to noxious stimulation. (Adapted from Vuyk J, Lim T, Engbers FH, et al: The pharmacodynamic interaction of propofol and alfentanil during lower abdominal surgery in women. Anesthesiology 83:8–22, 1995.)