Figure 3-21 Fentanyl and alfentanil arterial concentrations (circles) and electroencephalographic (EEG) response (irregular line) to an intravenous infusion. With each drug, there is a time lag between the rise and fall in arterial concentration and the EEG response, but the time lag is much greater for fentanyl than for alfentanil. (Adapted from Scott JC, Ponganis KV, Stanski DR: EEG quantitation of narcotic effect: The comparative pharmacodynamics of fentanyl and alfentanil. Anesthesiology 62:234–241, 1985.)