Clinical endpoints for inhalation anesthetics with and without fentanyl administration

The quantal concentration-response curves of the steady-state end-tidal partial pressure (F_ET) of volatile anesthetics. All curves are meant to explain concepts and are not directly based on patient data, except when specifically mentioned.

(A) As the concentration (expressed as a fraction of minimum alveolar concentration [fMAC]) is increased, the probability of rendering patients unconscious (blue), immobile (green), and blunting autonomic responses (BAR) (red) increases and reaches 50% at MAC_awake, minimum alveolar concentration (MAC), and MAC_BAR, respectively (colored circles on the x-axis).

(B) Effect of a 2-ng/ml effect-site fentanyl concentration on fMAC required to ensure the three different clinical endpoints. Opioids do not reduce MAC (a fixed point in the x-axis) but reduce the steady-state FET (or fMAC) needed to attain the same no-response probability (Pno-response). This graph is adapted from data by Katoh et al., 30–32 with the percentages being only approximate changes used for illustrative purposes.

(C) We propose that anesthesia requires a 99.99% or greater, 95%, and 85% probability of unconsciousness, immobility, and blunting of adrenergic responses, respectively. The corresponding required steady-state fMAC can be back-extrapolated to the x-axis.

(D) If a 2-ng/ml effect site fentanyl (or equivalent) concentration is maintained, 0.7 fMAC can ensure that all three clinical goals are achieved. Although based on patient data (in the work of Katoh et al. 30–32), this cartoon only illustrates the general concept.