Surrogate Measures of Opioid Potency
Because a high-resolution measure of analgesia is not available,
opioid potencies are usually estimated by some surrogate measures. Reduction of
the MAC required to produce lack of movement to skin incision has been a frequently
utilized surrogate measure in the estimation of opioid potency ( Fig.
11-18
).[62]
[78]
[79]
[80]
However
MAC is not useful as a surrogate measure of opioid potencies outside the operation
room.
The EEG has been another widely utilized surrogate measure in
estimating opioid potency.[74]
[77]
The EEG is advantageous, because it is noninvasive and can be used as an effect
measure when an experimental subject is unconscious or apneic. When processed by
Fourier spectral analysis, the raw EEG changes translate into a significant
Figure 11-18
The reduction in isoflurane concentration to prevent
movement at skin incision in 50% of patients by increasing measured remifentanil
whole blood concentrations. F represents a patient who moved and S, a patient who
did not move. The solid line is the logistic regression solution for a patient 40
years of age. (From Lang E, Kapila A, Shlugman D, et al: Reduction of isoflurane
minimal alveolar concentration by remifentanil. Anesthesiology 85:721–728,
1996.)
decrease in the value of the spectral edge, a parameter that quantitates the frequency
below which a given percentage (usually 95%) of the power in the EEG signal is found.
Although the clinical meaning of the EEG changes produced by opioids is unclear,
the opioid potencies estimated using the EEG as a surrogate measure appear to be
clinically reliable because they relate to clinically determined potencies in a proportional,
reproducible fashion. For opioids, because the surrogate measures do not always
assess the drug effect of clinical interest (analgesia), estimations of potency based
on surrogate measures must be interpreted with caution.
