We investigated whether
fentanyl decreases the serum concentrations of the
steroid anesthetic eltanolone effective in producing
loss of consciousness in 50% of patients (EC50induction) and in preventing movement at skin incision in 50% of patients (EC50incision). For
anesthetic induction, patients received effect-site target concentrations of
fentanyl 0.0, 1.5, 3.0, or 4.5 ng/mL and
eltanolone 500, 750, 1000, or 1200 ng/mL. Loss of response to verbal command was assessed after 10 min. For incision, patients received effect-site target concentrations of
fentanyl 0.5,1.5, 3.0, or 4.5 ng/mL and
eltanolone 547-2926 ng/mL. Movement at incision was assessed at least 10 min after new targets were entered. Probability of
loss of consciousness and of movement versus arterial serum concentration combinations were analyzed by logistic regression. Dixon up-down analysis was used to estimate ET50incision effective target concentration combinations. In the absence of
fentanyl,
anesthesia was induced in only 1 of 12 patients, which suggests that the EC50induction is >1500 ng/mL at
fentanyl 0.0 ng/mL. With
fentanyl (38 patients),
eltanolone EC50induction was independent of
fentanyl concentration, calculated as 628 ng/mL. For the incision phase (52 patients), logistic regression failed to generate a valid model. Dixon analysis (43 patients) produced an
eltanolone ET50incision of 2288 ng/mL at
fentanyl targets of 0.5 ng/mL, 754 ng/mL at 1.5 ng/mL, 735 ng/mL at 3.0 ng/mL, and 645 ng/mL at 4.5 ng/mL.
Fentanyl reduced the serum concentration of
eltanolone required to produce
loss of consciousness and the target concentration of
eltanolone required to prevent movement to skin incision.
IMPLICATIONS: