The comparative amnestic effects of midazolam, propofol, thiopental, and fentanyl at equisedative concentrations

The authors evaluated the effects of midazolam, propofol, thiopental, and fentanyl on volunteer participants' memory for words and pictures at equisedative concentrations.

Sixty-seven healthy volunteers were randomized to receive intravenous infusions of midazolam (n = 11), propofol (n = 11), thiopental (n = 10), fentanyl with ondansetron pretreatment (n = 11), ondansetron alone (n = 8), or placebo (n = 16) in a double-blind design. Three increasing and then two decreasing sedative concentrations were achieved by computer-controlled infusion in each volunteer. Measures of sedation, memory, and drug concentration were obtained at each target concentration. Drug concentrations were normalized to equisedative effects using both Emax and logistic regression methods of pharmacodynamic modeling. The serum concentrations at 50% memory effect (Cp50s) were determined using four different memory end points. The relative potencies compared with midazolam for memory impairment were determined.

Equisedative concentrations were midazolam, 64.5 +/- 9.4 ng/ml; propofol, 0.7 +/- 0.2 microg/ml; thiopental, 2.9 /- 1.0 microg/ml; and fentanyl, 0.9 +/- 0.2 ng/ml. The Cp50s for 50% loss of memory for words were midazolam, 56 +/- 4 ng/ml; propofol, 0.62 +/- 0.04 microg/ml; thiopental, 4.5 +/- 0.3 microg/ml; and fentanyl, 3.2 +/- 0.4 ng/ml. Compared with midazolam, relative potencies (with 95% confidence intervals) were propofol, 0.96 (0.44-1.78); thiopental, 0.76 (0.52-0.94); and fentanyl, 0.34 (0.05-0.76). Large effects on memory were only produced by propofol and midazolam.

At equal sedation, propofol produces the same degree of memory impairment as midazolam. Thiopental has mild memory effects whereas fentanyl has none. Ondansetron alone has no sedative or amnesic effects.