Modelling the cardiovascular effects of ephedrine

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Abstract
Recent reports have called into question the safety of ephedra supplements especially with regards to their cardiovascular effects. The purpose of this analysis was to characterize, via pharmacokinetic/pharmacodynamic modelling, the cardiovascular effects of ephedrine, the main active ingredient of ephedra, in apparently healthy, overweight volunteers. In a randomized, double-blind, crossover, placebo-controlled study, eight subjects received either placebo, 0.25, 0.5 or 1.0 mg kg(-1) ephedrine sulphate by mouth with a 7-day washout between treatments. Plasma ephedrine concentrations, heart rate and blood pressure were determined for 8 h postdose. The pharmacokinetics of ephedrine were best described by a one-compartment model with first-order absorption and elimination. The percentage change in heart rate was described by a linear model with a resulting slope of 0.14%.l microg(-1) (CV = 59%). The percentage change in systolic blood pressure demonstrated clockwise hysteresis, and a sigmoidal tolerance model was used to describe the data. The mean maximum predicted effect (Emax) was 53.7% (CV = 41%) with an EC50 of 107 microg.l(-1) (CV = 65%) and an inhibitory maximum (Imax) of 39.8% (CV = 60%). Tolerance developed with a mean half-life of 15 min (range 6-140 min). This is the first study to apply a comprehensive pharmacokinetic/pharmacodynamic model to the cardiovascular effects of orally administered ephedrine. Although systolic blood pressure increases quickly after administration, the increase is nearly abolished by compensatory mechanisms.