Schematic representation of the pharmacokinetic–pharmacodynamic model used to describe the effect of ropivacaine on current perception and pain thresholds after a femoral nerve block in orthopedic patients. A_d = amount in the depot compartment; A_e = amount in the effect-site compartment; CI = clearance; C_p = ropivacaine plasma concentrations; CV² = variance of the input time distribution; F₁ = fraction estimated for the fast input (I(t)_fast); γ = shape factor; k_a = first-order absorption rate constant; k_e0 = first-order depot-effect site equilibrium rate constant; MAT = mean absorption time for the fast input rate; t = time after dosing; t₅₀ = time to achieve 50% of the maximum input rate; V = volume of distribution.