Temporal delay of the current perception threshold (CPT) in two representative patients having received a single injection (20 ml) of 0.5% ropivacaine for femoral nerve block. Counterclockwise hysteresis loop of the estimated amount in the depot compartment versus CPTs (full lines; A). The arrows indicate the time course. Corresponding CPT versus estimated effect-site ropivacaine amount and model fit (broken lines; B).