Relationship of the train-of-four ratio to plasma atracurium concentration

The electromyographic response to a short infusion of atracurium 0.25 mg kg-1 was recorded using the train-of-four (TOF) technique, and the plasma atracurium concentration profile measured, in 10 healthy patients. The TOF ratio (T4:T1) was depressed over a time course which did not conform to the predictions of an effect compartment model; the fit of such a model, when possible, was associated with large residual errors. In contrast, the absolute height of the fourth response of the TOF (T4:T0) may be fitted by a standard effect compartment model with smaller errors. This residual error was reduced further on fitting a threshold effect compartment model to the data set T4:T0. The parameter values of such a model were related closely to those for the first response (T1:T0). The kinetics of the effect compartments for the first and fourth response of the TOF were similar, whilst the CPss50 for the fourth response was approximately 67% that for the first response.