"It has been reported in the literature that dynamic cell models, which account for changes in intracellular ion concentrations, show a nonphysiological drift in computed parameters (Yehia et al., 1999; Endresen et al., 2000; Rappel, 2001). This drift occurs during rapid pacing for prolonged periods of time (Yehia et al., 1999; Rappel, 2001). The results of the present study establish that such drift is due to a nonconservative implementation of the stimulus, and not to an intrinsic property of the LRd model. The drift disappears when ions carried by the stimulus current are accounted for in the computation of ion concentrations. In general, when using a dynamic model, any source of charge (such as the stimulus) must also be considered a source of ions. Failure to do so violates conservation and may produce a nonphysiological behavior due to drift of model parameters. In this study, the problem is easily corrected by incorporating the stimulus current into the total K
⁺current in the LRd formulation. Assuming that other ions in the system (Na⁺ and Ca²⁺) to be the stimulus charge carrier is also consistent with the conservation principle, as long as these ions are accounted for in the formulation."