If the drugs dissociate slowly from the sodium channel, then only a relatively small fraction of the drug will leave during diastole and with each subsequent impulse, progressively more drug (shown here in red) will remain bound to the channel until equilibrium is reached. Thus, the effects of the drug on the sodium inward current, on the maximum rate of rise of the action potential upstroke (dv/dtmax) and on the QRS complex will be more pronounced at rapid than at slow rates.