The role of active monetary policy is to avoid the need for price flexibility - that is, to prevent the fiscal decision from either temporarily lowering output below its full employment level or permanently increasing prices. In an interest-rate regime, holding the funds rate unchanged following a shift in the IS curve would result in an escalating disequilibrium.
For this reason, an interest-rate regime has to be modeled in 3  BIS Review 16/2000  terms of a policy reaction function.