A device such as a cardiac pacemaker or other implantable biomedical device, and method of operation thereof, containing one or more switchable circuits. A control system, preferably incorporating a microprocessor, controls predetermined changes of the operating characteristics of the switchable circuits. In a cardiac pacer embodiment, a circuit such as the input amplifier can be switched to have different characteristics, e.g. filter setting and sensitivity, during predetermined portions of the pacer cycle. Further, the operation of a circuit can be monitored over a plurality of operating cycles, with controlled switching of the circuit characteristics as a function of cumulative monitored circuit performance. A specific example of the invention is shown for changing characteristics of a filter circuit used in a pacer which adjusts the pacer rate as a function of determined stimulus-T wave time interval.