In an implantable cardioverter-defibrillator and/or pacemaker, each having DDD pacing capabilities, an improved method of operation is described which dramatically increases the longevity of the implanted device by conserving battery power. The method comprises deactivating at least one unnecessary, power-consuming feature of the device until such feature is needed and then reactivating said feature only for so long as it is required by the patient. In a particular embodiment, the atrial sense amplifier is deactivated during normal operation of the implantable device, resulting in single-chamber sensing and pacing. Upon the occurrence of a predefined event, indicative of a need for dual-chamber sensing and pacing, the atrial sense amplifier is reactivated, the need for DDD pacing confirmed, and if appropriate, DDD pacing is begun. Once the patient's heart rate has returned to an acceptable level, the atrial sense amplifier is again deactivated and single-chamber sensing/pacing continued. In addition, the atrial sense amplifier of an ICD/pacemaker is deactivated during normal operation of the device and reactivated immediately following the detection of ventricular tachycardia. In this embodiment, DDD sensing/pacing is preferably automatically begun following this detection. Also contemplated herein, are improved devices employing the improved methods.