Abstract: A programmable pacemaker and program therefor are disclosed, wherein the pacer is programmed to operate in a first pacing mode whenever the measured atrial rate is less than a certain threshold switching rate R.sub.T, and to operate in a second pacing mode whenever the measured atrial rate is greater than the threshold switching rate R.sub.T. The threshold switching rate R.sub.T is varied based on either a programmed algorithm or the measured value of one or more sensed parameters. In another embodiment, a hysteresis is introduced, so that the rate at which the pacer switches back to its low-activity mode is lower than the rate that triggers a switch to the high-activity mode.

Abstract: An implantable endocardial lead with retractable fixation device. The fixation device comprises a sharpened helix which can be repeatedly both retracted within a distal end of the lead and displaced outside the lead. A threaded stylet passes through a lumen from a proximal end of the lead to the distal end of the lead, where the threaded stylet is screwed into a piston supporting the helix. When the helix is in an exposed position, torque can be transmitted from the proximal end of the lead through the distal end to the piston and thence to the helix to screw the helix into the endocardial tissue.