I've always thought that a pacemaker worked by "jumping in" when the heart stops. I now understand that there are defibrillator versions which "jump start" the heart. However, I didn't realise that a pacemaker could actually control the heart rate. My DM had her pacemaker adjusted and her heart rate dropped by 50 bpm to a near normal rate. It's a new pacemaker which had been adjusted but could it have been over adjusted and her heart had been struggling to keep up?