Morton Mower, a cardiologist who helped invent an implantable defibrillator that has saved many lives by returning potentially fatal irregular heart rhythms to normal with an electrical jolt, died on April 25 in Denver. He was 89.

Dr. Mower and Dr. Michel Mirowski, a colleague at Sinai Hospital in Baltimore, began work in 1969 on a device that would be small enough that it could be implanted under the skin of the abdomen and quickly correct a heart’s rhythms when they go dangerously awry. “We were the crazy guys who wanted to put a time bomb in people’s chests,” Dr. Mower said in 2015 in an interview with the medical journal The Lancet, which noted at the time that two million people around the world had received the implantable device.

Dr. Mower’s work in resetting the heart’s rhythms didn’t end with the implantable defibrillator. He and Dr. Mirowski went on to invent cardiac resynchronization therapy, which uses an implantable device much like a pacemaker to send electrical impulses to the right and left ventricles of the heart in order to force them to contract in a more efficient, organized pattern.