Wilson Greatbatch is credited with inventing the first medical cardiac implantable pacemaker. Greatbatch was an engineer and inventor who held more than 325 patents. Incredibly, his pacemaker resulted from a mistake; he was reassembling an oscillator and mistakingly installed an incorrect resistor. The machine created a rhythmic electrical pulse, giving him the idea for an implantable pacemaker. He made a small device and tested the device on dogs. When it worked well, he developed a new version for use on humans. Today, it is estimated that 500,000 to 3 million people in the U.S. have an implantable pacemaker. There are also over 3000 different models of pacemakers available. Over seventy percent of all pacemakers are used by people 65 years or older.