Electrical pacing of the heart has been utilized in human medicine for more than two decades. Cardiac pacing was used initially for cases of complete heart block that were unresponsive to medical therapy. During the 14-year period following the first human implant in 1958, an estimated 120,000 units were implanted with 60% of patients being over 65 years old and with a 50-60% survival rate five years after implantation.