How did the earlier versions of the implantable cardioverter-defibrillator function?

Similar: The first icds required surgeons to place patches on the surface of the heart and the patches were connected by wires to the generator which was a large bulky device placed under the skin in the abdomen.

I Agree, and...: I agree with dr. Hahn's answer and would add that the earlier devices used an electrical pulse which is inferior to the one currently used. The result was that more energy was required for a successful shock and more patients could not be successfully shocked out of a dangerous rhythm on the first attempt. The earlier devices also had no ability to tell safe from dangerous rhythms.