A system for internally sensing the cardiac electrogram in a patient with a cardiac pacemaker is disclosed comprising an implanted electrode that, in combination with the metallic case of the pacemaker which acts as an indifferent electrode, senses cardiac electrical signals and provides them as an input...http://www.google.com/patents/US4387717?utm_source=gb-gplus-sharePatent US4387717 - Pacer internal cardiac electrogram sensing system
