Apparatus is disclosed for sensing and transmitting the stimulating pulse of an implanted pacemaker over readily available, low-bandwidth transmission media to a remote receiver capable of measuring the pulse width to provide an indication of the condition of the pacemaker energy source, e.g. a battery....http://www.google.de/patents/US3986496?utm_source=gb-gplus-sharePatent US3986496 - Apparatus for sensing and transmitting a pacemaker's stimulating pulse
