Defibrillator electrodes, often referred to as paddles, are conductors that deliver a powerful but controlled electric shock designed to bring a heart back to its normal rhythm. They are connected by wires to a power source that can provide an appropriate charge for a given situation. Defibrillator electrodes come in numerous designs, including those that attach directly to heart muscle and others that affix to the outside of the chest.
The first defibrillators were invented around the turn of the 20th century by Swiss scientists who realized that controlled electric shocks could stop and re-start a heart. Until 1950, defibrillator electrodes could only be used directly on the heart during surgeries where the chest cavity was open. These early electrodes were metal ovoid disks roughly the diameter of a hockey puck, and worked on alternating current (AC) directly from a wall outlet.
Later in the 1950s, defibrillators were developed that used direct current (DC). These models relied on banks of capacitors that were charged up and could deliver a more controlled shock of predictable length and power. DC-powered defibrillators are still the standard design, though refinements to the actual electronic pulses have greatly decreased power consumption as well as the risk of burns and other tissue damage as the shock passes through the defibrillator electrodes.
Such advances allowed defibrillators to be far less bulky, and the first portable models entered the market in the 1960s. They were adopted quickly as standard equipment for ambulances and emergency responders, and portable defibrillators fundamentally changed the prospects for individuals with heart conditions. In cases of cardiac arrest, modern defibrillator electrodes can restore a normal heartbeat 90% of the time on the first charge.
For individuals with a history of heart problems, an implantable cardioverter-defibrillator (ICD) can be surgically inserted into the chest cavity. Similar in mechanism to the very first defibrillators, its electrodes are attached directly to the heart muscle. Complex electronics can detect irregular rhythms as well as cardiac arrest, and also automatically issue a corrective charge.
Where defibrillator electrodes are placed has a great deal to do with how effective the charge is at restoring normal rhythm. The two recommended arrangements are anterio-apical placement and anterior-posterior placement. Anterio-spical placement is preferred for external defibrillators, and anterior-posterior placement is recommended for internal devices. For implanted permanent devices, precise measurements of the heart muscle are taken to ensure optimal attachment.