Electrode

An electrode is a conductor that makes electrical contact with a non-metallic part of a circuit, in a medical context typically the body’s tissues. Electrodes detect or deliver electrical signals and are fundamental to cardiac monitoring, diagnostic testing and rhythm management devices.

Surface electrodes are adhesive patches applied to the skin. In electrocardiography (ECG), 10 surface electrodes placed on the chest, arms and legs detect the tiny electrical signals generated by cardiac depolarisation and repolarisation, which are then displayed as the familiar waveform trace. Defibrillator pads (used with an AED or manual defibrillator) are large adhesive electrodes placed on the chest to deliver a controlled shock that can restore a normal rhythm during cardiac arrest. These external electrodes conduct a high-energy current across the heart through the chest wall.

Internal (implanted) electrodes are the sensing and pacing tips at the ends of the leads used in cardiac implantable electronic devices (CIEDs). A pacemaker lead carries an electrode at its tip that sits in contact with the inner surface of a heart chamber: the right ventricle for a standard ventricular pacemaker, and additionally the right atrium for dual-chamber devices, or the coronary sinus for cardiac resynchronisation therapy (CRT). The electrode detects the heart’s intrinsic electrical activity (sensing) and, when needed, delivers a small electrical stimulus to trigger a heartbeat (pacing). ICD leads carry the same sensing and pacing function but also include a defibrillation coil capable of delivering a high-energy shock to terminate ventricular fibrillation.

Electrode integrity is critical for device function. Electrode fracture, dislodgement, insulation failure or fibrosis at the electrode-tissue interface can all cause sensing or pacing abnormalities. Any device check or remote monitoring appointment that flags an electrode problem should be followed up promptly with the implanting centre.