A standard twelve-lead heart tracing uses only ten electrodes
The classic hospital heart tracing is called a 12-lead ECG, yet only ten sticky pads go on the limbs and chest. A lead is not a wire but a viewpoint: the machine compares pairs of points, and even imaginary ones, to watch the heart's electrical wave from twelve angles during a recording that usually lasts ten seconds.
Several of those viewpoints use a virtual electrode. Called Wilson's central terminal, it is simply the average of the three standard limb leads, and the chest readings are measured against it. Each heartbeat starts with pacemaker cells in the sinoatrial node, spreads across the upper chambers, pauses at the atrioventricular node, then races down specialized fibres through the lower chambers. The tracing turns that sequence into three familiar shapes: the P wave as the upper chambers fire, the tall QRS complex as the lower chambers fire, and the T wave as they reset.
Before electronics, cardiographs were mechanical. Nineteenth-century instruments passed the chest's movements to a spring and air chamber, and a lever scratched them onto a smoked, rotating cylinder, picking up every other movement of the body too. After scientists found the heart's electrical activity late in that century, Willem Einthoven built a string galvanometer in 1903 that measured it precisely, winning him the 1924 Nobel Prize.
The engineering remains demanding. Signals on the skin are tiny, so machines need low-noise amplifiers and shielding, plus a right leg driver circuit to cancel the hum of 50 or 60 hertz mains power. They must also survive a defibrillator shock or static discharges up to 18,000 volts. Modern units record all leads at once and offer automatic readings, which count as preliminary because software still misreads tracings.
Recording has left the hospital bed. The portable Holter monitor appeared in 1962, later inspiring implantable loop recorders whose batteries last years, and wire-free patches now stick to the chest. Watches record a simple single lead from two electrodes, beginning with the fourth-generation Apple Watch in 2018 and followed by Samsung's Galaxy Watch 4 in 2021.
Source: Electrocardiography
