12-lead ECG
A 12-lead ECG is the standard, full version of the test, which looks at the heart's electrical activity from twelve different angles at once. Imagine photographing a statue from a dozen positions: each shot reveals features the others miss, and together they let you build a three-dimensional picture. Each lead is one such viewpoint of the same heartbeat.
The twelve views come from ten electrodes: six placed across the chest and four on the limbs. From these, the machine computes six limb leads that look at the heart in the vertical (frontal) plane and six chest leads that look at it in the horizontal plane. Because different leads face different walls of the heart, a problem in one region — say the front wall versus the bottom wall — shows up most clearly in the leads pointing at it.
This multi-angle approach is what makes the 12-lead ECG so powerful for locating disease. It can pinpoint which part of the heart muscle is starved of blood during a heart attack, characterise an abnormal rhythm, and reveal chamber enlargement or conduction blocks. A simpler one- or three-lead tracing, as used on a bedside monitor, watches the rhythm well but cannot map the heart in this detailed regional way.
When chest pain raises concern for a heart attack, a 12-lead ECG is usually obtained within minutes, because ST elevation in specific leads tells the team which artery is likely blocked.
The 12-lead ECG localizes injury to a heart region.
Despite the name, only ten electrodes are needed; the twelve leads are mathematical combinations of the voltages between them.