ECG lead

An ECG lead is a single electrical viewpoint of the heart — one particular angle from which the recording machine looks at the same heartbeat. It is a common point of confusion that a lead is not the same as an electrode wire: a lead is the measured difference in voltage between electrodes, and from a handful of electrodes the machine derives many leads.

Each lead measures the heart's electrical activity along its own line of sight. When the wave of activation travels toward a lead, that lead draws an upward deflection; when it travels away, it draws a downward one. Because the various leads face the heart from different directions, the very same beat looks different in each, and that is exactly what makes the multi-lead ECG informative.

Leads are grouped by the region they best survey: certain leads look mainly at the bottom wall, others at the front wall, and others at the side. By noting which leads show an abnormality, a reader can infer which part of the heart is affected — for instance, which coronary territory is involved in a heart attack. Understanding leads is therefore the foundation for localizing almost any ECG finding.