An electrocardiogram (ECG) is a record of the electric activity of the heart. A standard ECG is produced by sensing electric potentials in six leads from the limbs and six leads from the chest.
Electric signals of the heart spread in all directions. However each standard lead can accurately represent only a small spatial sector around its axis (axes are shown as green arrows). When projected onto an imaginary sphere surrounding the heart, such a conic sector would look like a small circle or an oval.
When an ECG is taken, twelve standard ECG leads may produce normal tracings (gray ovals) while a pathologic focus (black spot) may remain unnoticed. This happens, because electric signals (red arrow) from the pathologic focus do not propagate along (are not collinear with) the axes of any of standard ECG leads and therefore their magnitude does not reach diagnostic thresholds to be properly detected. In such cases a correct diagnosis is missed.
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