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An Electrocardiogram (abbreviated as either ECG or EKG) is graphic produced by an electrocardiograph, which records the changes in the electrical current in the heart during heartbeats[?] in the form of a continuous strip graph. The results of the ECG are used to tell whether the heart is performing normally or suffering from abnormalities (eg.extra or skipped heartbeats - Cardiac arrhythmia).

A typical ECG tracing of a normal heartbeat consists of a p wave, a qrs complex and a t wave. The p wave is the electrical signature of the atrial contraction[?]. Both the left and right atria contract simultaneously. The qrs complex corresponds to the contraction of the ventricle[?], which is much more forceful than that of the atria and results in a much greater ECG deflection. The t wave is the repolarization of the ventricles. Electrically, the cardiac muscle cells are like loaded springs. A small impulse sets them off, they depolarize and contract. Setting the spring up again is repolarization (more at action potential).

A typical ECG report shows the cardiac cycle from 12 different vantage points, like viewing the event elecrically from 12 different directions. Understanding the usual and abnormal directions, or vectors, of depolarization and repolarization yields important diagnostic information. The directions are known as leads. The inferior leads are II, III and aVF, the lateral leads are I and aVL. The chest, or anterior leads are V1 through V6. aVR is rarely used for diagnostic information, but indicates if the ECG leads were placed correctly on the patient. Inferior, lateral and anterior refer to portions of the heart. The inferior leads record events from the apex of the left ventricle. The lateral and anterior leads record events from the left wall and front walls of the left ventricle, respectively. The right ventricle is small and electrically weak. It leaves only a small imprint on the ECG, making it more difficult to see changes in the right ventricle on the ECG.

The device has become so familiar with its depiction in various television medical dramas[?] to the point where the reading of no cardiac electrical activity nicknamed flatline is often used as a symbol of death or at least extreme medical peril. This is technically known as asystole.

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