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Electrocardiography i-!lek-trO-+k@r-dE-u-!graf

by Michael J. Shea, MD

Electrocardiography (ECG) is a quick, simple, painless procedure in which the heart’s electrical impulses are amplified and recorded on a piece of paper. This record, the electrocardiogram (also known as an ECG), provides information about the part of the heart that triggers each heartbeat (the pacemaker, called the sinoatrial or sinus node), the nerve conduction pathways of the heart, and the rate and rhythm of the heart.

Usually, an ECG is obtained if a heart disorder is suspected. It is also obtained as part of a routine physical examination for most middle-aged and older people, even if they have no evidence of a heart disorder. It can be used as a basis of comparison with later ECGs if a heart disorder develops.

To obtain an ECG, an examiner places electrodes (small round sensors that stick to the skin) on the person's arms, legs, and chest. These electrodes measure the magnitude and direction of electrical currents in the heart during each heartbeat. The electrodes are connected by wires to a machine, which produces a record (tracing) for each electrode. Each tracing shows the electrical activity of the heart from different angles. The tracings constitute the ECG. ECG takes about 3 minutes, is painless, and has no risks.

ECG: Reading the Waves

An electrocardiogram (ECG) represents the electrical current moving through the heart during a heartbeat. The current's movement is divided into parts, and each part is given an alphabetic designation in the ECG.

Each heartbeat begins with an impulse from the heart's pacemaker (sinus or sinoatrial node). This impulse activates the upper chambers of the heart (atria). The P wave represents activation of the atria.

Next, the electrical current flows down to the lower chambers of the heart (ventricles). The QRS complex represents activation of the ventricles.

The electrical current then spreads back over the ventricles in the opposite direction. This activity is called the recovery wave, which is represented by the T wave.

Many kinds of abnormalities can often be seen on an ECG. They include a previous heart attack (myocardial infarction), an abnormal heart rhythm (arrhythmia), an inadequate supply of blood and oxygen to the heart (ischemia), and excessive thickening (hypertrophy) of the heart's muscular walls.

Certain abnormalities seen on an ECG can also suggest bulges (aneurysms) that develop in weak areas of the heart's walls. Aneurysms may result from a heart attack. If the rhythm is abnormal (too fast, too slow, or irregular), the ECG may also indicate where in the heart the abnormal rhythm starts. Such information helps doctors begin to determine the cause.

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