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Here is an example of an ECG:

The ECG consists of 12 views of the electrical impulse generated by the heart. The 6 views on the left half of the ECG (called I, II, III, aVR, aVL, and aVF) are generated by the electrodes on the arms and legs; the 6 views on the right half (V1 through V6) are generated by the electrodes on the chest. From these 12 views, various cardiac abnormalities can be localized to specific areas of the heart.

Each heartbeat generates a "complex" consisting of 3 parts:

The "P" wave represents the electrical impulse traveling across the atria of the heart. Abnormalities of the P wave, therefore, reflect abnormalities of the right and/or left atrium.

The QRS complex represents the electrical impulse as it travels across the ventricles. Abnormalities of the QRS are often seen when there has been prior damage to the ventricular muscle, such as in a prior myocardial infarction (heart attack.)

The "T" wave represents the recovery period of the ventricular muscle after it has been stimulated.

The portion of the ECG between the QRS complex and the T wave is called the ST segment. Abnormalities of the ST segment and the T waves are often seen when the heart muscle is ischemic - that is, when it is not getting enough oxygen, usually because there is a blockage in a coronary artery.

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