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Electrocardiogram (ECG) (cont.)

Heart Function and the ECG

Electrode leads on the chest wall are able to detect electrical impulses that are generated by the heart. Multiple leads provide many electrical views of the heart. By interpreting the tracing, the physician can learn about the heart rate and rhythm as well as blood flow to the ventricles (indirectly).

Rate refers to how fast the heart beats. Normally, the SA node generates an electrical impulse 50-100 times per minute. Bradycardia (brady=slow+cardia=heart) describes a heart rate less than 50 beats per minute. Tachycardia (tachy=fast+cardia=heart) describes a heart rate faster than 100 beats per minute.

Rhythm refers to the type of heartbeat. Normally, the heart beats in a sinus rhythm with each electrical impulse generated by the SA node resulting in a ventricular contraction, or heartbeat. There are a variety of abnormal electrical rhythms, some are normal variants and some are potentially dangerous. Some electrical rhythms do not generate a heartbeat and are the cause of sudden death.

Rhythm strip showing a normal 12-lead ECG.

Examples of heart rhythms include:

There can also be delays in transmission of the electrical impulse anywhere in the system, including the SA node, the atria, the AV node, or in the ventricles. Some aberrant impulses cause normal variants of the heart rhythm and others can be potentially life threatening. Some examples include:

  • 1st degree AV block
  • 2nd degree AV block, type I (Wenckebach)
  • 2nd degree AV block, type II
  • 3rd degree AV block or complete heart block
  • Right bundle branch block
  • Left bundle branch block

There can also be short circuits that can lead to abnormal electrical pathways in the heart causing abnormalities of rate and rhythm. Wolfe-Parkinson-White (WPW) syndrome is a condition where an abnormal accessory pathway at the AV node can cause tachycardia.

The ECG tracing can also provide information about whether the heart muscle cells are conducting electricity appropriately. By analyzing the shape of the electrical waves, the physician may be able to determine if there is decreased blood flow to parts of the heart muscle. The presence of an acute blockage associated with a myocardial infarction or heart attack can be determined as well. That's one of the reasons that an ECG is done as soon as possible when a patient presents with chest pain.

Medically Reviewed by a Doctor on 12/4/2014

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