Stanford Cardiac Arrhythmia Service


An Electrocardiogram (ECG or sometimes called an EKG) is the most commonly used test to diagnose arrhythmias. It’s a simple non invasive test that detects and records the heart’s electrical activity, showing how fast it’s beating and its rhythm (regular or irregular). The ECG also records the strength and timing of electrical signals as they pass through each part of the heart. This information from the ECG may be used to make the diagnosis of a heart rhythm problem.  The information gathered from this simple noninvasive test may sometimes provide evidence of increased thickness of the heart muscle, damage to the various parts of the heart muscle, or enlargement of the heart chambers.  It can also help your doctor determine If you are likely to develop disturbances of the heart rhythm.

What to expect?  You may be asked to remove all jewelry from your neck and wrists. Prior to having an ECG you may want to avoid putting on skin cream or oil since it tends to interfere with obtaining a good ECG recording. Men are usually bare-chested during the test. Women are offered a gown. If you are wearing stockings, you should take them off. You will lie on a bed or table. Small, sticky patches (electrodes) are placed on your chest, wrists and ankles. Areas on your arms, legs, and chest where the electrodes will be placed are cleaned and may be shaved to provide a good contact with your skin.The electrodes are connected to a machine that monitors and records your heart’s electrical activity from 12 different views. You’ll be asked to lie quietly while the ECG is recording. The test, which takes approximately 10 minutes, allows your physician to see your “baseline ECG” and determine if there’s any abnormal conduction through your heart while you’re at rest.

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Last Updated: March 26, 2008
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Your physician uses the ECG to determine the heart rhythm and information about the heart’s electrical signals. 
The ECG records the electrical signals as they move through the heart.  In normal rhythm the electrical signals starts at the heart’s own pacemaker, called the sinus node.  The electrical signal first spreads through the heart’s upper chambers called the atria.  The electrical signal spreading through the atria produce a noticeable bump or deflection called the “P wave”. 

Next, the normal signals travel from the heart’s top chambers, the atria (plural of atrium), to the lower chambers by traveling through a connector or staircase called the A-V node.  The time the electrical impulse takes to travel through the A-V node make up the major part of the period of time to go from the atrium to the ventricle and is measured as the “P-R interval”.  In some conditions, this time may be increased.   In some situations, some of the signals do not make it from the atrium to ventricle, a condition called A-V block or heart block.

Next the impulse travels to the lower heart chambers, the ventricles. On the ECG, this produces a signal called the QRS complex.  The time that the electrical signals take to reach all parts of the ventricles determines the width of the QRS complex.  In some cases, an extremely wide QRS complex can result in a decreased efficiency of the heart’s electrical system.

One the ventricles have been activated, the ventricles start to get ready to be activated again. This process called repolarization produces a deflection called the T wave.  The time period from the beginning of the QRS complex to the end of the T wave is known as the QT interval.  The QT interval may be prolonged due to a variety of conditions including the effect of medications, heart muscle abnormalities, heart attack, or genetic conditions that may lead to serious lower chamber fast rhythms called ventricular tachycardia (Long QT syndrome).

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