What Is an Electrocardiogram?

An electrocardiogram (ECG or EKG) is one of the simplest and fastest procedures used to evaluate the heart. An EKG records the electrical activity of the heart, shows abnormal rhythms (arrhythmias or dysrhythmias), and detects heart muscle damage. An EKG may be conducted in several ways and used in combination with other tests, such as a stress or exercise test, to evaluate heart function and rhythms.

Your physician uses the EKG to determine the heart rhythm and information about the heart’s electrical signals.

Electrodes (small, plastic patches) are placed at certain locations on the chest, arms, and legs. When the electrodes are connected to an EKG machine by lead wires, the electrical activity of the heart is measured, interpreted, and printed out for the physician's information and further interpretation.

How an EKG works

The EKG 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 EKG, 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).

Related procedures

Other related procedures that may be used to assess the heart include: