An electrocardiogram, also called an ECG or EKG, is a quick, painless test that measures the heart’s electrical activity and records any disturbances in heart rhythm. It transforms the heart’s electrical activity into a tracing on paper. These tracings can help determine if the rhythm of the heart is normal and can give clues to if the heart’s chambers are enlarged or under strain. Certain patterns in the ECG may also be associated with certain types of congenital heart disease.

The Heart’s Electrical System

The individual muscle cells of the heart are capable of creating electrical impulses. Together, these individual cells generate the synchronized and rhythmic patterns that result in the coordinated pumping and filling of the heart.   Besides the muscle cells, there is specialized tissue that help the electricity pass throughout the heart. Some of this tissue acts as the initiator of the electrical impulses; other tissue acts like electrical wiring to send the impulses between the upper and lower chambers.

The tissue that generates electrical activity is called the sinus node (or sinoatrial node). The sinus node, located high in the heart’s upper right chamber (atrium), generates an electrical impulse each fraction of a second. The impulse travels through the atrial muscles, causing them to contract. It then travels to the AV node (atrioventricular), which is located between the heart’s atria (upper chambers) and the ventricles (lower chambers). The AV node is the gatekeeper that regulates which impulses are passed to the lower chambers. It is typically the only pathway for electrical impulses to travel to the ventricles.  

The AV node conducts the electrical impulses more slowly than other nodes to allow time for the ventricles to receive blood from the atria before they contract and send it out of the heart. If the electrical impulses traveling from the atria to the ventricles come early or late, the balance between blood filling the lower chambers and the timing of the heart’s contraction is disturbed. The AV node can prevent some of these extra impulses that may throw off the normal synchronized rhythm between the upper and lower chambers from going through to the lower chambers. 

The electrical activity of the heart is very elegant and efficient. Since all heart tissue is capable of creating an electrical impulse, this creates a back-up system should one part of the normal conduction system fail. This is why it is very unusual for the heart to completely stop beating suddenly. If the sinus node were to fail, other areas of the atria can pick up, typically at a slightly slower rate. If that were to fail, specialized tissue in the area between the atria and ventricles will then activate, again usually at a slightly slower rate. If that were to fail, areas within the lower pumping chambers (ventricles) can then create the rhythm.

In some people, extra tissue is present that can function to create a short circuit between the chambers. This may result in the sudden episodic development of abnormally fast heart rates. These are sometimes called accessory pathways. In others, the signals from the top chambers are not passed through to the lower chambers, resulting in what is called heart block. This sometimes will require placement of a pacemaker. Other abnormal rhythms may be generated within the individual heart chambers. These can be detected on an electrocardiogram.

How Is an ECG Performed?

An ECG may be performed at rest or during exercise. During the ECG, small, sticky patches (electrodes) are placed on different areas of the chest and body. Wires leading from the patches to a computer carry a signal that traces the heart’s electrical activity on paper or on a computer. Doctors analyze the ECG to learn more about the heart’s rhythm and condition.    

An ECG shows three “waves” of signals.

•  The “P” wave indicates the electrical impulse in the upper chambers of the heart.
•  The “QRS” wave records electrical activity in the lower chambers. 
•  The “T” wave reflects the heart’s electrical return to rest.