Electrocardiogram

Overview

An electrocardiogram (ECG or EKG) is a quick, painless test that measures the heart’s electrical activity and records any disturbances in heart rhythm in both children and adults. The heart’s electrical activity determines if it keeps a normal rhythm. An ECG shows three “waves” of signals.

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

The shape and size of the waves, the time between each wave, and the rate and regularity of beating provide valuable information to doctors. In addition to providing insight into the heart’s rhythm, the ECG helps doctors with the following:

• Determine the size of the heart chambers
• Detect heart muscle damage
• Identify abnormal levels of certain electrolytes, such as potassium and calcium, in the blood

The heart’s electrical system

Specialized tissues within the heart are capable of generating electrical impulses. These impulses cause the heart muscles to contract. With each contraction, the heart sends blood out to the lungs to pick up oxygen or to the body to deliver oxygen to the cells.

The tissues that generate electrical activity are called the sinus node (or sinoatrial node). The sinus node, located 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 (atrioventricular) node, which is located between the (upper chambers) and the ventricles (lower chambers). The AV atria node is 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 result is reduced cardiac output—or a reduced amount of blood ejected from the heart.