The Cardiac Electrical System and How the Heart Beats

The heart generates its own electrical signal (also called an electrical impulse), which can be recorded by placing electrodes on the chest. This is called an electrocardiogram (ECG, or EKG).

The cardiac electrical signal controls the heartbeat in two ways. First, since each electrical impulse generates one heartbeat, the number of electrical impulses determines the heart rate. And second, as the electrical signal "spreads" across the heart, it triggers the heart muscle to contract in the correct sequence, thus coordinating each heartbeat and assuring that the heart works as efficiently as possible.

The heart's electrical signal is produced by a tiny structure known as the sinus node, which is located in the upper portion of the right atrium. (The anatomy of the heart's chambers and valves includes two atria at the top of the heart with two ventricles at the bottom.)

From the sinus node, the electrical signal spreads across the right atrium and the left atrium (the top two chambers of the heart), causing both atria to contract, and to push their load of blood into the right and left ventricles (the bottom two chambers of the heart). The electrical signal then passes through the AV node to the ventricles, where it causes the ventricles to contract in turn.

Figure 1: The components of the heart's electrical system including the sinus node (SN) and atrioventricular node (AV node) are illustrated here. From an electrical standpoint, the heart can be thought of as being divided into two portions: the atria (upper chambers), and the ventricles (lower chambers). Separating the atria from the ventricles is a fibrous "disc". This disc (labeled AV disk in the figure), prevents the passage of the electrical signal between the atria and the ventricles. The only way the signal can get from the atria to the ventricles is through the AV node.

In this figure:

• SN = sinus node
• AVN = AV node
• RA = right atrium
• LA = left atrium
• RV = right ventricle
• LV = left ventricle
• TV = tricuspid valve (the valve that separates the right atrium from the right ventricle)
• MV = mitral valve (the valve that separates the left atrium from the left ventricle)

Figure 2: The electrical impulse originates in the sinus node. From there, it spreads across both atria (indicated by the blue lines in the picture), causing the atria to contract. This is referred to as "atrial depolarization."

As the electrical impulse passes through the atria, it generates the so-called "P" wave on the ECG. (The P wave is indicated by the solid red line on the ECG off to the left side).

Figure 3: When the wave of electricity reaches the AV disc, it is stopped, except in the AV node. The impulse travels through the AV node only slowly. The solid red line on the ECG in this figure indicates the PR interval.

Figure 4: The specialized AV conduction system consists of the AV node (AVN), the "His bundle," and the right and left bundle branches (RBB and LBB). The AV node conducts the electrical impulse very slowly and passes it to the His bundle (pronounced "hiss"). The His bundle penetrates the AV disk, and passes the signal to the right and left bundle branches. The right and left bundle branches, in turn, send the electrical impulse to the right and left ventricles, respectively. The figure also shows that the LBB itself splits into the left anterior fascicle (LAF) and the left posterior fascicle (LPF).

Because the impulse travels only very slowly through the AV node, there is a pause in the electrical activity on the ECG, referred to as the PR interval. (The PR interval is illustrated on the ECG in Figure 3.) This "pause" in the action allows the atria to contract fully, emptying their blood into the ventricles before the ventricles begin to contract.

Problems anywhere along this route from the AV node can cause abnormalities in the ECG (and heart rhythm). 

AV block (heart block) is one of the two major causes of a low heart rate (bradycardia). There are different degrees, with third-degree heart block the most severe and usually requiring a pacemaker.

Bundle brain block occurs in either the right bundle branch or left bundle branch, with those in the left bundle branch usually most serious. Bundle branch blocks may occur for no apparent reason but often occur when the heart is damaged due to a heart attack or other cardiac conditions.

Figure 5: This figure shows the electrical impulse spreading throughout the right and left ventricles, causing these chambers to contract. As the electrical signal travels through the ventricles, it generates the “QRS complex” on the ECG. The QRS complex is indicated by the solid red line on the ECG below.

In this manner, the electrical system of the heart causes the heart muscle to contract and send blood to either all the organs of the body (via the left ventricle) or to the lungs (via the right ventricle).

Bottom Line

From the initiation of a heartbeat in the SA node, through contraction of the ventricles, the cardiac electrical system causes the heart to contract in a coordinated manner, maximizing the efficiency of the beating heart.