The Cardiac Electrical System and How the Heart Beats

The heart’s electrical system is critical to the function of the heart. The electrical system determines the heart rate (how fast the heart is beating), and also coordinates and organizes the beating of the heart muscles, so that the heart works efficiently with each heartbeat.

Abnormalities in the heart’s electrical system can lead to problems with the heart rate (too fast or too slow), or can entirely disrupt the normal functioning of the heart—even if the heart’s muscles and valves themselves are entirely normal.

Talking about the cardiac electrical system and abnormal heart rhythms can be very confusing. When we talk about heart disease, many people think of blocked coronary arteries that can result in a heart attack or the need for bypass surgery. Yet, problems with the electrical system may occur even if your heart muscle is normal.

It's helpful to picture your heart like a house, and your cardiac electrical system as the electrical wiring in your home. You may have problems with your home's wiring even if your home as a structure is completely normal. Likewise, your heart could be normal but an electrical problem may occur causing an abnormal heart rhythm.

Heart disease can lead to abnormalities in your heart's electrical system, much as a house damaged in a tornado or flood might have problems with the electrical system. In fact, damage to the electrical system of the heart is often the cause of sudden death with a heart attack, even if the damage to the heart caused by the heart attack is only mild or moderate. This is one of the reasons behind performing CPR and having access to defibrillators. If the heart rhythm can be restored, some of these heart attacks (and other causes of arrhythmias) are survivable.

Let's take a look at how the cardiac electrical system works to make your heart beat, as well as medical conditions which can affect your pulse.

Introduction to the Cardiac Electrical Signal

cardiac anatomy showing the electrical system of the heart — Encyclopaedia Britannica/UIG/Getty Images

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.

The Components of the Cardiac Electrical Signal

cardiac anatomy showing sinus node and chambers — Fogoros

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)

The Cardiac Electrical Signal Spreads Across the Atria

atrial depolarization, when the electrical impulse spreads across the atria — Fogoros

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).

Sinus bradycardia ("brady" means slow) is the most common cause of a low heart rate and is caused by the SA node firing at a reduced rate. Sinus tachycardia ("tachy" means fast) refers to a rapid heart rate and can be caused by the SA node firing at an increased rate.

The Cardiac Electrical Signal Reaches the AV Node

heart's electrical system showing the PR interval — Fogoros

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.

The Cardiac Electrical Signal Passes to the Ventricles

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. In fact, a left bundle branch from a heart attack is an important cause of sudden cardiac death.

The Cardiac Electrical Signal Spreads Across the Ventricles

cardiac electrical system during ventricular contraction — Fogoros

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.