Cardiac Conduction System

What is the cardiac conduction system?

Your heart’s conduction system is the network of nodes (groups of cells that can be either nerve or muscle tissue), specialized cells and electrical signals that keep your heart beating.

Two types of cells control your heartbeat:

• Conducting cells carry the electric signals.
• Muscle cells control your heart’s contractions.

Signals tell your heart when to pump blood through your body.

What does the cardiac conduction system do?

Your heart (cardiac) conduction system sends the signal to start a heartbeat. It also sends signals that tell different parts of your heart to relax and contract (squeeze). This process of contracting and relaxing controls blood flow through your heart and to the rest of your body.

Ideally, the electrical conduction system keeps up a steady, even heart rate. It also helps your heart speed up when you need more blood and oxygen or slow down when it’s time to rest.

Steps of cardiac conduction

For each heartbeat, electrical signals travel through the conduction pathway of your heart. It starts when your sinoatrial (SA) node creates an excitation signal. This electrical signal is like electricity traveling through wires to an appliance in your home.

The excitation signal travels to:

1. Your atria (top heart chambers), telling them to contract
2. The atrioventricular (AV) node, delaying the signal until your atria are empty of blood
3. The bundle of His (center bundle of nerve fibers), carrying the signal to the Purkinje fibers
4. The Purkinje fibers to your ventricles (bottom heart chambers), which causes them to contract and deliver blood out to your body

These steps make up one full contraction of your heart muscle. Your heart’s electrical conduction system sends out thousands of signals per day to keep your heart beating.

What are the parts of your heart’s conduction system?

The conduction system in your heart contains specialized cells and nodes that control your heartbeat. These are the:

• Sinoatrial node
• Atrioventricular node
• Bundle of His (atrioventricular bundle)
• Purkinje fibers

Sinoatrial node

Your sinoatrial (SA) node is your heart’s natural pacemaker. It sends the electrical impulses that start your heartbeat. When your sinoatrial node isn’t working well, the lower segments of your conduction system act as backup pacemaker cells.

The SA node is in the upper part of your heart’s right atrium. It’s at the edge of your atrium near your superior vena cava (a large vein that brings oxygen-poor blood from your body to your heart).

Your autonomic nervous system controls how quickly or slowly your SA node sends electrical signals. This part of your nervous system directs hormones that control your heart rate based on what you’re doing. For example, your heart rate increases during physical activity and slows when you’re asleep.

The autonomic nervous system includes your:

• Sympathetic nervous system (fight or flight response), which makes your SA node work faster and increases your heart rate
• Parasympathetic nervous system (rest and digest response), which makes your SA node work slower and decreases your heart rate

Atrioventricular node

Your atrioventricular (AV) node, located near the central area of your heart, delays the SA node’s electrical signal. It delays the signal by a consistent amount of time (a fraction of a second) each time.

The delay ensures that your atria (upper heart chambers) are empty before the contraction stops. Your atria receive blood from your body and empty it into your ventricles (lower heart chambers).

Bundle of His (atrioventricular bundle)

The bundle of His (sounds like “hiss”) is a branch of fibers (nerve cells) that extends from your AV node. This fiber bundle receives the electrical signal from the AV node and carries it to the Purkinje fibers.

The bundle of His runs down the length of the septum (wall) that separates your right and left ventricles. The atrioventricular bundle has two branches:

• Left bundle branch sends electrical signals through the Purkinje fibers to your left ventricle.
• Right bundle branch sends electrical signals through the Purkinje fibers to your right ventricle.

Purkinje fibers

The Purkinje fibers are branches of specialized nerve cells. They send electrical signals very quickly to your heart’s right and left ventricles. Purkinje fibers are in your ventricle walls in the inner layer of tissue that lines your heart’s chambers.

When the Purkinje fibers deliver electrical signals to your ventricles, the ventricles contract. Blood flows from your right ventricle to your pulmonary arteries and from your left ventricle to your aorta. Your pulmonary arteries take blood to your lungs to get oxygen. Your aorta, your body’s largest artery, sends blood from your heart to the rest of your body.

What does the cardiac conduction system look like?

The sinoatrial node is about 15 millimeters (mm) long (about as long as a staple) and 4 mm wide. It looks like a key, with a head and a smaller, jagged lower part. The AV node is about 5 mm long and 5 mm wide. This is about the size of a light switch tip, but the AV node looks like a spindle. The Bundle of His is 20 mm long and about 4 mm around.

Together, the Bundle of His, its branches and the Purkinje fibers look like an upside-down tree. The Bundle of His and its branches are like a tree trunk. The Purkinje fibers spread upward and outward to look like a tree’s canopy.

What conditions and disorders affect your cardiac conduction system?

Several different conditions can affect your heart’s electrical system. Cardiac conduction problems cause issues with your heart’s rhythm.

Some common heart rhythm disorders include:

• Arrhythmia. An irregular heart rhythm, including atrial fibrillation (Afib).
• Bundle branch block. A block in the Purkinje fibers on one side of your heart, causing arrhythmia.
• Heart block. Impaired electrical signals between your heart’s atria and ventricles.
• Long QT syndrome (LQTS). Your ventricles contract and release too slowly, sometimes leading to fainting (syncope) or cardiac arrest.
• Premature ventricular contractions. A too-early heartbeat in your ventricles, causing heart palpitations or a “skipped heartbeat.”
• Cardiac arrest. A severe malfunction in your heart’s rhythm that causes your heart to stop. Without immediate treatment, this is fatal.

Signs/symptoms of a problem

If you have an issue with your cardiac conduction system, you may experience:

• Shortness of breath
• Chest pain
• Heart palpitations
• Dizziness
• Fainting
• Fatigue

You should seek emergency medical care if you have these symptoms. A healthcare provider can use an electrocardiogram (EKG) to check your heart rhythm. If they want to look at your heart’s electrical activity longer, your provider may ask you to wear a heart monitor for a few days or weeks. An electrophysiology study can give even more information about your cardiac conduction system.

Medicines and procedures like a pacemaker placement can treat issues with your cardiac electrical system.

How can I keep my heart conduction system healthy?

You can’t change the genetics that cause many problems with heart rhythm. But you can help keep your entire heart well with healthy habits. You can:

• Reach and maintain a weight that’s healthy for you
• Stop using tobacco products and avoid secondhand smoke
• Take medications as directed
• Drink beverages with alcohol or caffeine only in moderation
• Eat nutritious foods like fruits, vegetables and whole grains
• Be physically active for at least 150 minutes per week
• Manage stress with healthy coping techniques like talk therapy or meditation