IN THIS ISSUE

From the Summer 2004 Issue

Learning Center

The Heart's Electrical System

Did you know that your heart has an electrical system? It's a bit like the electrical wiring in your home. The heart's electrical system creates the signals that tell your heart when to beat. And your heartbeat is what pumps blood throughout your body. The heart's electrical system is also called the cardiac conduction system.

You may know that the heart is a four-chambered muscle, about the size of your fist. The two upper chambers are the right and left atria, and the two lower — and larger — chambers are the right and left ventricles (Figure 1).

Figure 1 Heart Chambers Click to enlarge and interact

Parts of the Electrical System

Your heart's electrical system ../includes three important parts (Figure 2):

• S-A node (sinoatrial node) — Known as the heart's natural pacemaker. The S-A node initiates each heartbeat.
• A-V node (atrioventricular node) — The bridge between the atria and ventricles. Electrical signals are passed from the atria down to the ventricles through the A-V node.
• His-Purkinje system — Carries the electrical signals throughout the ventricles. The His-Purkinje system consists of the following parts:
• His Bundle (the start of the system)
• Right bundle branch
• Left bundle branch
• Purkinje fibers (the end of the system)

Figure 2 The Heart's Electrical System Click to enlarge and interact

The Path of an Electrical Signal

A system of electrical pathways in your heart connects one part to another — the S-A node to the A-V node, for instance. This movement of the signals causes your heart to contract (or beat) and relax. The number of electrical signals controls the speed of your heartbeat. The more signals passing through the heart, the faster the heartbeat.

Usually 60-100 signals per minute travel these pathways. That means a heart rate of 60-100 beats per minute. Your doctor or nurse may check your heart rate by checking your pulse.

You can see the steps in the electrical system pathway below. You can also play an animation that shows the pathway in action (Figure 3).

Figure 3 Start the Heart Click to enlarge and interact

Note that the atria contract a fraction of a second before the ventricles do. After your heart contracts, it relaxes for a moment before the process begins again.

When working right, your conduction system automatically responds to your body's changing need for oxygen:

• When you climb stairs, carry heavy groceries, or take a walk, you need more oxygen; therefore, your heart beats at a faster heart rate.
• When you are sitting or sleeping, you need less oxygen; therefore, your heart beats at a slower rate.

Your conduction system senses your need for oxygen and responds with the proper heart rate.

When Your Heart Doesn't Work as It Should

Sometimes there are problems with the electrical conduction system:

• The S-A node doesn't produce the right number of signals.
• Another part of the heart takes over as the natural pacemaker.
• The electrical pathways are interrupted.

Sometimes the heart beats too slowly or too rapidly. Heartbeats (or rhythms) that are too fast or slow are called arrhythmias.

• Slow Arrhythmias — When the heart beats too slowly it's called bradycardia (brady = slow, cardia = heart). Slow arrhythmias can be a problem because they cause the oxygen- and nutrient-rich blood to travel more slowly to your organs and other tissues. Your body may not receive enough oxygen and nutrients to function properly.
• Fast Arrhythmias — When the heart beats too fast it's called tachycardia (tachy = fast, cardia = heart). During tachycardia the heart isn't able to pump blood to the body as well as it should. Fast rhythms in the upper chambers may not be life-threatening in themselves. But they may contribute to other problems that are serious. Fast arrhythmias in the lower chambers, the ventricles, can be dangerous and even fatal.

What Causes These Problems?

Interruptions in the pathways can occur for a number of reasons:

• Heart disease causes changes in the heart tissue.
• Aging of the heart muscle can also change the heart tissue.
• Physical problems, such as diabetes, smoking, and excessive alcohol or drug use, can affect the heart tissue.
• There could be an inherited heart problem.
• There is evidence of heart failure or a heart attack.

People who have had a heart attack or have heart failure often have hearts that don't pump blood as well as they should. The ejection fraction (EF) is a measure of how well the heart is pumping. A low EF also may mean that these people are at higher risk for arrhythmias.

How to Find Out If You May Have a Problem

Of course it's normal for your heart rate to vary widely during the course of a day, depending on whether you're active or resting. For example, you can expect your heart rate to increase when you're exercising, but not when you're sitting still. What's not normal is a heart rate that is fast or slow because of a problem with the conduction system. If you have certain symptoms, your doctor may want to test your conduction system.

Conduction problems like arrhythmias can sometimes go unnoticed. Other times they can cause the following symptoms:

• Reduced energy levels
• Fatigue
• Shortness of breath
• Palpitations (feeling your heart skip a beat or pound quickly)
• Chest pain
• Dizziness
• Lightheadedness
• Fainting

The good news is that tests can show if you have conduction problems. That's why you should check with your doctor if you have any of these symptoms. You know your body well enough to tell when something doesn't feel right.

To check for problems, your doctor might recommend one or more tests. But keep in mind that if you are referred for testing, it doesn't automatically mean that you have conduction problems. These tests can also help determine if you don't have conduction problems.

Tests for the Conduction System

ECG or EKG (Electrocardiogram)

In this fairly fast and simple test, up to 12 electrodes are placed on your chest, arms, and legs. The electrodes sense the heart's electrical activity, which is printed on a strip of paper. By interpreting your ECG, your doctor can:

• Measure heart rate
• Look for arrhythmias
• Identify enlargement of the heart's chambers
• Help diagnose whether you've had a heart attack

Electrophysiology (EP) Test

This test is performed by an electrophysiologist (a cardiologist who specializes in conduction). During the test, several thin tubes (catheters) are inserted into a blood vessel, usually in your groin area. Your doctor will gently push the catheters through the vessel, all the way into your heart.

The EP test can do the following:

• It tests your heart to see if you have a problem in your electrical system.
• If you do have an electrical problem, it helps your doctor choose the best treatment.

What the Tests Show

Based on the results of the test(s), your doctor might conclude that:

• Your heart's electrical, or conduction, system is fine.
• You are at a higher risk for electrical conduction problems.
• You have some type of problem with electrical conduction in your heart.

If there is a conduction problem, treatment may include medication. Sometimes there is also a need for an implanted device:

• A pacemaker to treat slow heart rates (bradycardia)
• An implantable cardioverter defibrillator (ICD) to treat fast heart rates (tachycardia)

Talking to Your Doctor

Don't hesitate to talk to your doctor and/or nurse if you:

• Feel symptoms
• Have had a heart attack
• Have heart failure
• Are diagnosed with an electrical problem
• Have questions about your treatment

It helps everyone — you, your family, your doctor — when you get the answers you need.

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