Electrical signals created by the S-A node follow a natural electrical pathway through your heart walls. The movement of the electrical signals causes your heart's chambers to contract and relax. When a signal passes through a chamber wall, the chamber contracts. When the signal has moved out of the wall, the chamber relaxes. In a healthy heart, the chambers contract and relax in a coordinated way, or in rhythm.
When your heart beats in rhythm at a normal rate, it's called sinus rhythm. A problem in your heart's electrical system can disrupt your heart's normal rhythm. Any kind of abnormal rhythm or heart rate is called an arrhythmia. It's normal and healthy for your heartbeat to speed up or slow down during the day as your activity level changes. But it's not normal for your heart to beat out of rhythm. When your heart beats out of rhythm, it may not deliver enough blood to your body.
Learn more about different types of arrhythmias in the Conditions and Treatments section: tachycardia, bradycardia, and atrial fibrillation.
The Path of an Electrical Signal
You can read about the path of an electrical signal below, and you can also play an animation of the heart's electrical system in Figure 2.
Q. What is it called when your heart beats in rhythm at a normal rate?
Recording Electrical Activity in Your Heart The Electrocardiogram (ECG)
If your doctor suspects a problem in your heart's electrical system, he or she may schedule you for an electrocardiogram (ECG). An ECG is a recording of the electrical activity in your heart. You've probably seen an ECG on television or when visiting someone in the hospital. The ECG monitor is usually next to the patient's bed. It shows what looks like an electric current on its monitor, and it makes a "blip, blip, blip" sound.
Parts of an ECG
The hills and valleys on an ECG recording are called waves. Each wave tells your doctor a story about how your heart is working (Figure 1):
The P-wave shows your heart's upper chambers (atria) contracting
The QRS complex shows your heart's lower chambers (ventricles) contracting
The T-wave shows your heart's lower chambers (ventricles) relaxing
Play the animation below (Figure 2) to see your heart's electrical system in action. You can click the Step Through button repeatedly to see the animation play one step at a time.
Next: Intro to Blood Vessels
Q. What does an ECG show?
Blood flow through your heart
Electrical activity in your heart