Find Information About:

Drugs & Supplements

Get information and reviews on prescription drugs, over-the-counter medications, vitamins, and supplements. Search by name or medical condition.

Pill Identifier

Pill Identifier

Having trouble identifying your pills?

Enter the shape, color, or imprint of your prescription or OTC drug. Our pill identification tool will display pictures that you can compare to your pill.

Get Started

My Medicine

Save your medicine, check interactions, sign up for FDA alerts, create family profiles and more.

Get Started

WebMD Health Experts and Community

Talk to health experts and other people like you in WebMD's Communities. It's a safe forum where you can create or participate in support groups and discussions about health topics that interest you.

  • Second Opinion

    Second Opinion

    Read expert perspectives on popular health topics.

  • Community


    Connect with people like you, and get expert guidance on living a healthy life.

Got a health question? Get answers provided by leading organizations, doctors, and experts.

Get Answers

Sign up to receive WebMD's award-winning content delivered to your inbox.

Sign Up

Heart Health Center

Font Size

Electrocardiogram (EKG) Components and Intervals


An electrocardiogram (EKG, ECG) is a test that measures the electrical signals that control heart rhythm. The test measures how electrical impulses move through the heart muscle as it contracts and relaxes.

The electrocardiogram translates the heart's electrical activity into line tracings on paper. The spikes and dips in the line tracings are called waves.

  • The P wave is a record of the electrical activity through the upper heart chambers (atria).
  • The QRS complex is a record of the movement of electrical impulses through the lower heart chambers (ventricles).
  • The ST segment shows when the ventricle is contracting but no electricity is flowing through it. The ST segment usually appears as a straight, level line between the QRS complex and the T wave.
  • The T wave shows when the lower heart chambers are resetting electrically and preparing for their next muscle contraction.

ByHealthwise Staff
Primary Medical ReviewerRakesh K. Pai, MD, FACC - Cardiology, Electrophysiology
Specialist Medical ReviewerGeorge Philippides, MD - Cardiology

Current as ofMarch 12, 2014

WebMD Medical Reference from Healthwise

Last Updated: March 12, 2014
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

Today on WebMD

hdl letters stacked up
How to boost your ‘good’ cholesterol.
Learn the causes.
Compressed heart
5 habits to change.
heart rate
What’s normal? What’s not?
Lower Cholesterol 02
Heart Foods Slideshow
Compressed heart
doctor looking at xrays
Heart Disease And Ed
Lowering Cholesterol Slideshow
lowering blood pressure
Wide Awake For Heart Surgery