What do the tracings on an electrocardiogram mean?

During the electrocardiogram (ECG), sticky electrode patches are applied to your chest, arms and legs. Wires connect the electrodes to a computer, which translates the electrical activity in your heart into tracings on a monitor and/or special ECG graph paper. Your doctor will analyze the ECG to learn more about your heart’s rhythm and the overall condition of your heart.

An ECG shows several types of “waves” of electrical signals.

• The “P” wave is the first small peak in the ECG. It indicates the electrical impulse in the upper chambers of your heart.
• The “QRS” complex is made up of the “R” wave, which is the tallest peak in the ECG, plus the small notches in the ECG that come before it (the “Q” wave) and after it (the “S” wave). The QRS complex records electrical activity in the lower chambers of your heart.
• The “T” wave is the final small peak in the ECG. It reflects the heart’s return to a resting state.

The shape and size of the waves, the time between each wave and the rate and regularity of beating provide valuable information to your doctor. In addition to providing insight into the heart’s rhythm, the ECG helps your doctor determine the size of the heart chambers, detect heart muscle damage and identify abnormal levels of certain minerals in the blood, such as potassium and calcium, which alter the ECG. The results of an ECG can be normal when the patient is in a resting state, which is why many patients also undergo a stress test to evaluate the heart during exercise.