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.