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An electrocardiogram, also called an ECG or EKG, is a quick, painless test that measures the heart’s electrical activity and records any disturbances in heart rhythm. The heart’s electrical activity determines if it keeps a normal rhythm.

The Heart’s Electrical System

Specialized tissues within the heart are capable of generating electrical impulses. These impulses cause the heart muscles to contract. With each contraction, the heart sends blood out to the lungs to pick up oxygen or to the body to deliver oxygen to the cells.

The tissues that generate electrical activity are called the sinus node (or sinoatrial node). The sinus node, located in the heart’s upper right chamber (atrium), generates an electrical impulse each fraction of a second. The impulse travels through the atrial muscles, causing them to contract. It then travels to the AV (atrioventricular) node, which is located between the (upper chambers) and the ventricles (lower chambers). The AV atria node is the only pathway for electrical impulses to travel to the ventricles.

The AV node conducts the electrical impulses more slowly than other nodes to allow time for the ventricles to receive blood from the atria before they contract and send it out of the heart. If the electrical impulses traveling from the atria to the ventricles come early or late, the balance between blood filling the lower chambers and the timing of the heart’s contraction is disturbed. The result is reduced cardiac output – or a reduced amount of blood ejected from the heart.

How Is an ECG Performed?

An ECG may be performed at rest or during exercise. During the ECG, small, sticky patches (electrodes) are placed on different areas of the body. Wires leading from the patches to a computer carry a signal that traces the heart’s electrical activity on paper or on a computer. Doctors analyze the ECG to learn more about the heart’s rhythm and condition.

How Does an ECG Work?

An ECG measures the heart’s electrical activity through electrodes, and a computer translates the heart’s activity into a printed or onscreen readout. You may have seen images of an ECG readout from television or movies: the electrical activity image looks like a line graph with peaks and valleys.

An ECG shows three “waves” of signals.

  • The “P” wave indicates the electrical impulse in the upper chambers of the heart.
  • The “QRS” wave records electrical activity in the lower chambers.
  • The “T” wave reflects the heart’s return to rest.

The shape and size of the waves, the time between each wave, and the rate and regularity of beating provide valuable information to doctors. In addition to providing insight into the heart’s rhythm, the ECG helps doctors determine the size of the heart chambers, detect heart muscle damage and identify abnormal levels of certain electrolytes, such as potassium and calcium, in the blood.

Is It Safe?

Electrocardiograms do not pose any safety risks.

Questions to Ask Your Doctor About Electrocardiograms

The following questions can help you talk to your physician about having an electrocardiogram. Consider printing out or writing down these questions and taking them with you to your appointment. Taking notes can help you remember your physician’s response when you get home.

  • What can an electrocardiogram tell us about my heart health?
  • What happens next if an electrocardiogram shows that I have an abnormal heart rhythm?
  • What happens next if an electrocardiogram shows damage to my heart muscle?
  • Can an ECG show if I have blockages or not?

Please print this list of questions here. Take them with you to the doctor and share them with friends and loved ones when you are encouraging them to see their doctors.

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