Whether during routine examinations or heart diagnostics, many people have already had an electrocardiogram (ECG or EKG). But what does it actually measure, and what does the ECG curve show us?
Our nerve and muscle cells communicate with each other using electrical and chemical signals. Regular electrical signals also control our heartbeat. These signals are sent by a group of cells in the right atrium of the heart known as the sinoatrial node (SA node), and they spread through the heart muscle tissue as tiny electrical impulses. This causes first the atria and then the ventricles of the heart to contract. The way that these signals spread through the heart can also be measured on the surface of our skin. An ECG measures these changes in electrical signals (or, in fact, voltage) on different areas of skin and plots them as a graph. The resulting ECG graph is called an electrocardiogram.
If the heart is beating steadily, it will produce the typical ECG pattern: The first peak (P wave) shows how the electrical impulse (excitation) spreads across the two atria of the heart. The atria contract (squeeze), pumping blood into the ventricles, and then immediately relax. The electrical impulse then reaches the ventricles. This can be seen in the Q, R and S waves of the ECG, which is called the QRS complex. The ventricles contract. Then the T wave shows that the electrical impulse has stopped spreading, and the ventricles relax once again.
Heart diseases and irregular heartbeats can be detected in ECGs. What they look like and how they develop can help us find out what the underlying causes are.
Course of the QRS complex in a normal heartbeat
What does the test involve?
The electrical activity of the heart can be measured on the surface of the skin – even as far from the heart as on your arms or legs. The standard “12-lead ECG” uses a total of ten electrodes: six on your chest, and then one each on your lower arms and calves. If there is too much body hair, these areas are shaved first; other than that, no preparation is needed. These electrodes are connected by cables to an ECG machine. The machine converts the signals it receives into an ECG graph and saves it. Some machines can also print the graphs out.
What types of ECG tests are done?
Resting ECG: This involves lying still on your back with a bare chest. It is important that you lie calmly and comfortably during the test because tensing your muscles, moving, coughing or shaking can affect the results. The actual measurement takes about one minute, or five minutes at most.
Exercise ECG: Here the electrical activity of your heart is measured while you are physically active. This usually involves riding an exercise bike. The amount of exertion is steadily increased to a high level by making it increasingly difficult to turn the pedals. The test is stopped earlier if any irregularities in the ECG occur. In addition to the ECG graph, this test also provides data on the power that was generated in Watts. Your blood pressure is also checked regularly.
Holter monitor: The electrical activity of the heart is typically recorded over a period of 24 hours. Three or four electrodes are attached to your chest, and a small recording device is worn on a belt or hung around your neck. The ECG data are then transferred to a computer later on at the doctor's office for analysis. To do this, the doctor also needs information about your daily schedule (like unusual events, physical activity and sleep). A Holter monitor may be used if, for instance, you only have an irregular heartbeat some of the time and it doesn't show up in a “normal” ECG.
What do the results of a 12-lead ECG show?
The 12-lead ECG takes advantage of the fact that signals sent by the heart don't travel evenly over your skin. The device compares the strength of the signals between two electrodes – doctors call these measurements “leads.” For example, one of the leads is measured based on the two electrodes on your arms. A 12-lead ECG, as its name implies, is used to measure twelve leads.
Depending on which lead shows irregularities, experts can find out things like in which part of the heart muscle an infarction has occurred, or whether a heart rhythm problem is coming from the left or right ventricle.
Normal ECG – Left: the leads from the arms and legs, Right: the leads from the chest wall. Source: CCB Frankfurt a.M.
Schmidt R, Lang F, Heckmann M. Physiologie des Menschen: mit Pathophysiologie. Berlin: Springer; 2017.
Trappe HJ, Schuster HP. EKG-Kurs für Isabel. Stuttgart: Thieme; 2017.
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