What happens during electrocardiography (ECG)?

Whether in routine examinations or to clarify heart problems: Many people have ever had an electrocardiogram (ECG) done. But what is actually measured and what does the ECG curve show?

Nerve and muscle cells communicate via electrical and chemical signals. Regular electrical impulses also control the heartbeat. They are triggered by the so-called sinus node in the right atrium of the heart and spread like small surges through the heart muscle. This causes the atria and then the heart chambers to contract first. The spread of power surges in the heart muscle is still measurable on the skin. An ECG measures these current fluctuations (actually: voltage fluctuations) at different skin sites and displays them as a curve. The ECG curve is called the electrocardiogram.

When will the investigation be offered?

An ECG is made to check the function of the heart. It records, in particular, how often the heart beats per minute (heart rate) and how regularly it beats (heart rhythm). The examination can provide important information, for example on a narrowing of the coronary vessels, a heart attack or on arrhythmias such as atrial fibrillation.

What does an ECG show?

When the heart beats evenly, the typical ECG pattern results: The first peak (p-wave) shows how the electrical impulse (excitation) propagates across the atria. The atria contract, pumping blood into the ventricles, and immediately relax. The arousal then reaches the heart chambers. On the ECG this is visible as the Q, R and S waves, the so-called QRS complex, in which the heart chambers contract. Thereafter, the T-wave indicates that the arousal recedes and the heart chambers relax again.

Cardiac disorders and arrhythmias show changes in the ECG. Their appearance and course can give indications of the causes of the disorder.

How is the examination performed?

The heart currents can be “derived” on the surface of the skin – even on the arms and legs. The standard is the “12-lead ECG”, which uses a total of ten electrodes: six on the chest, one each on the forearms and calves. With strong hairiness, these areas are shaved before; otherwise, no preparation is necessary. The electrodes are connected via cable to the ECG device. The device converts the received data into the ECG waveforms and saves them. Depending on the device, the curves can also be printed out.

Which types of examination are there?

With the resting ECG, one lies relaxed with bare torso on the back. Being calm and comfortable during the exam is very important because muscle tension, movement, coughing or shaking can distort the result. The measurement takes about one to a maximum of five minutes.

The exercise ECG measures cardiac currents during physical exertion. Mostly one enters the pedals of a bicycle ergometer. The load is continuously increased to a high performance, as the pedals kick harder and harder. If changes in the ECG occur, the examination is terminated earlier. In addition to the ECG curve, the study also provides data on the power output in watts. The blood pressure is also checked regularly.

A long-term ECG usually records the cardiac currents over 24 hours. For this purpose, three or four electrodes are glued to the chest, a small recording device is worn on the belt or hung around the neck. The ECG data will later be transferred to a computer and evaluated in the doctor’s office. For this, the doctor also needs information on the daily routine (such as special events, physical activity, sleep). For example, a long-term ECG is done if cardiac arrhythmias are temporary and therefore not noticeable on a normal ECG.

What do the curves of a 12-lead ECG describe?

The 12-lead ECG takes advantage of the fact that heart impulses do not spread evenly across the skin. The device compares the strength of the pulses between every two electrodes – experts speak of “derivatives”. A derivative corresponds for example to the measurement of the two electrodes on the arms. For a 12-lead ECG, a total of twelve leads are taken.

For example, depending on which derivative shows deviations, those skilled in the art may recognize in which region of the heart muscle an infraction has taken place, or whether an arrhythmia originates from the right or left ventricle of the heart.

ECG with one infographic!

ECG infographics


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