The heart is controlled electrically. Before every beat, an electrical impulse spreads across the atria and ventricles, starting from the sinus node, the natural pacemaker. The voltages produced are tiny but can be measured on the skin. The electrocardiogram, ECG for short, records them as a trace. It only measures and does not emit any current itself.
For a resting ECG, usually ten electrodes are attached to the chest, arms and legs. From these, the device calculates twelve leads – twelve viewing angles on the same heartbeat. The recording shows how fast and how regularly the heart beats and whether the impulse is conducted without disruption. Signs of a recent or previous heart attack, a thickened heart wall or changes in blood salts can also be found in it. However, anything that occurs only intermittently can easily be missed by this snapshot.
This is where the ambulatory ECG comes in. A small recorder, connected to the chest by a few adhesive electrodes, usually runs for 24 to 48 hours, including at night. In a diary, you note when symptoms occurred and what you were doing at the time. This helps to clarify whether skipped beats, a racing heart, dizziness or a brief faint are caused by the heart rhythm. After a stroke, the recording looks for undetected atrial fibrillation. If symptoms are rare, longer recordings may be considered, for example with an event recorder.
In an exercise ECG, known as ergometry, you pedal on a stationary bike against a resistance that increases in stages. ECG, pulse and blood pressure are monitored continuously, and a doctor is present. The test shows how much exertion you can manage and how your circulation and rhythm respond to it. Signs of reduced blood flow to the heart muscle under load point to narrowed coronary arteries. To confirm such a narrowing, the German National Disease Management Guideline on coronary heart disease now tends to recommend imaging methods. During treatment, however, the exercise ECG still provides important information.