An electrocardiogram is a recording of the electrical activity of the heart. It is performed by attaching 10 small slightly sticky electrodes to the chest, arms, and legs whilst the patient lies comfortably on a couch. The ECG machine records the tiny electrical signals created by the heart as it beats. It is a very routine test done after listening to your heart, and most patients hardly notice that it is being done.
Please note the ECG machine simply records the electrical activity from the patient it does not apply any electrical current to the patient. Indeed, the machines are specifically designed to isolate the electrical currents in the machine from the patient-usually this isolation is optical.
It records the electrical activity of the heart as shown here-
How the ECG is formed-
Benefits of a resting ECG
It is a very easy test for a patient to have done; it is quick, cheap. It gives useful information about the electrical timing mechanism of the heart and any marked structural change in the heart. It is therefore part of the routine assessment of the heart.
The ECG can supply useful information in patients with chest discomfort, shortness of breath, palpitations, blackouts and high blood pressure.
It is especially useful for heart rhythm disturbances such as atrial fibrillation as in the illustration above. It sometimes gives important information in tachycardias (fast heart rhythms) and bradycardias (slow heart rhythms).
The ECG can also help detect-
Previous heart attacks (Myocardial infarction)
Lack of blood supply to part of the heart (Myocardial ischemia)
Enlargement of heart chambers which can be caused by heart valve lesions (Left ventricular hypertrophy)
Raised pulmonary artery pressures (Pulmonary Hypertension)
Side effects of medication (eg Digoxin)
Effect of abnormal blood salts on the activity of the heart (eg low or high potassium levels)
Genetic abnormalities of cardiac electrical activity (eg Brugada syndrome, Long QT syndrome, Wolf-Parkinson-White syndrome with a delta wave on the ECG)
Limitations of a resting ECG
Although the ECG may show that a previous heart attack has occurred it does not tell us how mild or severe the damage to the heart is. That requires further scans such as a cardiac ultrasound “echocardiogram”.
Due to the distribution of the leads on the body not all areas of the heart are well represented on the ECG.
An ECG at rest can be normal in the presence of coronary artery disease as the flow of blood down the arteries may be adequate for the heart at rest but not when the heart is working hard.