As a widely used clinical test, ECG has the advantage of being quick, simple, convenient and efficient. By recording and monitoring the electrical activity of the heart, ECG tells the doctor certain signals to assist and guide the diagnosis and treatment. Electrocardiography has the following important functions: I. Diagnose various arrhythmias For various arrhythmias such as tachyarrhythmia, bradyarrhythmia, ectopic arrhythmia and so on, they can be diagnosed and differentiated by electrocardiography. Diagnosing myocardial infarction When chest pain occurs and myocardial infarction is suspected, early electrocardiography can help early detection and early diagnosis. In addition, it is also helpful for the diagnosis of angina pectoris, cardiomyopathy and acute pericarditis. Monitoring side effects of drugs and electrolyte disorders In clinical practice, certain drugs (some anti-arrhythmic drugs, antibiotics, tumor drugs, etc.) or electrolyte disorders (e.g., abnormalities of blood potassium) may have an effect on the heart, and ECG examination can help diagnosis and treatment at this time. When myocardial injury, atrial or ventricular hypertrophy and enlargement are caused by various reasons, ECG examination can indirectly get the hint, combined with other necessary examinations such as cardiac ultrasound, etc., which can help the diagnosis. In addition, it can be used to rule out anxiety. It is important to note that ECG is not an all-encompassing test, and the diagnosis of a disease must be based on a combination of history, physical examination, and other ancillary tests such as hematology, ultrasound, and CT.