When performing an ECG, it is important to consider the practical application value. ECG records the form of electrical activity, and many diagnoses, such as myocardial ischemia and ventricular hypertrophy, in addition to arrhythmia, are presumptive judgments. Therefore, it is necessary to confirm the diagnosis with other auxiliary tests. The ECG does not directly measure the function of the heart and may be normal if the pumping function of the heart is impaired without affecting the electrical circuits. Experienced professors always use the analogy: the heart is like a house, with a structure, circuits, and waterways. The EKG is to look at the circuitry. If the house is on fire and the wires are not burned, the circuitry is not significantly altered. Similarly, if the water pipes are not burned, the water circuits will not be a problem. This is not uncommon. For example, in acute pulmonary edema, the EKG may be normal even though it affects the pumping function of the heart. The ECG does not respond directly to structural abnormalities, but only records circuit abnormalities due to structural changes. For example, in mitral valve disease, it may cause an arrhythmia of atrial fibrillation. Some structural abnormalities that do not affect the circuitry may also have a normal ECG. The normal body ECG does not record the electrical activity of all heart cells. The heart actually has an electrically resting zone. If an abnormality occurs in this zone, the ECG does not record electrical changes. The atrioventricular junction zone is more clearly recorded on the ECG with Hitchcock bundle electrodes. Recording of the left atrium is more pronounced in esophageal ECG recordings. Intermittent episodes of ECG. No abnormalities are recorded when the disease does not attack, when the ECG may be normal. Summary: Symptoms, signs and symptoms are evident on the ECG. Although seemingly simple, try to consider the whole picture. Acute infarction or pericarditis? No symptoms, no signs, and a difficult to interpret abnormalities on the EKG. Going back to the history ECG becomes very important, some people may have had ECGs like this all their lives, some may have age-related changes. Some may also have a genetically related disease. Symptoms and signs, the ECG does not show typical changes, the clinician’s experience is the main reason to change the examination strategy, or other systemic diseases, or take other auxiliary tests. Focus: ECGs with changes must be carefully searched for causes.