Sinus rhythm is a normal rhythm with a typical frequency of 60-100 beats per minute. The heart is composed of numerous cardiac myocytes, which can be divided into normal and special myocardium according to their function. The special myocardium, which consists of the sinus node, intersegmental bundle, atrioventricular node, Hitchcock’s bundle, left and right bundle branches and Purkinje fibers, is the conduction system of the heart and is self-regulating, with the function of “giving and conducting instructions”, directing the activity of the atria and ventricles. Under normal conditions, the entire heart beats rhythmically according to the electrical impulses from the sinus node. Therefore, the normal heart rhythm is called sinus rhythm. Sinus rhythm has the following ECG characteristics: 1. P waves in I, II, aVF upright, aVR inverted; PR interval 0.12-0.20 seconds; 2. The frequency of normal sinus rhythm is generally 60-100 beats/min; the difference of PP interval in the same lead should be less than 0.16 seconds. Sinus rhythm types 1, sinus tachycardia: In addition to automatic work, the heart will also receive regulatory signals from the brain and the body, so that the blood supply can adapt to the needs of the body, such as running, tension, lack of oxygen, etc., the heart will speed up, at this time the ECG conclusion is “sinus tachycardia”. Clinically, a heart rate of more than 100 beats/min is considered as tachycardia; 2, sinus bradycardia: clinically, the heart rate is less than 60 beats/min as sinus bradycardia, some people have a quiet heart rate of only 50 times/min, but will not show any uncomfortable symptoms, this is because the amount of blood supplied by the heart contraction per minute is enough to meet the body’s needs, which is also a normal phenomenon.