The electrocardiogram examination of sinus rhythm must meet the following two criteria: (1) P wave stands in lead I, II, and aVF, and is inverted in aVR. (2) P-R interval is 0.12-0.20 seconds. There is a unique small nodule on the upper part of the right atrium of the body, composed of unique cells, known as the sinoatrial node.

It can automatically and regularly generate an electrical current, which is transmitted to each position of the heart in the order of transmission of the conducting tissue, thereby causing the contraction of the body and the left ventricle. The normal heart rate is transmitted from here, which is called the ‘pacemaker’ of the heart. The sinoatrial node generates a desire once, and the heart beats once, which is called ‘sinus rhythm’ in medicine.

Therefore, the normal heartbeat of the heart should be sinus rhythm. The sinoatrial node is the largest ‘command center’ of the heart’s heartbeat. Therefore, a normal heart must have a normal sinoatrial node, which has strong intrinsic control ability. Any heart rate generated by the excitation transmitted from the sinoatrial node is called sinus rhythm. The sinoatrial node frequency is 60-100 times per minute, but 25% of young people have a heart rate of 50-60 times per minute, children under 6 years old can exceed 100 times per minute, and newborns can reach 100-150 times per minute.

Any arrhythmia caused by a change in the intrinsic control ability of the sinoatrial node is called sinus arrhythmia. Factors affecting the change in the intrinsic control ability of the sinoatrial node include nervous elements, blood elements, and intrinsic elements of the sinoatrial node itself, but the key factors are the nervous and blood elements, and only in a very few cases is the change in the intrinsic control ability of the sinoatrial node due to intrinsic organic damage.

In the nervous elements, the main influence is from the sympathetic nervous system, followed by the central nervous system. Therefore, tachycardia can occur during emotional excitement, physical activity, after meals, and fever. Similarly, some blood elements can also affect the intrinsic control ability of the sinoatrial node, causing sinus arrhythmia.