A sinus rhythm slower than 60 beats per minute is called sinus bradycardia. It can be seen in healthy adults, especially in athletes, the elderly and during sleep. Other causes are increased intracranial pressure, high blood potassium, hypothyroidism, hypothermia and the use of drugs such as digitalis, beta-blockers, reserpine, guanethidine and methyldopa. In organic heart disease, sinus bradycardia is seen. 1. Etiology 1. Intracardiac factors (1) Vagal excitation mostly acts through the nerve (mainly vagal excitation), humoral mechanism via extracardiac nerves, or directly on the sinus node and causes sinus bradycardia. (2) Impaired sinus node function refers to sinus bradycardia caused by damage to the sinus node (e.g., inflammation, ischemia, toxicity, or degenerative damage). In addition, it can be seen in myocardial damage such as myocarditis, pericarditis, myocardial sclerosis, etc. It may also be caused by transient sinus node inflammation, ischemia and toxic damage. (3) The incidence of sinus bradycardia in acute myocardial infarction is 20% to 40%, with the highest incidence in the early stage of acute myocardial infarction (especially inferior wall infarction). 2, extracardiac factors Sinus bradycardia caused by extracardiac factors, most of them are accompanied by the phenomenon of vagal hyperactivity, which is neurological, and the heart rate is not very stable. When the autonomic tone changes, such as deep breathing, exercise, atropine injection, etc., there are often changes in heart rate, and the P-R interval may be slightly prolonged. Clinical manifestations vary in severity and may be intermittent. The symptoms of insufficient blood supply to the heart, brain, kidneys and other organs due to slow heart rate are the main ones. In mild cases, there is weakness, dizziness, poor memory and slow reaction, while in severe cases, there may be blackout, syncope or A-Syndrome. In some severe cases, in addition to palpitations, it may also aggravate the symptoms of existing heart disease and cause heart failure or angina. Low cardiac output can seriously affect the perfusion of kidneys and other organs, and can lead to oliguria.