Sudden low heart rate and chest tightness may be caused by heart disease or pulmonary embolism. 1. Cardiac diseases: in case of sudden myocardial ischemia, bradyarrhythmia or myocardial infarction, due to insufficient blood supply to the myocardium or conduction block, there is a sudden decrease in heart rate and tightness in the chest, and in serious cases, there may be chest pain, profuse sweating, and a sense of dying. 2. Pulmonary embolism: it occurs in people who have been lying in bed for a long time and suddenly exercise, when getting up or standing up, due to the embolus dislodging and pulmonary embolism, there is a sudden decrease in heart rate and chest tightness. If the symptoms of sudden heart rate reduction, chest tightness, must go to the hospital in a timely manner, such as the symptoms are more serious, it is recommended to call the 120 medical treatment, to prevent the delayed condition.