Whether a cardiac arrest is life-threatening or not depends on different clinical situations. First, it is generally not life-threatening. Patients often have no previous underlying heart disease, and electrocardiograms, cardiac ultrasounds, and even coronary angiograms are completely normal. A small number of premature heart beats, such as atrial premature beats, ventricular premature beats and junctional premature beats, can be detected when doing an ambulatory ECG, and there are no particularly serious arrhythmias. The patient needs to be reassured and the anxiety and nervousness needs to be changed. When symptoms do occur, medications can be taken for symptomatic management, such as metoprolol. When there are no symptoms, even no medication can be used. Second, life-threatening, which is often seen in patients with acute myocardial infarction or severe heart disease. An ambulatory electrocardiogram can reveal frequent ventricular premature heart beats, multi-source ventricular premature heart beats, and in some cases can even induce recurrent ventricular tachycardia. Aggressive pharmacological treatment is necessary to control the primary disease and premature heart beats and effectively reduce the risk of sudden death.