The diagnosis of cardiac arrest must be made quickly and decisively, preferably within 30 seconds, and can be confirmed by the following signs: 1) sudden loss of consciousness in a conscious patient, who does not respond to calls; 2) loss of aortic pulsation, often touching the carotid or femoral artery; 3) dilated pupils; 4) respiratory arrest or wheezing-like breathing. The first and second criteria are the most important and can be used to confirm the diagnosis of cardiac arrest. It is important not to delay resuscitation by repeatedly measuring blood pressure and auscultating heart sounds in patients suspected of cardiac arrest or waiting for ECG tracing. Although pupil dilation is an important indication of cardiac arrest, the response is delayed and easily influenced by drugs and other factors, so clinically we should not wait for pupil changes before confirming cardiac arrest.