Cardiogenic syncope occurs due to arrhythmia (bradycardia and tachycardia) or decreased cardiac output, and if proper treatment is not provided, it can lead to acute sudden death. A detailed medical history and physical examinations are required to determine the cause of syncope, and clinical approaches, including 12-lead ECG, are important. The 12-lead ECG does not have a chest lead in the posterior wall of the left ventricle; therefore, ECG of the isolated posterior wall myocardial infarction caused by left circumflex artery occlusion is not observed with ST elevation. Therefore, the significantly higher appearance of ST depression and R waves than S waves from V1 to V3 of the chest lead must be interpreted meaningfully. Isolated posterior wall myocardial infarction is small in the area of myocardial necrosis, and tension is increased in the necrotic area due to the contraction of the normal myocardial muscle, which can cause ventricular wall rupture. Therefore, it is necessary to additionally check Beck’s triad, such as jugular venous distension and decreased heart sound, in patients with low blood pressure with an isolated posterior wall myocardial infarction on 12-lead ECG in patients with syncope.