The danger of left anterior branch conduction block is mainly caused by the primary disease, which may be manifested as dizziness, fainting and sudden death. Left anterior branch block is mostly seen in heart disease patients with left ventricular involvement, such as congestive heart failure, acute myocardial infarction, hypertensive heart disease, aortic stenosis, etc.; in addition, acute infections, drug intoxication such as quinidine, electrolyte disorders, etc., can also cause left anterior branch block. Simple left anterior branch block usually has no obvious symptoms and is basically harmless, and patients usually have a good prognosis and can be observed and followed up. If combined with heart disease, the danger of left anterior branch block mainly depends on the severity of the existing heart disease, which may lead to a significant slowing of the ventricular rate, accompanied by dizziness, fatigue, fainting and other symptoms, or even sudden death. Therefore, it is recommended that the patient consult a medical professional and standardize the diagnosis and treatment in a timely manner.