Left-axis deviation refers to the leftward shift of the electrical axis of the heart during an electrocardiogram, which is usually caused by physiological or pathological factors. Physiological factors include obesity, hypertension, hypertrophic obstructive cardiomyopathy, or organic cardiomyopathy, such as heart valve disease or valve stenosis with incomplete closure. In the case of hypertensive heart disease, left-sided cardiac axis also occurs. If left-sided cardiac axis occurs, patients are advised to have their cardiac ultrasound examined to clarify the function of the heart.