The normal ST segment of the EKG tracing begins at the isoelectric line, is typically isoelectric (flat), extending from the end of the S wave and curving gradually upward to the beginning of the T wave. Causes of ST segment depression include myocardial ischemia, digoxin effect, ventricular hypertrophy, acute posterior myocardial infarction, pulmonary embolus, left bundle branch block.