The above is technically a sinus tachycardia with a rate of 107 beats per minute with ST depression of approximately 3 mm (3 small boxes). The arrow is pointing at the ST segment of the EKG. If you look at Normal Heart Rhythm, you will note that the ST segment is neither elevated or depressed which is referred to as being isoelectric. A thorough evaluation of ST changes requires the use of a 12 lead EKG. ST segment depression can suggest the presence of ischemia, or lack of blood flow to the heart muscle itself. Patients may complain of chest tightness, pressure or pain and/or lightheadedness or dizziness in these situations.