ST Elevation

ST segment elevation of 3 mm or more represents acute MI. This finding is also observed in other rare cardiac diseases such as Prinzmetal angina, Brugada syndrome, LVH, and left ventricular overload (lead V₂). It can also be found in benign early repolarization. Patients with acute pericarditis may have ST elevation, but the ST segment elevations do not exceed 2 mm.

ST Elevation Within QT Interval

ST Depression

ST depression is an indication of ischemia. When trying to confirm coronary artery disease, clinicians attempt to elicit ST depression during a 12-lead exercise (treadmill) stress test. Anteroseptal wall ischemia cannot be identified on an ECG, but significant ST depression in leads V₂ or V₃, and sometimes in V₁, likely represents a mirror image of impulse activity during an acute posterior wall MI.

ST depression at rest in precordial leads V₅ and/or V₆ and in limb leads I and/or aVL can be attributed to left ventricular overload with or without LVH. A mirror image in lead V₁ and/or V₂ may be present. 

Digitalis drugs may also elicit a 1 mm ST depression resembling a tub configuration on ECG. In most cases, a 1 mm ST depression is insignificant.

Related Video – ST Segment Elevation Explained