ST segment elevation
The ST segment represents the period of time during which the ventricles are depolarized. Since there is usually no electrical activity during this period of time, the ST segment is usually isoelectric with the PR and TP segments (i.e., baseline). In a normal EKG, the junction of the QRS complex with the ST segment (sometimes called the "J point") is approximately ninety degrees.
Myocardial injury is defined as reversible damage to myocardial cells due to lack of adequate oxygenation. It occurs after prolonged ischemia and before necrosis (infarction). Myocardial injury can cause abnormal electrical activity during the otherwise electrically silent ST segment. This injury will cause deviation of the ST segment in the leads aligned with the resultant abnormal ST vector.
When an area of injury is confined to the subendocardium, the ST segment deviates downward causing ST depression in the leads facing the area of injury.
| In the setting of myocardial infarction, injury is more likely to be transmural (i.e., involving the full thickness of the ventricle). In this case, the ST segment deviates upward causing ST elevation in the leads facing the area of injury. | ST segment elevation |
Reciprocal ST depression can sometimes be found in the leads opposite to the area of injury (e.g., anterior transmural ischemia can produce ST elevation in the anterior leads and ST depression in the inferior leads).
Since myocardial injury, if not reversed, will often lead to myocardial infarction, this same ST segment deviation is usually seen in the early phases of an MI. Therefore, since Q waves are usually not seen for hours to days into a new infarction, ST segment deviation is usually the best early electrocardiographic indicator of a acute (vs. old) infarct.
Marriott's Practical Electrocardiography sets forth the following criteria for diagnosing acute myocardial injury from ST deviation (p.140).
1. Elevation of the origin of the ST segment at its junction with the QRS(J-point) of greater than or equal to 1.0 mm (0.10mV) in two or more limb leads OR 2.0 mm (0.20mV) in two or more precordial leads.
OR
2. Depression of the origin of the ST segment at the J point of greater than or equal to 2.0 mm (0.20mV) in at least two of the first three precordial leads (V1-V3)
However, be careful! Just like other EKG diagnostic features, ST deviation is not completely specific for cardiac injury and/or MI. ST elevations can be found in some normal, healthy individuals as well as individuals with acute myocarditis, hyperkalemia, hypothermia, acute cor pulmonale, idiopathic hypertrophic subaortic stenosis, cerebrovascular hemorrhage, cardiac tumor or cardiac sarcoidosis.
ST depressions can also be found in certain normal, healthy individuals as well as in cases of hyperventilation, hypokalemia, mitral valve prolapse, cardiomyopathy and as an effect of the cardiac glycoside, digitalis.
References: Gersh, p.167, Harrison's p. 961, Wagner pp. 46, 138-142;Goldberger p.222, 243-252