The criteria for MI are divided into two categories: ST-segment elevation MI (STEMI), and ST-segment depression/T-wave change MI (NSTEMI). The J point is located at the juncture of the QRS complex and the ST segment, and it is used to measure the magnitude of the ST-segment deflection as compared with the baseline of the ECG.
A new J-point elevation of 0.1 mV or greater is required in all leads except V2 and V3 to meet the criteria for STEMI. J-point elevations of up to 0.25 mV may be seen in leads V2 and V3 in healthy men younger than 40 years of age; however, this finding decreases with age and is less prominent in women. For this reason, a range of J-point elevation criteria for MI are defined for V2 and V3 leads: 0.2 mV or greater for men 40 years old and older, 0.25 mV or greater for men younger than 40 years of age, and 0.15 mV or greater for women. he J-point elevations must be seen in two or more contiguous leads for the satisfaction of ST-segment elevation criteria. New horizontal or downsloping ST-segment depressions of 0.05 mV or greater or T-wave inversion of 0.1 mV or greater in two contiguous leads with an R-wave–to–S-wave ratio greater than 1 satisfy the criteria for NSTEMI. However, ST-segment elevations are more specific than ST-segment depressions and/or T-wave inversions for localizing the site of ischemia. ST-segment elevation generally suggests greater degrees of myocardial damage than ST-segment depression or T-wave changes. Previously inverted T waves may pseudonormalize during episodes of acute myocardial ischemia.
With predominant subendocardial ischemia the resultant ST-segment vector is directed toward the inner layer of the affected ventricle and the ventricular cavity. Overlying leads therefore record ST-segment depression.(A):
With ischemia involving the outer ventricular layer (transmural or epicardial injury), the ST-segment vector is directed outward. Overlying leads record ST-segment elevation. Reciprocal ST-segment depression can appear in contralateral leads (B):
The criteria for myocardial ischemia with ST-segment changes described previously are used in conjunction with clinical symptoms and confirmation with elevation of biomarkers to diagnose acute coronary syndromes.
