Depression of the ST segment and inversion of the T wave are common electrocardiographic abnormalities. Knowing the various ischemic and nonischemic morphologic features is critical for a timely diagnosis of high-risk myocardial ischemia and electrolyte- or drug-related abnormalities. Moreover, it is important to recognize that true posterior infarction or subtle ST-segment elevation infarction may masquerade as ST-segment depression ischemia, and that pulmonary embolism may masquerade as anterior ischemia. These common electrocardiographic abnormalities are summarized in Table 1.
THE ST SEGMENT AND THE T WAVE: A PRIMER
The ST segment corresponds to the plateau phase of ventricular repolarization (phase 2 of the action potential), while the T wave corresponds to the phase of rapid ventricular repolarization (phase 3). ST-segment or T-wave changes may be secondary to abnormalities of depolarization, ie, pre-excitation or abnormalities of QRS voltage or duration.
On the other hand, ST-segment and T-wave abnormalities may be unrelated to any QRS abnormality, in which case they are called primary repolarization abnormalities. These are caused by ischemia, pericarditis, myocarditis, drugs (digoxin, antiarrhythmic drugs), and electrolyte abnormalities, particularly potassium abnormalities.
ST-segment deviation is usually measured at its junction with the end of the QRS complex, ie, the J point, and is referenced against the TP or PR segment.1 But some prefer to measure the magnitude of the ST-segment deviation 40 to 80 ms after the J point, when all myocardial fibers are expected to have reached the same level of membrane potential and to form an isoelectric ST segment; at the very onset of repolarization, small differences in membrane potential may normally be seen and may cause deviation of the J point and of the early portion of the ST segment.2
Although a diagnosis of ST-segment elevation myocardial infarction (STEMI) that mandates emergency reperfusion therapy requires ST-segment elevation greater than 1 mm in at least two contiguous leads,3 any ST-segment depression or elevation (≥ 0.5 mm, using the usual standard of 1.0 mV = 10 mm) may be abnormal, particularly when the clinical context or the shape of the ST segment suggests ischemia, or when other ischemic signs such as T-wave abnormalities, Q waves, or reciprocal ST-segment changes are concomitantly present. On the other hand, ST-segment depression of up to 0.5 mm in leads V2 and V3 and 1 mm in the other leads may be normal.1
In adults, the T wave normally is inverted in lead aVR; is upright or inverted in leads aVL, III, and V1; and is upright in leads I, II, aVF, and V2 through V6. The T wave is considered inverted when it is deeper than 1 mm; it is considered flat when its peak amplitude is between 1.0 mm and −1.0 mm.1
As we will discuss, certain features allow the various causes of ST-segment and T-wave abnormalities to be distinguished from one another.
SECONDARY ST-SEGMENT AND T-WAVE ABNORMALITIES
- The ST segment and T wave are directed opposite to the QRS: this is called discordance between the QRS complex and the ST-T abnormalities. In the case of right bundle branch block, the ST and T are directed opposite to the terminal portion of the QRS, ie, the part of the QRS deformed by the conduction abnormality.
- The ST segment and T wave are both abnormal and deviate in the same direction, ie, the ST segment is down-sloping and the T wave is inverted in leads with an upright QRS complex, which gives the ST-T complex a “reverse checkmark” asymmetric morphology.
- The ST and T abnormalities are not dynamic, ie, they do not change in the course of several hours to several days.
Thus, in cases of left ventricular hypertrophy or left bundle branch block, since the QRS complex is upright in the left lateral leads I, aVL, V5, and V6, the ST segment is characteristically depressed and the T wave is inverted in these leads (Figure 2). In cases of right ventricular hypertrophy or right bundle branch block, T waves are characteristically inverted in the right precordial leads V1, V2, and V3.
Left bundle branch block is always associated with secondary ST-T abnormalities, the absence of which suggests associated ischemia. Left and right ventricular hypertrophy, on the other hand, are not always associated with ST-T abnormalities, but when these are present, they correlate with more severe hypertrophy or ventricular systolic dysfunction,4 and have been called strain pattern. In addition, while these morphologic features are consistent with secondary abnormalities, they do not rule out ischemia in a patient with angina.
Some exceptions to these typical morphologic features:
- Right ventricular hypertrophy and right bundle branch block may be associated with isolated T-wave inversion without ST-segment depression in precordial leads V1, V2, and V3.
- Left ventricular hypertrophy may be associated with symmetric T-wave inversion without ST-segment depression or with a horizontally depressed ST segment. This may be the case in up to one-third of ST-T abnormalities secondary to left ventricular hypertrophy and is seen in hypertrophic cardiomyopathy, particularly the apical variant, in leads V3 through V6.5