In the last chapter we learned about the dramatic ECG findings of acute ST elevation myocardial infarction (STEMI). We also reviewed other ischemic syndromes that produce alterations of the ST segment and T wave. But not every case of ST segment elevation represents an acute myocardial infarction. In this chapter, we’ll examine the many important conditions that can affect the ST segment, first focusing on items that produce ST segment elevation, imitating the findings of acute MI. At the end of the chapter, we’ll describe some infarction imitators that do not involve ST segment elevation.
THE NORMAL AND ABNORMAL ST SEGMENT
You know by now that the ST segment is the interval between the end of the QRS complex and the beginning of the T wave (Figure 11-1). Recall that the ST segment is normally isoelectric, but may be slightly elevated or depressed by <1 mm relative to the baseline. The ST segment should be measured at the J point compared to a baseline of either the PR segment or TP segment, using whichever appears the most stable and accurate.
ECG segments and intervals.
Clinically important abnormalities of the ST segment include displacement from the baseline and changes in morphology. Abnormal ST segment displacement is defined as elevation or depression of ≥1 mm from the baseline in at least two contiguous leads. In leads V2 and V3 the criteria for abnormal ST segment elevation is ≥2 mm in men and ≥1.5 mm in women. Displacement of the ST segment may take different morphologies that may be described as horizontal, upsloping, downsloping, or with more complex patterns.
Thinking back to Chapter 3 you will recall that ST segment changes ultimately reflect alterations of repolarization. The J point represents the end of early repolarization (phase 1) and the ST segment corresponds to the plateau (phase 2) of the ventricular action potential (Figure 11-2).
Electrocardiogram matched with phases of the ventricular action potential. Phase 0: Rapid depolarization. Phase 1: Early repolarization. Phase 2: Plateau phase (delayed repolarization). Phase 3: Terminal rapid repolarization. Phase 4: Resting phase.
ST ELEVATION INFARCTION IMITATORS
Abnormal ST segment elevation unrelated to MI may be either a primary or secondary phenomenon. Primary ST segment elevation is caused by clinical conditions that directly affect repolarization, whereas secondary ST segment elevation results as a consequence of alterations of depolarization.
Primary ST segment elevation unrelated to acute STEMI and other ischemic syndromes is characteristically seen with the following:
Secondary ST segment elevation unrelated ...