In most infarctions, the EKG will reveal the correct diagnosis. An EKG should be performed immediately on anyone in whom an infarction is even remotely suspected. However, the initial EKG may not always be diagnostic, and the evolution of electrocardiographic changes varies from person to person; therefore, it is necessary to obtain serial cardiograms once the patient is admitted to the hospital. During an acute myocardial infarction, the EKG evolves through three stages: 1) T-wave peaking followed by T-wave inversion, 2) ST-segment elevation, 3) the appearance of new Q waves.
With the onset of infarction, the T waves become tall and narrow, a phenomenon called peaking. These peaked T waves are often referred to as hyperacute T waves. Shortly afterward, usually a few hours later, the T waves invert. These T-wave changes reflect myocardial ischemia, the lack of adequate blood flow to the myocardium. Ischemia is potentially reversible: if blood flow is restored or the oxygen demands of the heart are eased, the T waves will revert to normal. On the other hand, if actual myocardial cell death (true infarction) has occurred, T-wave inversion will persist for months to years. T-wave inversion by itself is indicative only of ischemia and is not diagnostic of myocardial infarction. T-wave inversion is a very nonspecific finding. Many things can cause a T wave to flip. One helpful diagnostic feature is that the T waves of myocardial ischemia are inverted symmetrically, whereas in most other circumstances, they are asymmetric, with a gentle downslope and rapid upslope. In patients whose T waves are already inverted, ischemia may cause them to revert to normal, a phenomenon called pseudonormalization. Recognition of pseudonormalization requires comparing the current EKG with a previous tracing. In young persons without any cardiac-related symptoms, T-wave inversion isolated to one geographic region of the heart, particularly one or two midprecordial leads, for example, V3 and V4, is usually a normal variant.
- Peak and inversion
- Only reflecting ischemia
ST-segment elevation is the second change that occurs acutely in the evolution of an infarction. ST-segment elevation signifies myocardial injury. Injury probably reflects a degree of cellular damage beyond that of mere ischemia, but it, too, is potentially reversible, and in some cases the ST segments may rapidly return to normal. In most instances, however, ST-segment elevation is a reliable sign that true infarction has occurred and that the complete electrocardiographic picture of infarction will evolve unless there is immediate and aggressive therapeutic intervention.
Even in the setting of a true infarction, the ST segments usually return to baseline within a few hours. Persistent ST-segment elevation often indicates the formation of a ventricular aneurysm, a weakening and bulging out of the ventricular wall. Like T-wave inversion, ST-segment elevation can be seen in a number of other conditions in addition to an evolving myocardial infarction. There is even a type of ST-segment elevation that can be seen in normal hearts. This phenomenon has been referred to as early depolarization or J point elevation.
The J point, or junction point, is the place where the ST segment takes off from the QRS complex. J point elevation is very common in young, healthy individuals. The ST segment usually returns to baseline with exercise. J point elevation has long been thought to have no pathologic implications. However, some research has reported a slightly increased risk of death from cardiac causes in patients with J point elevation in the inferior leads. How can the ST-segment elevation of myocardial injury be distinguished from that of J point elevation? With myocardial injury, the elevated ST segment has a distinctive configuration. It is bowed upward and tends to merge imperceptibly with the T wave. In J point elevation, the T wave maintains its independent waveform.
- Reflecting true infarction
- Return to baseline within a few hours
- J point elevation
The appearance of new Q waves indicates that irreversible myocardial cell death has occurred. The presence of Q waves is diagnostic of myocardial infarction. Q waves usually appear within several hours of the onset of infarction, but in some patients they may take several days to evolve. The ST segment usually has returned to baseline by the time Q waves have appeared. Q waves usually persist for the lifetime of the patient.
Other leads, located some distance from the site of infarction, will see an appear increase in the electrical forces moving toward them. They will record tall positive R waves. These opposing changes seen by distant leads are called reciprocal changes. The concept of reciprocity applies not only to Q waves but also to ST-segment and T-wave changes. Thus, a lead distant from an infarct may record ST-segment depression.
Small Q waves can be seen in the left lateral leads (I, aVL, V5, and V6) and occasionally in the inferior leads (especially II and III) of perfectly normal hearts. These Q waves are caused by the early left-to-right depolarization of the interventricular septum. Pathologic Q waves signifying infarction are wider and deeper. They are often referred to as significant Q waves. The criteria fro significance include: 1) The wave must be greater than 0.04 seconds in duration, 2) the depth of the Q wave must be at least one-third the height of the R wave in the same QRS complex. Because lead aVR occupies a unique position on the frontal plane, it normally has a very deep Q wave. Lead aVR should not be considered when using Q waves to look for possible infarction.