Electrocardiographic Hallmarks of STEMI

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Figure 9.2 illustrates the following three ECG hallmarks of a classic STEMI:

1. ST-segment elevation

2. T wave inversion

3. Q wave formation

These three changes in the ECG typically evolve over a period of minutes to hours, with ST elevation usually appearing first, followed variably by T wave inversion and Q wave formation. Subsequently, the changes may show slow resolution, usually over a period ranging from days to months. Q waves, however, may persist indefinitely, producing ECG evidence of a scar.

Wave Formation
Figure 9.2. The three ECG hallmarks of AMI, including ST elevation, T wave inversion, and Q wave formation,

This sequence of changes is called electrocardiographic evolution of an infarction (Figure 9.3). It is important to recognize that the ECG diagnosis of AMI is much more accurate when made on the basis of evolution over a series of tracings than when made on the basis of a single ECG. Keep in mind also that the ECG may not reveal clear patterns of infarction in the earliest stages of evolution.

On occasion, the earliest change of AMI, occurring even before ST segment elevation, may actually be an increase in the height of the T wave called

Segment Types Ami

Figure 9.3. The evolution of an inferior wall myocardial infarction, as seen in lead III of a 55-year-old white male, Note that the admission tracing shows only ST elevation, A Q wave is beginning to form by 1 hour, and ST elevation is on the way down, By 24 hours, Q wave formation is complete, and the T wave is fully inverted, By 1 year, a pathologic Q wave is the only remaining evidence of infarction,

Figure 9.3. The evolution of an inferior wall myocardial infarction, as seen in lead III of a 55-year-old white male, Note that the admission tracing shows only ST elevation, A Q wave is beginning to form by 1 hour, and ST elevation is on the way down, By 24 hours, Q wave formation is complete, and the T wave is fully inverted, By 1 year, a pathologic Q wave is the only remaining evidence of infarction, hyperacute T wave changes. Hyperacute T waves alone are insufficient evidence with which to make the diagnosis of AMI. When seen, however, they should increase your index of suspicion for AMI in a patient presenting with signs and symptoms that are compatible with AMI.

You may have deduced from this discussion that it is extremely important to correlate clinical signs and symptoms with the ECG before making the diagnosis of AMI. More on that later.

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