12-Lead ECG case: Is this a STEMI?

Review the subtle signs of an acute ST-elevation MI with this case of a man presenting with "heavy" chest pain


Mr. Jackson is a 65-year-old man who called 911 because of chest pain. He describes his pain as "heavy" and indicates that it is substernal, radiating to his back. He is diaphoretic and nauseous. He denies prior myocardial infarction, but admits to smoking a pack of cigarettes per day and having no regular medical care.

In his ECG, the ST segments are elevated at the J points in Leads II, III, and aVF, but the amount of elevation may look subtle to some. When the amount of elevation seems small, what other signs can help us recognize acute ST-elevation MI?

Patient history and presentation
This patient has acute chest pain and is over the age of 50. Because he is a poor historian, and has not had much medical care, we do not know much about his past medical history. We do know he is a smoker. It would also be significant if he had a history of coronary artery disease, past MI, diabetes, metabolic syndrome or strong family history of heart disease.

ST-segment elevation distribution
In acute STEMI, the elevation will be seen in related leads. That is, the leads that show signs of injury will reflect a region of the heart that is supplied by one artery. Some myocardial infarctions are larger than others, affecting more leads, because some obstructions are more proximal in the artery. Become very familiar with the ECG as a map of the heart, so that the patterns of MI jump off the page at you.

This ECG shows ST-segment elevation in the inferior wall leads: II, III and aVF. The culprit artery for this patient is the right coronary artery, which supplies the inferior and posterior walls of the left ventricle, the right ventricle and the right atrium in the majority of people.

Reciprocal ST-Depression
ST depression in the leads that are OPPOSITE the leads with elevation is a very reliable sign confirming acute MI. In fact, often the reciprocal depression is stronger or easier to see than the elevation.

Leads I and aVL are reciprocal to Lead III. Leads V1, V2 and V3 are opposite the posterior wall. 

This patient has reciprocal changes in V1 through V3, indicating that the inferior wall damage extends up the posterior wall. You would, of course, be able to see this directly if you put electrodes on the patient’s back. But, there is a way to approximate what you would see if you placed electrodes on the patient’s back. Print the ECG out, turn it upside down and look at V2 and V3 through the back. V2 and V3 will look like a classic STEMI.

ST-segment shape
A normal ST-segment is concave upward, like a smile. Of course, even an ST-segment with a normal shape can be abnormal if it is elevated. But, when the shape is abnormal, even a slight amount of elevation can mean injury.

What is abnormal? Convex upward or frowning is abnormal. A straight ST-segment is also abnormal.

In this patient's ECG, you see that the ST-segments in II, III and aVF are straight, shooting off the QRS in a straight line. Interestingly, the reciprocal ST depression is straight, too, as they mirror the ST elevations. Look at Leads V1, V2 and V3 to see examples of flat ST depression.

Associated ECG Signs
ST-elevation is a sign of acute injury and it may be accompanied by T wave inversion, a sign of ischemia or reperfusion of an occluded artery which may be a transient event.

Pathological Q waves are an ominous sign of necrosis. When seen with ST elevation, they are considered acute. When seen with no ST changes, they are considered a sign of previous MI.

There is a small Q wave in Lead III in this ECG, but Lead III often has Q waves. They are not considered pathological unless they are also present in II and aVF. Pathological Q waves are an indication of dead tissue, like an electrical hole in the heart.

Dysrhythmias
The presence of dysrhythmia does not indicate an acute MI and the absence of dysrhythmia does not exclude MI. However, certain rhythms are often associated with acute MI due to impairment of blood supply to parts of the electrical system of the heart or to left ventricular weakening and failure. 

In an inferior wall MI, for example, it is common to see sinus bradycardia and AV node blocks, such as Type I second-degree AV block and third-degree AV block with junctional escape. These blocks reflect ischemia or damage to the SA or AV nodes, which usually have the same blood supply as the inferior wall.

AV blocks from below the AV junction, such as Type II second-degree AV block or third-degree AV block with ventricular escape, are often attributed to septal damage. Ventricular tachycardia and ventricular fibrillation are always a danger in acute STEMI, due to altered cellular function in the damaged tissues.

Key takeaways: Is this a STEMI?
When you are confronted with an ECG with subtle signs of acute ST-elevation MI, consider the following:

  1. Patient age and presenting symptoms.
  2. Patient medical history.
  3. Amount of ST elevation, shape of ST segments, and distribution of elevation.
  4. Presence of reciprocal ST depressions.
  5. Presence of associated signs of MI such as T wave inversion and pathological Q waves.
  6. Presence of dysrhythmias.

Mr. Jackson was taken to the cath lab, where a right coronary artery lesion was repaired and stented.

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