Acute Anterior STEMI: A Challenging Diagnosis

While many studies have confirmed paramedics can accurately identify ST-elevation on the 12-lead ECG, it is less clear if paramedics can identify true STEMI with a both a sensitivity and specificity adequate for the success of a regional system of care.

There are many reasons for this. In the first place, acute STEMI is not the most common cause of ST-elevation amongst chest pain patients, so it’s not enough to merely identify ST-elevation. We must identify the ST-elevation of acute myocardial infarction, and that is a much different and much higher standard.

In addition, while it’s tempting to think of all STEMIs as essentially the same, there is a tremendous variety of ECG findings that qualify for the title of “STEMI,” and some are much easier to identify than others.

One recent study showed 96% of paramedics correctly identified acute inferior STEMI, and that makes. Acute inferior STEMI is almost always an easy diagnosis and certain features, like a downsloping ST-segment in lead aVL, are almost always present to help confirm the diagnosis.

On the other hand, only 78% correctly identified acute anterior STEMI, and just 51% correctly identified acute lateral STEMI. The reality is acute anterior STEMI and acute lateral STEMI—of which there are two main varieties—are much more difficult to diagnose. That’s to say nothing of acute isolated posterior STEMI.

In this article we will concentrate on acute anterior STEMI. What makes it more challenging than acute inferior STEMI? In the first place, many of the so-called STEMI mimics, including early repolarization, left ventricular hypertrophy, left bundle branch block and paced rhythm, are primarily anterior STEMI mimics.

Consider the following 12-lead ECG. About a year ago I removed the computerized interpretive statement, posted it to my Facebook page and asked for interpretations, with the assumption that the patient was a 32-year-old male with a chief complaint of chest pain.

I was surprised by the incredible variety of interpretations that were offered. Some called it acute anterior STEMI because of the prominent T-waves in leads V2–V4. Others called it pericarditis or hyperkalemia. Some even called it WPW, although I’m not sure why.

Very few correctly identified this as the “male pattern” or early repolarization.

The features pointing away from acute anterior STEMI include:

• Intact R-wave progression (this is very important)
• Tall R-wave in lead V4
• A short QTc at 377 ms
• Absence of reciprocal changes
• Upwardly concave ST-segments
• Asymmetrical T-waves
• Prominent U-waves

There were also no changes on serially obtained ECGs. It’s important to note the classic “fish hook” J-points are not always present with early repolarization.

Now let’s look at the same ECG with the computerized interpretive statement.

I suspect most paramedics are influenced by the interpretive statement whether they realize it or not. In fact, this is now being studied by our colleagues in the U.K. in the RESPECT study (Recognition of STEMI by Paramedics and the Effect of Computerized inTerpretation).

Now let’s look at the ECG of a patient who was actually suffering acute anterior STEMI.

In this example we see that R-wave progression has been obliterated. This is extremely important, so if you take nothing else away from this article, take away this lesson. True acute anterior STEMI tends to attenuate both the R-waves and the S-waves in the anterior leads.

In addition, there are tiny Q-waves present in leads V2 and V3 which make these T-waves very suspicious for acute injury. The R-wave is present but very small in lead V4, whereas it tends to be well developed in early repolarization.

The low voltage and poor data quality in the limb leads make it difficult to assess for reciprocal changes. However, LAD occlusion does not always present with reciprocal changes, so we can’t always rely on this finding to shore up the diagnosis.

You’ll also note the QTc is prolonged at 472 ms—a not-uncommon finding with acute anterior STEMI—whereas the QTc in the early repolarization case was on the short side of normal (less than 400 ms).

Here is the same ECG with the computerized interpretation.

This was actually the third ECG taken on the call. The first ECG displayed the ***ACUTE MI SUSPECTED*** message, which went away after oxygen and nitroglycerin were administered—another excellent reason to obtain a 12-lead ECG with the first set of vital signs.

Now let’s consider another 12-lead ECG. Let’s assume the patient is a 68-year-old male with a chief complaint of shortness of breath.

Many paramedics will correctly identify ST-elevation in leads V1–V3. In addition, some might mistake the ST-depression in the lateral leads for reciprocal changes.

In reality, this is left ventricular hypertrophy with a so-called “strain pattern” (or secondary ST-T wave abnormality)—one of the most common STEMI mimics.

With left ventricular hypertrophy and “strain pattern,” the T-waves will be deflected opposite the majority of the QRS complex. This pattern is often referred to as “T-wave discordance”—a pattern also seen with left bundle branch block and paced rhythm.

Whenever you see T-wave discordance you should stop and think about it before calling a Code STEMI, because there’s an excellent chance you’re dealing with a mimic!

Sometimes you will hear this pattern referred to as a “widened QRS-T angle,” which essentially means there is more than 100 degrees difference between the QRS axis and T-wave axis in the frontal plane. If you look at the computerized measurements here you will see the QRS axis is -39° and the T axis is 159°. This is just a mathematical representation of the same phenomenon.

Let’s look at the same ECG with the computerized interpretation.

As you can see the computer correctly identifies this as left ventricular hypertrophy and does not give the ***ACUTE MI SUSPECTED*** message.

I know some of you are adding the depth of the S-wave in lead V2 and the height of the R-wave in lead V6 to see if the total is ≥ 35 mm (in fact the total is closer to 50 mm).

Remember when I said acute anterior STEMI tends to attenuate the R-waves and the S-waves in the anterior leads? It is extremely rare for acute anterior STEMI to meet the voltage criteria for left ventricular hypertrophy in the precordial leads for this very reason.

Hopefully now you see that the ST-depression in the lateral leads is the result of the left ventricular hypertrophy and does not represent the reciprocal changes of acute STEMI.

Acute anterior STEMI is a much less intimidating diagnosis when you can successfully rule out early repolarization and left ventricular hypertrophy!

Tom Bouthillet is a fire captain/paramedic with Hilton Head (SC) Island Fire & Rescue, developer of the 12-Lead ECG Challenge smartphone app, host of the Code STEMI web series on First Responders Network, a member of the EMS World Editorial Advisory Board and a member of the EMS Advisory Panel of the American Heart Association. He has taught nationally in the Critical Care Transport (CCEMT-P) program out of UMBC and his writings have been referenced in the American Heart Journal, the Journal of the American College of Cardiology: Cardiovascular Interventions and the EP Lab Digest.