Usefulness of the QRS/T angle when diagnosing acute STEMI

A reader asks, “Have you ever seen a patient with a QRS/T angle of greater than 100 degrees who was experiencing acute STEMI?”

To give you some background, I often suggest to aspiring ECG aficionados that they consider the QRS/T angle when analyzing a 12-lead ECG. When the QRS/T angle (the difference between the QRS axis and the T axis in the frontal plane) is ≥ 100 degrees, I say, “Stop. Think about it! There’s an excellent chance it’s not a STEMI.”

The reason is simple.

A QRS/T angle of ≥ 100 degrees is nothing more than a mathematical representation of a general pattern of T-wave discordance. Generally speaking, it means the T-waves on the ECG are deflected opposite the majority of the QRS complex, which is a normal finding in left bundle branch block, paced rhythm, and left ventricular hypertrophy with strain.

In other words, it’s a normal finding with a secondary ST/T-wave abnormality.

See also: “Should you activate the cardiac cath lab?

To answer the question, “Yes! I have seen patients with a QRS/T angle of greater than 100 degrees who were experiencing STEMI!”

Let me show you two examples.

Example 1

lad occlusion qrs t angle greater 100 A wm

In this case we have a 68 year old male with a chief complaint of chest pain.

The ECG shows excellent data quality but a tachycardia is present (which increases the odds of a false positive computerized interpretive statement of ***ACUTE MI SUSPECTED***). The QRS duration is 108 ms and the QRS/T angle is 130 degrees.

At first glance, even though the rate is 134 and not 150, I find myself wanting to make the case for 2:1 atrial flutter. However, when I find the middle point between the inverted P-waves, I don’t find any sign of a hidden flutter wave buried in the T-wave.

There is significant ST-segment elevation in leads V1 and V2 with slightly less ST-segment elevation in lead V3. The S-waves are about 17 mm deep in lead V2, which makes them fairly prominent, but < 20 mm. The voltage criteria for LVH in the precordial leads are not met.

To my eye, there is not enough S-wave depth to explain 3 mm of discordant ST-segment elevation (measured at the J-point).

lad occlusion qrs t angle great than 100 A V2

Throw in the subtle, upwardly convex ST-segment elevation in lead III and the subtle ST-segment depression in lead aVL, and a picture emerges of probable LAD occlusion.

If you can understand why this example shows STEMI and this one doesn’t you’re in great shape!

Example 2

lad occlusion qrs t angle greater than 100 B wm

This was a 65 year old male patient who presented to EMS after experiencing 3 hours of substernal chest pain he initially attributed to indigestion. After taking antacids and belching several times the pain got worse.

The heart rhythm is sinus tachycardia. The QRS duration is 116 ms and the QRS/T angle is 107 degrees. Could this be a secondary ST/T-wave abnormality? After all, the QRS duration is very close to 120 ms. It feels almost like incomplete left bundle branch block!

A couple of points here.

First, even if this were complete left bundle branch block, it would be positive for acute STEMI!

The S-waves in lead V2 are only 10 mm deep! Using Smith’s modification to Sgarbossa’s criteria we would allow 2 mm of discordant ST-segment elevation in this lead but instead we see 4 mm of discordant ST-segment elevation! That’s a lot.

Similarly, the S-waves are only 5 mm deep in lead V3 so we’d only expect 1 mm of ST-segment elevation in this lead. Instead we see 3.5 mm of ST-segment elevation.

lad occlusion qrs t angle greater than 100 B V4

As compelling as this evidence may be, lead V4 removes all doubt. In lead V4 the S-waves are only 1.5 mm deep! We should not be able to appreciate any ST-segment elevation in this lead. Instead we see 2.5 mm of ST-segment elevation! That means there is more discordant ST-segment elevation (measured at the J-point) than S-wave depth.

Those with a very sharp eye will also notice that the ST-segment depression in the inferior leads looks very straight and more consistent with reciprocal changes than a secondary ST/T-wave abnormality.

Remember, no rule is 100%. When I ask you to look at the QRS/T angle it’s because I want you to consider the possibility that you’re looking at a secondary ST/T-wave abnormality! The key is “considering the company” that any ECG abnormality keeps (thank you Tomas Garcia, M.D.!) and interpreting the ECG in light of the history and clinical presentation.

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Tom Bouthillet
Tom Bouthillet - 25 posts

Tom Bouthillet (@tbouthillet) is Editor-in-Chief of ECGMedicalTraining.com (@ECGTraining) and Fire Captain/Paramedic in South Carolina where he is the Emergency Cardiac Care Program Manager and the STEMI and CARES Site Coordinator of his fire department.

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