Importance of Lead aVL in STEMI Recognition

In 2013 an article was published in Prehospital Emergency Care that measured the ability of paramedics to identify acute STEMI on the 12-lead ECG. The results showed that paramedic accuracy varied depending on the infarct location.

  • Inferior: 96%
  • Anterior: 78%
  • Lateral: 51%

This result did not surprise me.

I have long argued that acute inferior STEMI is the bunny slope of 12-lead ECG interpretation. One reason is that there is almost always a reciprocal change in lead aVL to shore up the diagnosis. This finding is so sensitive that the absence of a reciprocal change in lead aVL should make you doubt the diagnosis of acute inferior STEMI.

A downsloping ST-segment in lead aVL is a red flag that should get your attention!

Consider this example.


The QRS complexes are very small in the inferior leads so our threshold for what constitutes a significant amount of ST-elevation is less. This is known as the rule of proportionality. Because the ST-segment elevation is < 1 mm the computer is not giving the ***ACUTE MI SUSPECTED*** message.

Note the downsloping ST-segment in lead aVL! That should make you very suspicious.

There is also ST-depression in in the precordial leads. Do you see how the evidence is stacking up? This is what Tomas Garcia, M.D. (author of 12-Lead ECG: The Art of Interpretation) means when he says “consider the company it keeps” when referring to any ECG abnormality.

The patient was suffering acute inferior STEMI even though this ECG did not meet the “≥ 1 mm of ST-elevation in 2 or more anatomically contiguous leads” criterion. Many would call this a subtle acute inferior STEMI but others like Stephen Smith, M.D. would argue there’s nothing subtle about it.

The take-away point is that you should pay very close attention to a downsloping ST-segment in lead aVL.

Except when you shouldn’t.


In this case we might easily be fooled into thinking we have LAD occlusion with reciprocal changes in leads I and aVL. However, look at the ST-segment morphology! It’s more consistent with a strain pattern (secondary ST/T abnormality) resulting from left ventricular hypertrophy.

computer measurements

Note that the QRS/T angle (difference between the QRS axis and T axis) is 100 degrees! Whenever the QRS/T angle is ≥ 100 degrees you should stop and think because there’s an excellent chance it’s not a STEMI! This is a mathematical representation of a general pattern of T-wave discordance. In other words, generally speaking, the T-waves are deflected opposite the majority of the QRS complex.

You will see this with left ventricular hypertrophy, left bundle branch block, and paced rhythm — some of the most common causes of ST-segment elevation on the 12-lead ECG! It’s a sign that you’re probably dealing with a secondary ST/T-wave abnormality as opposed to the primary ST/T-wave changes of cardiac ischemia/injury.

As a side-note, the S-wave in lead V2 is 30 mm deep! It is rare for LAD occlusion to meet the voltage criteria for LVH in the precordial leads so you should always take ST-elevation in the anterior leads with a grain of salt when deep S-waves are present!

This is an unusual example because the GE-Marquette 12SL interpretive algorithm (used by the LIFEPAK 12) is rarely fooled by left ventricular hypertrophy.

Let’s look at another case.

lvh stemi 1

Here we have ST-elevation in the inferior leads (II, III, and aVF) and ST-depression in the high lateral leads (I and aVL). However, the QRS/T angle is > 100 degrees! It also meets the limb lead voltage criteria for LVH. Is this acute inferior STEMI or a strain pattern?

Look carefully at the ST-segment depression in lead aVL.

strain vs reciprocal change aVL

Do you see the difference? With a “strain pattern” (secondary ST/T-wave abnormality) the ST-depression is downwardly concave with asymmetrical T-wave inversion. I sometimes refer to this as “pouty lipped”.

Serial ECGs can frequently remove all doubt!

lvh stemi 2

After oxygen and nitroglycerin the patient was almost pain-free and the ST-elevation resolved. The patient was sent to the cath lab anyway and had a 99% occlusion of the RCA.


Mencl F, Wilber S, Frey J, Zalewski J, Maiers J, Bhalla M. Paramedic Ability to Recognize ST-segment Elevation Myocardial Infarction on Prehospital Electrocardiograms. Prehospital Emergency Care. 2013;17(2):203-210. doi:10.3109/10903127.2012.755585.


About author

Tom Bouthillet
Tom Bouthillet - 25 posts

Tom Bouthillet (@tbouthillet) is Editor-in-Chief of (@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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