The Six-Step Method for 12-Lead ECG Interpretation

I’m sometimes asked how I approach 12-lead ECG interpretation. I use what I call the “Six-Step Method” (which actually has seven steps).

It goes like this:

1.) Rate and rhythm
2.) Axis determination
3.) QRS duration (Intervals)
4.) Morphology
5.) STEMI mimics
6.) Ischemia, Injury, Infarct

“Step 7” is a rule I started throwing in to remind students that one should always interpret an ECG (or any other diagnostic test) in light of the history and clinical presentation.

Let’s break them down one at a time.

1.) Rate and rhythm

Are you dealing with a bradycardia or a tachycardia? If the exact rhythm is unknown, are we certain we’re dealing with a supraventricular rhythm?

This is critical because if the rhythm has wide QRS complexes (fast or slow) it’s ventricular until proven otherwise!

Failure to observe this simple rule can cause a lot of problems.

2.) Axis determination

Is the axis in the normal range?

Is it a left axis deviation (left superior axis), which might suggest left anterior fascicular block, inferior MI, or paced rhythm?

Is it a right axis deviation (right inferior axis), which might suggest left posterior fascicular block, lateral MI, right ventricular hypertrophy or acute right-sided strain?

Is it an extreme axis deviation (right superior axis), which might suggest VT, electrolyte derangement, or misplaced limb lead electrodes?

Examining the heart’s electrical axis in the frontal plane is one of the techniques I use to get a “feel” for a 12-lead ECG.

Similarly, while I don’t try to pinpoint the heart’s Z-axis (the horizontal plane), I do notice R-wave progression, the transition, and whether or not there is positive or negative concordance of QRS complexes in the precordial leads.

3.) QRS duration (and other intervals like the PR interval and QT interval)

If you’ve followed the first two steps there’s a good chance you’ve already picked up on a prolonged PR interval or wide QRS complex, but “Step 3” is the designated time to make sure you’re dealing with a narrow QRS rhythm (or a supraventricular rhythm with wide QRS complexes).

Time and time again I see paramedics who are new to 12-lead ECG interpretation saying things like “paced rhythm with left bundle branch block” or “VT with right bundle branch block.”

Maybe they mean “paced rhythm with left bundle branch block morphology” or “VT with right bundle branch morphology” but something like this is too important to be lazy with terminology!

This is also the designated time that you look at the QT/QTc and verify that the QTc is < 500 ms (and hopefully < 460 ms).

4.) Morphology

If the QRS complex is “wide” (the QRS duration is = or > 120 ms), what is the QRS morphology in lead V1?

Is it RBBB morphology or LBBB morphology? Typical or atypical? Now check lead I to confirm! That’s an important step, because if lead V1 shows LBBB morphology and lead I shows RBBB morphology (or vice-versa) then it’s a nonspecific intraventricular conduction block which may suggest an electrolyte derangement or drug overdose.

If it’s RBBB morphology in lead V1, combine with axis determination to determine whether or not bifascicular block is present (or at least bifascicular morphology).

Does anything look bizarre? This is your chance to examine the P/QRS/ST/T to see if anything stands out. This is where you might pick up on things like Brugada’s syndrome.

5.) STEMI mimics (QRS confounders, Imposters of AMI)

By now we’ve already determined whether or not a bundle branch block or paced rhythm is present, and there’s an excellent chance you’ve already picked up on several other abnormalities that could mimic or mask acute myocardial infarction.

However, this is where I explicitly rule out the STEMI mimics (paced rhythm, left bundle branch block, left ventricular hypertrophy, benign early repolarization, pericarditis, Wolff-Parkinson-White pattern, ventricular aneurysm, hyperkalemia).

6.) Ischemia, Injury, Infarct

Finally, I look for the obvious signs of acute STEMI (ST-elevation or hyperacute T-waves). I also look for ST-depression, T-wave inversion, abnormal Q-waves, and so on.

If an STEMI mimic is present, I look for acute STEMI in the presence of an STE-mimic using things like Sgarbossa’s criteria, the rule of appropriate T-wave discordance, and reciprocal changes.

To be honest, it’s not this linear in my mind because I’ve been doing this for a long time and my eyes often shoot straight to the most obvious abnormality on a 12-lead ECG.

However, I do not violate any of these principles!


  • Christopher says:

    An addition to #6 would be maintaining a consistent order to look for Ischemia, Injury, and Infarct: "I See All Leads".That is: – Inferior: II, III, aVF – Septal: V1, V2 – Anterior: V3, V4 – Lateral: V5, V6, I, aVLI also really liked Garcia's "Mine for Gold" step. The step where you take everything you've found and look for the one or two things which tie it all together into a field diagnosis and plan of action.

  • Tom B says:

    C.Watford – My favorite "Garcia-ism" is "consider the company it keeps". In other words, a funny little Q-wave in lead III by itself probably doesn't mean anything. But throw in a downsloping ST-segment in lead aVL and an R/S ratio > 1 in lead V2, then all of a sudden the funny little Q-wave in lead III amounts to something.Tom

  • Christopher says:

    Posterior MI changes?

  • Tom B says:

    C.Watford – If you're asking about the increased R/S ratio in lead V2, then yes, I'm referring to possible posterior involvement, which is consistent with acute inferior STEMI (RCA occlusion).But you could conceive of other examples, too. Certain ECG findings "go together" and point toward a pathological process. Other times, a single minor ECG finding, or even two minor unrelated ECG findings can be taken with a grain of salt.It's like Dr. Wes says, sometimes the hardest ECG diagnosis of all is "normal ECG." Tom

  • Mike says:

    As part of my interpretation, I use the "I see all leads" but also throw in papa smurf. The first 4 as per C Watford, the papa: posterior MI (V1, V2, V3), the smurf: S1Q3T3/signs of right ventricular strain (PE indicators)I see all leads papa smurf. It also helps that we have nick-named one of our dispatchers Papa Smurf:)

  • Excellent overview; the only addition I always add is to REPEAT the ECG if you've any worry about ongoing changes etc. and don't underestimate the value of getting old ECGs to compare with.Enjoying your site.

  • Tom B says:

    Mike – Who doesn't like the smurfs? Just remember that the most common ECG finding associated with PE is sinus tachycardia! Tom

  • Tom B says:

    Cardiology Cases -Excellent point about the value of serial ECGs and an "old" ECGs for comparison! I'm enjoying your site, too! Thanks for the comment.Tom

  • Donna says:

    I also like to turn the EKG around so your looking at the white side on the lifepack 12 It will give you a quick look and different perspective something might just POP out!

  • Mubeen Malik, MBBS, Dip.Card. ( Lond ) says:

    Good Summary: " How to read an ECG ". I really enjoyed reading all the article and would refer it my colleagues.

  • Collette Saxe says:

    I am currently just a basic EMT.  Would you have any suggestions of good web sites or books to read to learn more.  I am not ready to take ALS class yet and it isn't offered in my county this year.  I appreciate the tools and knowledge that the ALS providers use.  Thanks for your insights.  I am tucking them away..nuggets of truth. : )

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