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)
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).
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.) STE-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 STE-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 STE-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!