A few weeks ago on JEMS Connect there was a thread called Vectors, Axis and Cardiology.

In it, Dave M. asked:

I truly enjoy learning and studying the heart, how it works and why it works that way. I had the privilege of teaching a paramedic class today and going over the vectors and axis information for them. I am not strong in this area and tried to do the best I could as well as provide some online resources for them to look at. Then I got the dreaded question, “what good will this do for me when I am working with a patient?” I tried to make the point that it has to do with conduction pathways and determining where the impulse is coming from, but I know that it was not an effective answer.

Does anyone have a better answer that I can use and some resources that I can use myself and provide to the students on these subjects?

I was surprised how little discussion this thread generated but I did my best to answer Dave M.’s question. Since it will probably come up again I’m cross-posting it here with some examples.

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I’m reminded of what Jeff Beeson, DO, LP said in a breakout session at EMS Today 2010. “The eyes cannot see that which the mind does not know.” Without a proper understanding of axis and vectors you cannot really “see” a 12-lead ECG. Once you fully understand axis and vector it gives you a vocabulary that allows you can take ECG interpretation to the next level.

It doesn’t help that this topic is poorly taught at all levels.

Here are some examples where understanding the heart’s axis and vectors has helped me make wise decisions on actual emergency calls.

• The most common causes of left axis deviation are left anterior fascicular block and Q-waves from inferior MI. So when I see a left axis deviation it prompts me to consider these conditions. Many times I have caught subtle inferior STEMIs because the axis was slightly to the left and it prompted me to look at lead aVL for subtle reciprocal changes.



• A paced rhythm with a pacing lead in the apex of the right ventricle typically shows LBBB morphology in lead V1 and left axis deviation. So this prompts me to double-check for a pacemaker pocket on the patient’s chest and consider that the rhythm may be paced before I decide the patient is showing frequent PVCs or a run of slow VT.

Remember Rhythm Challenge #5?



• Conversely, it would be very unusual for LBBB or paced rhythm to show LBBB moprhology in lead V1 with a right axis deviation. That in turn further supports the dx of VT in a patient who happens to have a pacemaker. That helped me identify a run of VT at a rate of 140 when others called it a “runaway pacemaker.”

Here’s a similar example.



• A pulmonary disease pattern may pull the axis to the right. It may also cause right atrial enlargement. In addition many congenital heart defects cause right ventricular hypertrophy with an associated right ventricular strain pattern. So when you see right axis deviation, tall R-waves in lead V1, and T-wave inversion in the right precordial leads, you know it’s consistent with the patient’s history and not “anterior ischemia” requiring NTG. There is a young woman with a congenital heart defect in my jurisdiction who has received MONA for her anxiety attacks more than once because of her abnormal ECG.

Here’s the ECG if you’re curious.



• Q-waves from high lateral MI pulls the axis to the right. Left posterior fascicular block is rare as an isolated finding, but that also pulls the axis to the right. Combine left anterior fascicular block (left axis deviation) or left posterior fascicular block (right axis deviation) with RBBB morphology in lead V1 and it’s referred to as a “bifascicular pattern” which is one of the keys to understanding wide complex tachycardias, IMHO.

Here’s an example from the tutorial on wide complex tachycardias.



• An extreme right axis deviation (or right superior axis depending on what terminology you prefer) might suggest incorrect lead placement, electrolyte derangement, or help you rule-in a ventricular rhythm. I can’t discuss this topic without mentioning that failure to “rule-in” VT based on QRS morphology does not “rule-out” VT. Brugada and Wellens’ criteria are not well understood, IMHO, and have led far too many health care providers of all stripes to call a wide complex rhythm “SVT with aberrancy” when it was not warranted which can lead to clinical misadventure.

Never assume that a wide complex tachycardia is SVT with aberrancy based solely on QRS morphology! Just because it looks like LBBB doesn’t mean it isn’t VT.

I could give other examples, but the point is that you cannot develop a “trained eye” or a nuanced understanding of the 12-lead ECG if you don’t have tools to describe what you see. I was explaining the concept of appropriately discordant T-waves with bundle branch block to someone the other day and it would have been extremely difficult if he didn’t understand the concept of a terminal deflection.

If you want a dramatic illustration of this point, teach a 12-lead ECG class and at the beginning of the class ask the students to take out a blank piece of paper and draw a picture of a normal 12-lead ECG. If you don’t understand “normal” how can you possibly hope to identify “abnormal”?

So I would suggest that anyone who asks “why do we need to know that?” that it’s no different from considering a Mallampati score when evaluating a patient’s airway anatomy. You’re looking at the big picture and you’re seeing it. Therefore you retain more and you learn more with each patient encounter.

I’ve been collecting ECGs for 15 years, and I’m still amazed at what I can “see” now in ECGs that I collected 10 or 15 years ago. It scares me, actually, because I wonder how I was able to treat some of these patients without knowing what I know now, but experience is funny like that.

I hope this supplies at least a partial answer. Learning to read a 12-lead ECG is like learning a foreign language. If you only want to learn how to find the bathroom or order a beer, you can learn what you need to know in a couple of days. If you want to learn how to sing the national anthem and make the locals cry it takes a little longer.

See also:

Axis determination – Part I

Axis determination – Part II

Axis determination – Part III

Axis determination – Part IV

Axis determination – Part V

Axis determination – Part VI