By now you can predict the QRS axis in the frontal plane within 15 degrees as long as you have an equiphasic (or isoelectric) lead in the frontal plane. So what constitutes a normal QRS axis? What is a left axis deviation? A right axis deviation?
If you don’t have a copy of the hexaxial reference system, go back to Part IV and print yourself out a copy.
As a review, lead I cuts the hexaxial reference system in half horizontally and lead aVF cuts the hexaxial reference system on half vertically. You can think of this as an x and y axis that divides the hexaxial reference system into quadrants. Hence, you can use leads I and aVF to place the heart’s electrical axis into one of the four quadrants. This is sometimes called the Quadrant Method for axis determination.
Remember that the normal QRS axis goes from a right shoulder-to-left leg direction in most patients. In other words, it tends to point down and to the left, or toward the left inferior quadrant of the hexaxial reference system, which ranges from 0 to +90 degrees. When the QRS axis in the frontal plane is in the normal quadrant, you will have positive QRS complexes in lead I and positive QRS complexes in lead aVF.
When the QRS axis is 0 to -90 degrees, we call it a left axis deviation. This is the left superior quadrant of the hexaxial reference system. When the QRS axis is in the left superior quadrant, you will have positive QRS complexes in lead I and negative QRS complexes in lead aVF.
In reality, the QRS axis can be slightly into the left superior quadrant and still be considered normal.
When the axis is between 0 and -30 degrees, it is sometimes referred to as a physiological (as opposed to pathological) left axis deviation. With a physiological left axis deviation, lead II is usually equiphasic (remember that lead II is perpendicular to lead aVL and lead aVL points to -30 degrees on the hexaxial reference system). For a good example of this, see the ECG from Part V. Is this ECG normal? Absolutely not! But the axis is technically normal, even though it extends into the left superior quadrant at -26 degrees.
The most common causes of pathological left axis deviation are left anterior fascicular block or Q waves from inferior wall myocardial infarction. Some sources say that left ventricular hypertrophy pulls the axis to the left, and while this seems logical, in most cases patients with left ventricular hypertrophy have a normal QRS axis. Electrolyte derangements and ventricular rhythms may also present with a left axis deviation. Paced rhythms in particular should have a left axis deviation if the pacing lead is in the apex of the right ventricle.
If the QRS axis in the frontal plane is +90 to 180 degrees, it is a right axis deviation. This is the right inferior quadrant of the hexaxial reference system. With a right axis deviation, you will have negative QRS complexes in lead I and positive QRS complexes in lead aVF.
A right axis deviation is usually abnormal. It might indicate pulmonary disease, right ventricular hypertrophy, Q waves from lateral wall myocardial infarction, left posterior fascicular block, electrolyte derangement, or tricyclic antidepressant overdose, or a ventricular rhythm.
If the QRS axis is -90 to 180 degrees, something is very wrong (possibly your lead placement). This is the right superior quadrant of the hexaxial reference system, but in various publications it can be called an extreme right axis deviation, an indeterminate axis, or a right shoulder axis. It’s bad because it means the heart is depolarizing in the wrong direction. With an extreme right axis deviation, you will have negative QRS complexes in lead I and negative QRS complexes in lead aVF.
Finally, here’s a cheat sheet you can fall back on if all else fails. This one relies only on leads I, II, and III (although you can substitute lead aVF for lead III). This method works pretty good because, as we saw earlier, by looking for an equiphasic QRS complex in lead II we can distinguish between physiological and pathological left axis deviation.
Remember, QRS complexes in lead III are allowed to be negative. However, negative QRS complexes in lead I or lead II are abnormal.
Do I expect you to remember all this right now? No, I do not. Experience is the best teacher, and there’s nothing like holding an ECG in your hand and associating it with a particular patient. My goal is simple. I want you to start seeing it.
When you capture a 12 lead ECG with good data quality, in most cases you’ll get a computerized interpretive statement at the top. You’ll also get the computer measurements of the heart rate, PR interval, QRS duration, QT/QTc interval, and P-QRS-T axes. When you see that the QRS axis is -66 degrees and the interpretive statement says “Left axis deviation” I want you to take a good look at the ECG. Do you notice that the QRS complexes are positive in lead I and negative in leads II, III, and aVF?
A deeper understanding of axis determination helps you really see the 12 lead ECG, not just lead II, and it ultimately helps you consider various possibilities that you hadn’t considered before.
See also:
Axis determination – Part VI
What do you think of teaching the “thumbs method”?Is this too much an oversimplification that it obscures relevant information?
Thanks for the comment, James! I apologize, but I don’t know what the thumb method is.
Thumbs method for axis determination:Hold up your left and right hands and make a thumbs up/thumbs down position in the direction of the QRS in Leads I and aVFTwo thumbs up = NormalLeft thumb down, right thumb up = LADLeft thumb up, right thumb down = RADBoth left and right down = Extreme Axis Deviation
Okay, I gotcha. It’s sort of like the “turn signal” method for bundle branch block. Honestly, I think you have to use the method that works for you. Not everyone has the time or inclination to become an expert. Learning to recognize what quadrant the axis is in (however you do it) is better than not seeing it at all! Thanks for the follow-up.Tom
thanks tom! you are awesome…im a med student and this helps a ton….teacher hardly explained this stuff in class.
Same as above, I'm a medical student and your tutorial helps as another person explaining a concept key to such an important diagnostic tool.
JGu – Thanks for the kind words! Sorry I missed your comment in August. I think I was in Mexico! Tom
Dane – I'm glad you found the tutorial to be helpful.Tom
Great stuff — thanks for the help, it's been incredibleNow — what do the deviations mean? Are there pathological determinants that are case specific in the presence of these Axis Deviations?
Mike R. -Yes, for example Q-waves from inferior MI will pull the axis left. Q-waves in the high lateral leads will pull the axis right.Left anterior fascicular block will pull the axis left. Left posterior axis deviation will pull the axis right. Please note that left posterior fascicular block is an ECG diagnosis of exclusion.Right ventricular hypertrophy or acute right ventricular strain will pull the axis right. In some cases, left ventricular hypertrophy will pull the axis left. Left bundle branch block will pull the axis slightly left.Paced rhythms with the pacing lead in the apex of the right ventricle will show LBBB morphology in lead V1 with a left axis deviation.Then there are the bifascicular patterns, which in turn help you undertand wide complex tachycardias.Experience is the best teacher, but for experience to teach you, you have to know what you're looking at! So knowing how to read the axis must come first.Tom
nice sir….u made it so easy 4 me…..
Hey Tom! Man this tutorials been almost 3 years. Also a 2nd final year med student and just stumbled upon it and its AMAZING!! Our lecturers dont teach it well….its almost like they've forgotten how to do it as well!
We tend to use the quandrant method in australia, i.e. drawing a simple x y axis with lead 1, horizontal in the middle, and avf vertical and going down the middle of lead 1. Then If Avf is positive and lead 1 negative, its right axis. Is that an ok method as well?
hha the working out of the exact degree is still somewhat challenging without the cheat sheet.
Wow thank you SO much for this. I'm in medical school and today we went over ita nd it went ovr my head. I had to reserach stuff online and got to someone's blog that directed me here and OMG I AM FINALLY GETTING AXIS DETERMINATION. Took me a while but thank you SO MUCH
It helps a lot man….thank you for such an amazing work:)
Hey i found it very useful ! Thanks
You sir, are my hero.
This is BY FAR the best explination of axis determination I have yet to find (and I have done a lot of looking). I am and EMT-B learning to interperate ECGs outside of a formal setting for my own information, plus I hate being the only one on a scene that doesn't know. Axis determination had always been the one thing that I could not grasp and this made it as clear as it could be.
I do have one suggetion though. I know you did include some disorders that can cause deviation from the normal axis range for the QRS complex but, there was not a lot about what P and T wave deviations might indicate. I think that might make a good follow up article. And forgive me if that already exists and I just missed it, I am new to this site.