You’ve probably always heard the phrase “mean QRS axis” and if you want to go on saying it that way, that’s perfectly fine. I just want to point out that an axis is a just a fixed line on the hexaxial reference grid and that what you’re really tracking is the mean QRS vector. What we’re doing is determining which axis on the hexaxial reference grid most closely approximates the mean QRS vector.
OK, where do we start? We use only the limb leads – Leads I, II, III, aVR, aVL and aVF. We don’t use the precordial leads because they measure forces in the horizontal plane. The limb leads measure forces in the frontal plane and that’s the plane we are concerned with right now.
Let’s begin by making sure that we’re all on the same page with the terms that we’ll be using.
Isoelectric means that there is zero voltage recorded in that particular lead for the QRS interval. It doesn’t mean that the QRS interval doesn’t exist – it just indicates that the mean QRS vector is exactly perpendicular to that lead and is invisible to it.
Biphasic means that a deflection (in our case, the QRS complex, but it could also refer to the P wave or the T wave) has a positive portion and a negative portion. It doesn’t imply or address anything regarding the relative sizes of the two portions.
Equiphasic means that the two portions of the biphasic deflection are exactly equal. It also implies a net zero voltage for that deflection in that particular lead.
Now, the first thing we want to do is find the limb lead that has a QRS interval that most closely approximates zero voltage (isoelectric) or net zero voltage (equiphasic). Let’s call that the perpendicular lead. A QRS interval that is perfectly isoelectric is a very rare occurrence; a QRS that is equiphasic is not too uncommon. Either one will suit our purpose. Lead aVL (in the ECG shown above) is biphasic and almost equiphasic. It will serve as our perpendicular lead.
Now, after locating the isoelectric or equiphasic QRS complex, what’s the next thing we do?
We must determine if the almost equiphasic complex is slightly positive or slightly negative.
After doing so, we find the positive pole of the lead located 90° away. We will call that the vectorial lead or vector axis* since it more closely approximates the mean QRS vector.
Let’s pause right here while I editorialize a little bit…
Almost everyone who interprets ECGs at a beginners’ level thinks of the QRS with the greatest amplitude as being the QRS with the tallest R wave or the deepest S wave – actually, beginners usually don’t even think about the deep S waves – they’re all looking for tall R waves! Unfortunately, many of us who have progressed further also make the same mistake.
But that’s not what “amplitude” refers to.
Amplitude refers to the positive or negative area within the Q wave, R wave and S wave using the baseline as the enclosing boundary. It does NOT refer to height or depth!
Let’s look at this illustration (ECG Axis 2 at the top of the page)…
What we see are three deflections: two R waves – one tall and thin and the other short and wide – along with a biphasic complex.
Many people would say that, of the first two deflections, the first R wave has the greater amplitude because it’s taller. But look at the area enclosed by the short and wide R wave. There’s a lot more area there, so of the two, the second R wave has the greater amplitude.
Now let’s look at the third complex – the biphasic one. The R wave is prominent, but it certainly doesn’t contain as much area as the S wave. The S wave is going to cancel the R wave leaving a small, net negative amplitude.
The bottom line is, you can’t depend on the height of the R wave or the depth of a Q or S wave to tell you with any certainty if a deflection indeed has the greatest amplitude. Obviously, it is going to require integral calculus to determine the area under a curve, but don’t worry… it is being done for you by the ECG machine! The ECG machine happens to be very good at integral calculus and calculates the true amplitude for you! Just be aware that frequently the lead indicated by the ECG machine as the vector axis will not contain the tallest R wave or the deepest S wave. Again… the ECG machine indicates the actual amplitude – not height or depth.
So that is why you should always begin your determination of the axis of the mean QRS vector by first locating the lead with the QRS interval that is the most isoelectric or equiphasic. It will lead you to the QRS complex that indeed has the greatest amplitude – not just the tallest R wave or deepest S wave.
Getting back to locating the vectorial lead located 90° away from the perpendicular lead, you will immediately see that we have a problem: we know that the mean QRS vector, while not exactly coincident with the vectorial lead, will lie very close to it. But on which side of the vectorial lead?
If the nearly equiphasic QRS complex in the perpendicular lead is slightly NEGATIVE, the mean QRS vector in the frontal plane will be located between the NEGATIVE pole of the perpendicular lead and POSITIVE pole of the vectorial lead (the lead located 90° away), but much closer to the vectorial lead. This is indicated by the purple arrow in the diagram (ECG Axis 3) at the top of the page.
If the nearly equiphasic QRS complex in the perpendicular lead is slightly POSITIVE, the mean QRS vector in the frontal plane will be located between the POSITIVE pole of the perpendicular lead and the POSITIVE pole of the vectorial lead (the lead located 90° away), but much closer to the vectorial lead. This is indicated by the green arrow in the diagram (ECG Axis 3) at the top of the page.
In the ECG at the top of the post, the lead whose positive pole is 90° away from Lead aVL (the perpendicular lead) is Lead II. Lead II is our vectorial lead.
As a general rule (and using the ECG at the top of the page)…
If the QRS in the perpendicular lead is perfectly equiphasic, the QRS vector will coincide with the Lead II axis located 90° away. (If this were the case here, we would say that the QRS vector is located at exactly +60°.)
If the QRS in the perpendicular lead is biphasic and slightly positive, it will be located a short distance away from the Lead II axis that is 90° away (Lead II) and will be found between the positive pole of Lead II and the positive pole of Lead aVL. (If this were the case here, we would say that the QRS vector is located at approximately +55°. The subtraction of 5 degrees is a “guesstimate” which adds a little more accuracy but mostly indicates that the QRS vector does not perfectly coincide with Lead II.)
If the QRS in the perpendicular lead is biphasic and slightly negative, it will be located a short distance away from the lead axis that is 90° away (Lead II) and will be located somewhere between the positive pole of Lead II and the negative pole of Lead aVL. (Since this is the case here, we could say that the QRS vector is located at approximately +65°. The addition of 5 degrees is a “guesstimate” which adds a little more accuracy but mostly indicates that the QRS vector does not perfectly coincide with Lead II.)
Briefly, if the biphasic QRS in the perpendicular lead is slightly positive, the mean QRS vector will be located between the POSITIVE POLE of the vectorial axis and the POSITIVE POLE of the perpendicular lead, but much closer to the vectorial axis. (Green arrow)
If the biphasic QRS is slightly negative, the mean QRS vector will be located between the POSITIVE POLE of the vectorial axis and the NEGATIVE POLE of the perpendicular lead, but much closer to the vectorial axis. (Purple arrow)
If you wish, you can add or subtract 5 – 10° to/from the vectorial axis for a bit more accuracy (and to amaze your colleagues). That will be strictly a guess, however, and you will do just as well without it.
Well, I hope you learned how to locate the mean axis of the QRS vector in the frontal plane and I also hope that you now appreciate why we take the approach that we do – as odd as it may seem at first.
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*The terms perpendicular lead, vectorial lead and vector axis are proprietary to this discussion, i.e., they are not used universally. Call the two leads whatever you want but just be careful that you don’t get them confused.