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

* 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

* Equiphasic* means that the two portions of the biphasic deflection are

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

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

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

But that’s **not** what “amplitude” refers to.

* Amplitude* refers to the

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

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 the QRS in the perpendicular lead is * biphasic* and slightly

If the QRS in the perpendicular lead is * biphasic* and slightly

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.

If you would like to learn more about electrocardiography at an advanced level, please take a moment to visit the rest of our website.

Medicus of Houston produces continuing medical education for physicians and other healthcare providers with a focus on advanced training in 12-lead ECG interpretation and complex dysrhythmia analysis. We promote active participation in all our courses. During our * Advanced ECG Interpretation Boot Camp*, you will have up to 14 hours of active participation in the interpretation of complex ECGs – during the class – while being guided by myself or one of our experts. Our boot camps are all 4-day, live events held right here in the beautiful Galleria – Post Oak area of Houston – in the midst of hotels, restaurants, shopping and entertainment.

*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.