The 12 Leads of Christmas: aVF

This article is the fifth in our latest series, The 12 Leads of Christmas, where each day we examine a new finding particular to an individual electrocardiographic lead.

Lead aVF

I dug myself quite a hole on this one as there’s really not much to say about aVF. It’s located midway between leads II and III so any features you see here can usually be seen in one or both of those other leads. There is one trick that we can discuss pertinent to aVF, however…

In the second piece of this series examining lead II we discussed the lead II sign of right-arm/right-leg wire swap, why it displays the pattern that it does, and how the lower extremities are essentially equivalent from an electrocardiographic standpoint. I highly suggest reading that short article again before adding this trick to your practice.

There’s two keys to why this works:

  1. The left leg and right leg are essentially electrically equivalent.
  2. The left leg electrode is the only lower extremity electrode that contributes directly to the electrocardiogram. Depending on the particular electrocardiograph machine the right leg electrode can be used to perform common-mode rejection and acts as a ground, but it is not essential and some machines don’t even require the right leg electrode to be attached.

So what does that mean? You can swap the left and right leg wires with almost no effect on the resulting electrocardiogram!

Well, there will be an effect—and this is why you would even bother doing such a thing—but if you encounter a patient for whom movement or tremor artifact is affecting the ECG and it is mostly due to movement of their left leg or the left side of their body in general, switching the two leg wires can greatly improve the clarity of the tracing.

Here’s an example of a patient who was exhibiting a fine tremor, but mostly on the left side of her body.

1373 - 82yo F - 01

Even with an aggressive 0.5–40 Hz filter I wasn’t able to get a decent-quality tracing, but when I simply swapped the left and right leg wires…

1373 - 82yo F - 02


The final result isn’t perfect because she still had some left arm tremor affecting leads I, III, and aVL, but it’s certainly better than before with no discernible shift in axis. Leads II and aVF are now perfectly clear and lead III has improved substantially.

The machine’s measurements changed slightly but my eyes can’t see any difference. The different measurements might even be due to the cleaner baseline, meaning the the second ECG could have more accurate computerized numbers despite the leg-switch.

[Disclaimer: I haven’t tried this with Mason-Likar (torso) electrode positioning and I’m not sure how that setup would affect the final result. For now only use this trick if you’re placing the leg electrodes on the lower extremities.]

So go forth and try this out the next time you can’t get a clean tracing due to artifact in II, III, and aVF. It sounds like a niche trick but I end up using it more often than you would think. In fact, in the case above, it was clear the patient had a left-sided tremor so I actually ran the bottom EKG first and then captured the top one after to demonstrate that there were no adverse changes.


I hope you’re enjoying our 12 Leads of Christmas series. You can check out the rest of the posts below (updated as new posts come out):

12 Leads of Christmas: Lead I
12 Leads of Christmas: Lead II
12 Leads of Christmas: Lead III
12 Leads of Christmas: aVL
12 Leads of Christmas: aVR
12 Leads of Christmas: V1
12 Leads of Christmas: V2
12 Leads of Christmas: V3
12 Leads of Christmas: V4
12 Leads of Christmas: V5
12 Leads of Christmas: V6


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