The 12 Leads of Christmas: Lead II

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

The Lead II Sign

Yesterday, we covered the lead I sign, so it’s only fitting that we discuss the lead II sign today.

Lead II Sign

There’s a “lead I sign” in lead II.


It’s a commonly described finding, but I’ve never seen that term used anywhere else. Considering our last post, it seems fitting though. What we see here looks a lot like the lead I sign, except it’s in the wrong spot. Could this be a trick of the vectors again, where lead II just happens to be perpendicular to the three major vectors we see on the ECG (P, QRS, and T)?

What kind of pathology could be responsible for that?

Well there is something pathological going on here, but it’s not in the patient, it’s on her: the ECG electrodes! This is an example of a right-arm/right-leg (RA/RL) cable swap. We see this pattern on the ECG because, electrocardiographically speaking, the right leg and left leg are equivalent.

Einthovens Triangle Anatomical

Imagine what we are doing when we perform an ECG: We examine the change in voltage between two points as the heart depolarizes. If we’re monitoring lead II, we are looking at the change in voltage between the right arm (RA) and the left leg (LL) with each heart beat. Move the RA electrode to the right leg (a RA/RL swap), and we’re now examining the voltage difference between the right leg and the left leg. But—and this is where you need to trust me because it might not make intuitive sense at first—these two limbs are almost electrically equal, meaning that there is almost no voltage change observable between them. The result is an isolelectic, or nearly-isoelectric, line in lead II.

Put another way: If there is no change in voltage, there are no deflections on the ECG. Since there is almost no voltage difference between the two legs, if you try and measure one, you’ll simple see a flat line.

You can take advantage of this if you are seeing a lot of left leg artifact on a 12-lead. Simply swap the left and right leg wires (as long as you’re using stickers on the limbs themselves, and not Mason-Likar (torso) electrode placement). You’ll see no change in the P, QRS, or T waves, but it might get rid of some of that pesky artifact. As a proof-of-concept, try switching the left and right leg wires on your next few patients halfway through transport (reminder: this does not work when the lower extremity electrodes are placed on the torso). The ECGs should be identical pre- and post-swap.

Examine leads aVR and aVF

aVR and aVF look the same with a RA/RL swap.

aVR and aVF look the same with a RA/RL swap. Note the upright P-waves in both.

Not sure if you’re really looking at a lead II sign? Or do you just want secondary confirmation that you’re looking at a RA/RL swap? Look at leads aVR and aVF. With a RA/RL swap, aVR will now be “looking” at the right leg. aVF, as it should, “sees” the left leg. Since those two limbs are electrocardiographically equivalent, you’ll note that aVR and aVF are almost identical with one of these electrode swaps (see the figure above).

As a mental exercise, see if you can figure out why lead I looks just like the inverse of lead III.

03 - 0194 - 59yo F (I-III)

In RA/RL swap leads I and III are the inverse of one-another.


Exercise is no fun though, so here are some more pictures of RA/RL swaps.

RA/RL swap.

RA/RL swap.

RA/RL swap.

RA/RL swap.

RA/RL swap.

RA/RL swap.

RA/RL swap.

RA/RL swap.

RA/RL swap.

RA/RL swap.


So, that’s it: the lead II sign. You don’t need to call it that; just know what it means and check your wires when you see an isoelectric lead on the EKG.


I hope you’re looking forward to tomorrow’s post on lead III—spoiler: it won’t be called the lead III sign. You can check out the rest of the posts in this series below (updated as new posts come out).

12 Leads of Christmas: Lead I
12 Leads of Christmas: Lead III
12 Leads of Christmas: aVR
12 Leads of Christmas: aVL
12 Leads of Christmas: aVF
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


  • Bron says:

    Just spotted this in real life, saved me no end of trouble. Thanks!

  • Daniel says:

    I’m a NSW paramedic in Australia. On night shift the other night my partner requested to do a 12-lead on a patient. She has access to the patients lower body and chest and I placed the two upper limb leads on the anterior deltoids.
    My partner said to me that she didn’t have access to the patients thighs (since I placed the RA LA on shoulders) so she placed them onto the ankles and moved the upper limbs from shoulders to the wrists.
    Afterwards she said the that limbs need to be placed on one of two options shoulders/thighs OR wrists/ankles.
    I have researched this since that night and have found no documentation to support her argument.
    Can you provide any insight or point me into the right direction.
    Cheers Daniel

    • Thanks for the question! Your partner is mistaken in that belief, though it’s understandable why folks might think the electrodes need to be physically “equidistant” from the heart. As long as you’re on the actual arms and legs and not using Mason-Likar placement (subclavicular and iliac placement), it doesn’t much matter where the leads go, just wherever you get the best picture with the least amount of hassle. Though not physically equidistant from the heart, for our purposes pretty much any point on the actual arm is electrically equivalent to any other point (and the same with the legs). I could just leave it at that, or I could even walk through the mathematical equations that describe why that is (after wasting a lot of time looking them up), but how about I just prove it with a simple demonstration?

      I happen to have an electrocardiograph right here next to me (thanks to ebay) so I quickly ran a couple of six leads with different electrode placements. I’ll have to do a full post about this with more consistent positioning and less artifact (I had to run the machine and be the patient), but hopefully this shows that’s there’s no practical difference between the different arrangements.

      If someone really wanted they could tease out some minor differences, but those are just as likely to be due to respiratory variation and different positioning from test to test. So again, anywhere from the deltoids to wrists or from the thighs to ankles is fine, but changes will crop up once you start placing the electrodes on the torso. I personally almost always start at the deltoids and calves and then adjust from there depending on the artifact I’m seeing.

12 Trackbacks

Leave a Reply

Your email address will not be published. Required fields are marked *