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