The 12 Leads of Christmas: Lead III

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

Lead III

Lead III is great at identifying STEMI’s. In patients presenting with signs and symptoms concerning for ACS but no clear ischemia on the initial ECG, it’s one of the three leads I usually choose to continuously monitor (the other two are aVL and V3).

There’s three major reasons I like lead III in ACS.

First, when a patient is experiencing an inferior STEMI, it is usually the lead that shows maximal ST-deviation—conveniently in the form of ST-elevation.

01 - 1309 - 57yo M - 01

There’s a lot of ST-elevation all over, but lead III has the most.


It’s not as important when there’s giant ST-elevation, as in the case above, but when the ST-deviation is a bit more subtle it becomes vital that you know where to look.

02 - 1202 - 45yo M - 02

This inferior STEMI is a bit more subtle but lead III still has the most ST-deviation.

03 - 1414 - 58yo F - 07

This inferior STEMI is even more subtle but again lead III shows the most ST-elevation.


It’s no coincidence that lead III is great at identifying inferior STEMI’s. While it is commonly taught that II, aVF, and III are the “inferior leads,” lead III is really the star of the show. At an angle of 120 degrees, it is the lead best positioned to capture the bulk of the inferior, infero-septal, and infero-posterior walls.

If you think of the ST-elevation we see in STEMI as a vector, meaning the lead with maximal ST-elevation points to the area where the injury is occurring, in the case of a typical inferior MI the “injury vector” usually lands perfectly in the 110-120 degree range.

04 - Negatives + Heart + Vector

The typical ST-vector of a inferior or infero-posterior STEMI.


Consider the case below. It is clear that both the T-waves and ST-elevation reach their maximum size in lead III.

05 - 0170 - 51yo F - 01

In this case the huge ischemic T-waves point right to lead III.


As in the 360 Degree Heart series, we can re-imagine the limb leads arrayed around the heart in the frontal plane:

06 - Negatives + Heart + ECG


The cool thing about inferior STEMI’s is that most of them show that same pattern. It doesn’t matter whether there is concomitant posterior or right ventricular involvement. It also doesn’t matter if the culprit artery is the left circumflex (LCx) or the right coronary artery (RCA)—if the inferior wall is the main victim of the STEMI, it will usually produce an injury vector remarkably similar to what we see above. Of course there are exceptions, but once you start looking it’s amazing just how many different STEMI pattens show this same exact injury vector we see above.

The second reason I like to monitor lead III in ACS is that it also often shows maximal ST-deviation during “high lateral” infarctions; it’s just that it shows ST-depression. You would think aVL would be your best lead for spotting high-lateral STEMI but it turns out that’s not the case. Amongst the limb leads it will show the most ST-elevation, but lead III typically shows ST-depression that exceeds the amplitude of aVL’s elevation.

For more on this, check out Part 2 of the 360 Degree Heart series. Here are some more examples of high-lateral STEMI’s where the ST-depression in lead III is more striking than the elevation in the classic “high-lateral” lead, aVL.

07 - 1393 - 53yo M - 06

Postero-lateral STEMI due to a LCx lesion with maximal ST-deviation presenting at ST-depression in lead III. There is only subtle ST-elevation in aVL.

08 - 0703 - 59yo M - 01

Subtle antero-lateral STEMI presenting with ST-changes that are most visible as ST-depression in lead III.

09 - 0815 - 86yo F - 01

Isolated first-diagonal (D1) obstruction presenting with notable ST-depression in lead but only minimal elevation in aVL. There is also abnormal ST-elevation in V2, typical of this lesion.

10 - 1016 - 50yo M - 01

Another high-lateral STEMI with marked ST-depression in lead III. Starting to get the picture?

11 - 0054 - 52yo M - 01a

This case actually doesn’t show much ST-depression in lead III, however the down-sloping ST-segment with an inverted T-wave still helps identify the subtle STEMI here.


As if that wasn’t enough, the third reason I like to monitor lead III is that some anterior STEMI’s (mostly proximal LAD culprits) will also present with ST-depression in lead III. When it’s easy it’s not a big deal…

12 - 1418 - 63yo F - 01

Large anterior STEMI from a proximal LAD occlusion with reciprocal ST-depression in lead III.


…but it starts to matter a bit more when the EKG’s are subtle.

13 - 0827 - 71yo M - 01

Subtle anterior STEMI from a proximal LAD lesion with reciprocal ST-depression in lead III.

14 - 1386 - 80yo F - 01

Very subtle anterior STEMI. The inverted T-wave in lead III is normal (accompanying a large negative QRS complex) but the ST-depression there is not. Read the rest of the case description here.


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: 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



  • Cathy Glenn says:


  • Michael says:

    Hi Tom,

    Love your site and learning so much.

    I am a bit confused with regards to Lead II and III and at which angles they look at. According to this post, Lead III looks from the right and Lead II more from the Left. I was at a conference where they stated that the two is switched around, which makes your post a bit difficult to grasp. Could you please clarify this for me to better understand your post.

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