ECG Pointers: Limb Lead Reversal

Author: Ari Edelheit, MD (@EdelsMD – Resident Physician, John H Stroger, Jr Hospital of Cook County, Chicago, IL) // Edited by:  Jamie Santistevan, MD (@jamie_rae_EMdoc – EM Physician, Presbyterian Hospital, Albuquerque, NM), Manpreet Singh, MD (@MPrizzleER – Assistant Professor of Emergency Medicine / Department of Emergency Medicine – Harbor-UCLA Medical Center) and Brit Long (@long_brit  – EM Attending Physician, San Antonio, TX)

Welcome to this edition of ECG Pointers, an emDOCs series designed to give you high yield tips about ECGs to keep your interpretation skills sharp. For a deeper dive on ECGs, we will include links to other great ECG FOAMed!

The Case:

Your patient is a healthy 56 yo F who presents with atypical, reproducible chest pain. Vital signs and exam are unremarkable. Her ECG is below:

There is sinus bradycardia with a normal axis. The PR, QRS, and QTc intervals are all normal. There is an isolated Q-wave in lead III, as well as isolated T-wave inversion in aVL

Overall, these are some nonspecific findings in a patient with a history that is not suggestive of acute coronary syndrome. You continue to work up the patient’s chest pain, including an initial negative troponin. Given her age, you decide to repeat an ECG, which is below:

The rate/rhythm/axis and intervals remain unchanged. There appears to be some new T-wave inversions in III/aVF, now the T-wave in aVL is upright. This amounts to a “dynamic” ECG in a patient with chest pain.

The computer reads “consider inferior ischemia”. Easy admission, and on to the next patient. Right?? Wait…..

Lead Misplacement:

In our patients serial ECGs, lead III is now the exact reverse of itself from the previous ECG. That’s odd. Examining all of the leads and we see that lead I looks like the old lead II and vice-versa. aVF looks like the old aVL and vice-versa. What is going on? Here’s a close-up of lead III in the initial and repeat ECG:


This is a classic example of limb lead reversal. While there are many versions, in this case, the left arm and the left leg leads were switched. In order to understand why the leads look the way they do, we must recall Einthoven’s triangle:

The first triangle shows the leads as they are properly placed. Now with the left leg and left arm leads switched, we can see that what is reading as lead I is truly lead II, lead II is truly lead I, and lead III is reversed in polarity. By reversing just two stickers on the patient, many leads are affected.

As for the aVL/aVF reversal, we must remember that aVL/aVF/aVR are all augmented leads (hence the a) and rely on interpreting their view of the electrical activity of the heart as an average of two limb leads. This picture should help:

Key ECG Findings:

The findings of left arm and left leg reversal, as in our case, is a more difficult example because it relies on identifying reversed polarity in lead III, which can be admittedly subtle without a prior ECG for comparison. Luckily, the most common limb lead reversal is much easier to spot. This is the reversal of left and right arm limb leads. You will have one large clue:

1. The polarity in lead I will be predominantly negative or downwards.

While this can be an expected finding in some patients (i.e., severe pulmonary disease, acute PE, etc) a negative deflection in lead I should still catch your attention, no matter the cause. To confirm lead misplacement you will also notice:

2. Leads III and II will be the reverse of one another.

3. aVR and aVL are reversed.

4. aVF is unchanged.

As you can imagine, there are several more possible reversals. However, by understanding Einthoven’s triangle we can accurately predict these ECG patterns.

Back to our case…

This case highlights the importance of identifying limb lead reversal for two reasons. First, while not directly harmful, missing this diagnosis can profoundly impact patient care. This patient could easily be admitted for exertional testing and suspicion of unstable angina in the setting of a “dynamic” ECG. This places the patient at risk for false positive stress tests, hospital acquired infections, and the inconvenience of an unnecessary admission. Additionally, understanding why lead reversal produces the P-QRS-T pattern that it does demands a thorough understanding of the basics of ECGs and can only serve to assist your diagnoses of life threatening events.

The main ECG Pointers for Limb Lead Reversal:

  1. Lead reversals do happen; the most common is right and left arm reversals.
  2. Your first clue is a negative QRS complex in lead I.
  3. A predominantly upward P-QRS-T complex in aVR is another big clue.
  4. When in doubt, repeat the ECG!

And Here’s more great ECG FOAMed:


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