Thread by @DrAMcCullough

An #ACCEarlyCareer #ACCFIT #tweetorial from the @WeillCornell Graphics Lab on common ECG Lead Switches (boards ❓favorites).

In case you missed it - we are considering the infarct related artery responsible for the following pattern of injury in a 69yoM presenting with a STEMI:

Most guessed the LCx or D1 of the LAD from the STEs in leads 1 & L (as we did) with reciprocal depressions inferiorly - a high lateral (maybe posterolateral with V2 depression) infarction.

https://twitter.com/DrAMcCullough/status/1306594094978797571?s=20

The angiogram prompted a welcome phone call from our spicy🌶 interventionalist: "You ECG readers think you are *SO* perfect. Take this!😂It was the RCA all along!":

Fascinatingly, the post-PCI💉ECG correlated perfectly with the angiographic disease (Acute Inferior MI with residual ST elevation, Pearl: ST Elevations in III>II -> think RCA!):

The only way to explain the changing QRS and ST segment morphology on ECGs within 4 hours of each other is with a lead switch of some sort, but which?

The most common and easily recognizable lead switch is the R-arm L-arm lead switch. In this, the sinus P wave and native QRS morphology flips ⬇ in our right to left ➡ leads (1 and aVL), with the P wave and QRS complex upright in lead aVR:

🚨Always look at the P waves!🚨 An inverted P in lead 1 is almost never normal sinus rhythm, unless the patient has dextrocardia.

💥Dextrocardia electrically appears similar to a R-arm L-arm Lead Switch💥

🗝If the precordial R-wave voltage ⬆ normally, you have confirmed the lead switch. If poor or ⬇ precordial R-wave voltage, in all likelihood you have diagnosed dextrocardia, as here:

Now onto the more confusing but common lead switches, first the more recognizable of the two, the Right Arm - Right Leg lead switch:

As with the R-arm L-arm lead switch, our Sinus P wave in lead 1 is inverted.

🚨Always look at the P wave morphology🚨

When the right arm and right leg leads are switched, there is no cardiac voltage seen lead 2. The normal lead 2 is right arm to left leg. When the right arm lead is moved to the right ankle, Zero electrical potential is recorded (credit to: bit.ly/33BD29f for figure):

To make it simpler, if you see no potential in lead 2, odds are there is a R arm - R leg lead switch. There should be no electrical potential between the ankles (unless standing over a fallen power line 🔌).

Finally, the most nefarious😏 of lead switches, and the one we have already seen in the above ECG: The Left Arm Left Leg lead switch:

In the **SUBTLE** Left Arm Left Leg lead switch, lead 1 becomes lead 2, and lead 2 becomes lead 1. The most helpful hint, is that lead 3 becomes completely inverted (which means the P WAVE inverts in lead 3)! Again credit to bit.ly/33BD29f):

So to avoid good-spirited heckling by your cath colleagues (@SimranSinghMD et al), make sure to look out for these common lead switches!

Key Points:
🚨 P wave morphology clues you in
🗝In Rarm-Larm, lead 1 & the P wave are inverted with normal precordial R wave progression
🗝In Rarm-Rleg, there is no potential in lead 2
🗝In Larm-Lleg, look for an old ECG to see if leads 1&2 have switched and lead 3 is inverted

The following is a great resource to refer back to if you can't explain what's going on and you think there may be a lead switch: bit.ly/33BD29f

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