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The Lewis Lead

What exactly is the Lewis lead? Is there a difference between the Lewis Lead and the S5 lead? Why is this even relevant? These may be questions that pop into your head whenever someone brings up the Lewis lead. I recently had a call that I was able to use the S5 lead configuration to diagnose atrial flutter, which had recently converted from atrial fibrillation. Finding a P wave is important for multiple reasons when deciding on a treatment course for a cardiac patient, such as A-Flutter with variable conduction vs AV Blocks, differentiating wide-complex tachycardias, bundle branch blocks vs idioventricular rhythms, and the list goes on. Any time you are unsure if a P wave is present, try a Lewis Lead and find out! 

Sir Thomas Lewis (the guy above with more swagger than I can ever dream of having) discovered his lead configuration in 1931 when he was attempting to find the best ECG tracing of the atria, or what he called “auricular activity”, while decreasing the size of the ventricular activity. He was using this to study atrial flutter.

Multiple versions of the “Lewis Lead” have popped up over the years claiming to give the best view of atrial activity. The original configuration as written by Sir Lewis first changes the monitor calibration to 20mm/mV, and is placed as follows.

  • RA to 2nd intercostal space, right sternal border

  • LA to 4th intercostal space, right sternal border

  • RL and LL remain below the inguinal crease on their respective sides      

The second, and what seems to be the most common configuration (probably due to the prevalence in FOAMed), is also known as the S5 lead, which I’ve traced back to teachings by cardiologist and electrophysiologist Henry Marriott. The setup varies from Sir Lewis’ placement but still provides a great view of atrial activity. The calibration should also be set to 20mm/mV if possible. The placement of these leads makes the most sense to me, as it puts Einthoven’s triangle right over the atria.

  • RA over the manubrium

  • LA to the 5th intercostal space, right sternal border

  • LL to the right lower costal margin

  • RL remains below the inguinal crease on the right leg.

 S5   Lewis Lead

Both variations should be monitored in Lead I for best results, but don’t be afraid to look at Leads II and III.  

A third variation that I’ve read about is the anterior-posterior placement. This is done by placing the RA on the sternum and the LL on the back at the same area. I believe the other leads are left where they normally are and this should be viewed in Lead II. I really can’t find much literature to back this one up, so take it with a grain of salt.

Recently I had an Atrial Fibrillation patient that was most likely the result of mitral regurgitation from a prolapsed valve. The patient had undergone a cardizem infusion which had some adverse side effects and was discontinued. I wish I had a copy of the hospitals 12 lead prior to my arrival, but unfortunately I do not. The most important thing about it though was that it was irregularly irregular, and most definitely atrial fibrillation. Now after acquiring my own 12 lead, I noticed the patient was almost regularly irregular. I’ve most commonly seen this type of rhythm in atrial flutter (with or without variable conduction) or 2nd degree AV blocks, both types. Flutter waves were not easily discernible on my 12 lead so I decided to capture a S5 tracing. (I chose this over the classic Lewis Lead because I was more familiar with it due to EMcrit. Also, our monitors are not capable of changing the calibration)  Below is the 12 Lead and also the S5 strip. I marked the “saw tooth” appearance of the flutter waves with arrows. This is important because switching from an unorganised atria to now an organised atria means an increased risk of throwing clots depending how long the fibrillation was occuring. I believe the Lewis lead can be an asset in your ECG “toolbox” so give it a try on your next call. It doesn’t take long to place and the diagnostic use can be very helpful. Send me some pictures of some cool stuff you find!

AJ Beuscher (@AustinBeuscher)is a critical care paramedic, outdoor enthusiast, craft beer lover, huge proponent of FOAMed, & working towards a chemistry degree. 

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