This article covers the main parts of a 12-lead ECG and how to correctly identify rhythms.

The 7 Steps to ECG Interpretation

Paramedics get too in the weeds in ECG’s.

We aren’t cardiologists. There is no need to learn about axis deviation or cardiac hypertrophy. While it makes you sound smart it doesn’t actually make you a better paramedic.

It doesn’t even improve patient outcomes.

But that doesn’t help you pass medic class now does it?

Hey, I’m Shay. I’ve been working in EMS for over a decade now and today I’m going to cover some easy steps in ECG reading. I’ve also taught paramedic and EMT programs through a community college.

Here is everything you actually need to know for ECG interpretation.

• Rate
• Regularity
• ST Changes
• Origin
• PR Interval
• Heart Blocks
• ID That Rhythm

Rate

Determining Rate is the most important first step in reading an ECG.

Rate will quickly determine what treatment path you are going down. It also dramatically lowers the number of rhythms you could need to identify.

Just think of rate like Goldilocks and the three bears. It’s either too fast, too slow, or just right.

Remember, 60-100 BPM is a normal rate. Anything below 60 is bradycardia and anything above 100 is tachycardia.

Determining Rate

There are few pretty easy ways to determine a rate from an 12-lead strip.

Easiest is probably taking your 6 second strip, finding all the QRS complexes, and multiplying by 10. This is a pretty rough estimation but as paramedics we work a lot from rough estimates. Like when was the last time you actually knew a patient’s weight.

A second way is using the Box Method.

The large boxes on your strip can tell you how fast your patient’s heart is beating. It’s still an estimate, but it works well.

All you do is count the number of boxes between QRS complexes and divide 300 by that number.

For example

• One box between QRS would be 300/1 or 300 BPM
• 2 boxes is 300/2 or 150 BPM
• 3 boxes 100 BPM
• And so on..

Regularity

The next most important thing is looking at how regular your patient’s rhythm is.

Regularity tells us a lot. Both about what to call the ECG rhythm and how to treat it. Regular tachycardia is very different from irregular tachycardia.

With just rate and regularity there are a good number of rhythms you can identify right then and there.

All you need to look for is how many boxes between QRS complexes. Sounds pretty similar to figuring out rate by the Box Method, which is why I like that method for rate.

If there are the same number of boxes between complexes, you’re golden. The rate is regular.

If it’s dramatically different, your rate is irregular. Pretty simple.

ST-Segment Changes

Everyone wants to see the big Tombstone T’s on their ECG.

STEMIs (ST Elevated Myocardial Infarction) are one of the few issues we run into where seconds really do matter. That’s why we look for this third in our sequence.

We start by finding the isoelectric line- that flat part of the ECG with no electrical activity.

If all you’re seeing is an isoelectric line your patient is in asystole. Done. You actually should have figured that out before going through rate and regularity.

Specifically, you’re looking for elevation above the flatline or depression below.

ST elevation is 2 or more little boxes above flatline. And to call it a STEMI you need the elevation to be in two contiguous (right next to each other) leads.

ST depression is a drop below that isoelectric line.

This normally means you your heart muscle isn’t getting enough oxygen. Or you can be a fancy pants medic and call it myocardial ischemia. Same thing.

ST depression could also mean you have a posterior MI. But that’s a whole other ECG to look at.

Origin

In other words, where the beat starts.

We initially look for the P wave. P waves are the depolarization of the atria and exactly what we want when looking at an ECG.

Every P wave should have a QRS after it.

If you have no P waves but see QRS complexes, you’re looking at a ventricularly or junctionally paced rhythm. These can be bradycardic or tachycardic. Both are dangerous and have their own treatment algorithms.

Or you have too many P waves that don’t have a QRS associated with them.

This is typical for A-fib and A-flutter. Atrial fibrillation and atrial flutter are pretty common and rarely an issue in the EMS setting. It’s more so an issue when the rate becomes too fast.

A-fib is easy to see on a 12-lead.

Tons of P waves, irregularly irregular QRS complexes, and viola you have A-fib.

PR Interval

PR interval can tell you a few things about your patient’s heart but it’s not that important in the field.

Your P wave should end between 0.12 and 0.2 milliseconds (ms) before the R wave starts.

Intervals greater than 0.2 ms can mean a lot of things. This should clue you in to look for a heart block.

Shorter than 0.12 intervals can be pre-excitation syndrome. The one we hear about the most is Wolf-Parkinson-White (WPW) syndrome. For as often as it comes up, I’ve only seen one person with it in over 10 years.

Blocks

Bundle Branch Blocks and heart blocks have similar names but are very different.

Bundle Branch Blocks can be either left or right (LBBB or RBBB). BBB are nice to know about and can impress the cute nurse at the ER but otherwise you’re not doing anything with them.

Heart blocks are a different story.

There are 4 heart blocks you need to know.

• First Degree
• Second Degree Type 1
• Second Degree Type 2
• Third Degree

A long PR interval with no missing QRS is your first degree heart block. These are benign and just gee-wiz information.

2nd degree type 1 are also called Wenchenbach. Think “Walking Back.” The PR interval starts normal and gets wider and wider until you skip a QRS complex. Then it starts right up again like normal.

2nd degree type II or Mobitz Type II are harder to read on an ECG. You’ll have multiple normal beats with normal PR intervals for a while. Then you’ll have a sudden dropped beat.

3rd degree is the worst of the heart blocks. It’s called a complete heart block. The beat originating in the atria doesn’t reach the ventricles ever. These patients are super bradycardic.

Drugs might work but mostly you’ll be pacing 3rd degree heart block patients all the way to the operating table.

Identify

All the hard work is done, you just need to put it together.

And this starts with memorizing vocab. If you can describe what the different rhythms are, finding them on ECG strips becomes so much easier.

Spend a little more time learning the definition of each rhythm. There are really only a few you need to learn.

And going through the steps listed above will make it super easy to identify the rhythm.

Good Luck Out There!

P.S. If you want more paramedic resources check out our full list here.