006: 5 Tips for Rapid 12-Lead ECG Interpretation (Listener Question)

5 Tips To Improve Your 12-Lead ECG Reading Skills

This weeks' topic comes from one of our listeners Kris who asks about tips for reading 12-lead ECGs quickly in the field when caring for a sick patient.

As most paramedics already know, rapid 12-lead ECG interpretation is a skill that takes a long time and lots of practice to become good at.  In this episode I share with the 5 Step approach I use every time I read an ECG that makes the process simpler, faster, and more accurate.

Tips Shared in This Episode

  • Tip #1 - Read A Lot Of 12-Leads

  • Tip #2 - Know What a Normal 12-Lead Looks Like

  • Tip #3 - Develop A System For Reading a 12-Lead and Follow it Every Time

  • Tip #4 - Put Your Findings Into Context

  • Tip #5 - Follow Up With The Cardiologist


Tip #1 - Read A Lot of 12-leads

Fact #1 about reading ECG’s - The only way to get really good at reading ECG’s is to read them often.

The more ECG's you see, whether their totally normal or not, the better you'll become at reading ECGs.

With skill comes speed!

Just like everything else we do in this job, the better you master the skill the faster you'll become.

Fact #2 about reading ECG’s - Flights is not a great place to learn ecg’s.  The number of 12-leads we read is far less than what street medics or nurses in the hospital have access to. So it's up to you to seek out opportunities for looking at ECG's

One more thing...., in flights we often know what we’re looking for which tends to jade our thinking when we first pick up an ECG.  If at all possible, even if you've already been told what's going on with a patient by a doc, try to look at every ECG with a fresh set of eyes.

Tip #2 - Know What a Normal ECG Looks Like

It may sound trivial, but before you can start reading abnormal 12-leads it’s essential to know what a normal 12-lead looks like and what proper lead placement looks like so you can recognize when something's out of place.

  • Proper lead placement

    • Limb Leads must be on the limbs.  Doesn't matter where, just make sure their on the limbs for a true diagnostic 12-lead.   


  • Precordial (chest) leads


Proper intervals

  • P-wave

  • PRI - 0.12-0.20

  • QRS - 0.04-0.12

  • QT/QTc - <440 (men)/<460 (women)


Proper QRS orientation


Limb leads

  • Upright except in aVR

  • Precordial Leads

    • Negative in V1-V2

    • Negative or Isoelectric in V3 (point of transition)

    • Positive in V4-6


  • Intrinsic Rates

    • SA node 60-80

    • AV node 40-60

    • Ventricles 20-40

TIP #3 - Have a systematic process for reading ECG’s, and follow it every time.


Figure out a system for evaluating each part that works for you, and use it every time regardless of the patient.

Also, understand that there’s a lot of overlap between steps when reading an ECG, and the significance of a finding in one step may vary based on findings in another step.

Also, keep in mind that the steps below are my way of reading an ECG, but by no means is the only way.  


Use this process as a guide to get you started, but be willing to change it up if the flow doesn’t seem to work for you. 

The best way to read an ECG is the way that works best for you.

Step 1: Skim the ECG

I like to start by giving the ECG a quick skim looking for anything overtly abnormal.  I liken it to looking at the cover of a book and reading the synopsis.  It helps me mentally prepare to analyze the ECG in more detail, and ensure I don’t miss something that may require immediate attention like…

  • V-Tach

  • Hyperkalemia

  • Complete heart block

  • etc.

If you see something that’s abnormal, quickly decide

  • Is this life threatening - treat

  • Is this not life threatening - continue

  • Don’t know if this is life threatening?

    • Is the patient stable or unstable

      • Stable - move on and follow up

      • Unstable - Immediately seek consultation

Step 2: Analyze Rate, Rhythm, Width


Rate, Rhythm, and Width are best assess looking at the 10 second strip that prints below most ECG’s

Is the rate faster than 100 or slower than 60?

Is the rhythm regular or irregular?

Are the QRS complexes narrow or wide?


Most monitors will accurately determine the rate and QRS width and print it out with the ECG strip (top left corner.)

Two more ways to determine the rate is by taking the number of complexes displayed on the 10 second strip and multiplying it by 6,

or use the R-to-R method, counting the number of large blocks between R-waves.

1 small box = 0.04s

1 large box = 0.2s

There are 60 seconds in a minutes so 60seconds / 0.2s (that's the amount of time one large box represents) = 300 so if the R-R wave are 1 large box apart then the heart rate is 300.

Here's how the pattern goes


1 box | 2 boxes | 3 boxes | 4 boxes | 5 boxes | 6 boxes | 7 boxes | 8 boxes

300 150 100 70 60 50 43 38


If your ECG doesn’t print a 10 second, single lead, strip below the ECG consider looking at the basic rhythm strip.  

If neither of these are available, just make sure you’re comparing complexes from the same column when evaluating whether the rhythm is regular or irregular.

Most monitors obtain the ECG over a 10 second period, capturing 4 x 2.5 second snapshots of the heart's electrical activity. Each color coded column below represents a separate 2.5 second period of time.

Put a different way, Leads I, II, and III are captured at the same time, followed by lead aVR, aFL, and aFV, followed by V1-V3, and finally V4-V6.

So if you happen to not have the 10 second rhythm strip shown at the bottom of the below ECG, just make sure when you compare complexes that your looking at complexes from the same column, otherwise you may erroneously end up thinking the rhythm is irregular.



Lastly, take a quick look at the QRS complexes.  Do they appear narrow or wide?

I don’t worry about determining what the exact QRS duration is right now, I’m just trying to get a general feel for what I might see as I dig into the ECG more.

Now take a bit closer look at the ECG and begin analyzing its individual parts, but remember that everything we see on the ECG is connected to everything else in one way or aother.

Step 3: Evaluate the Intervals / Morphology

Now is when I start looking at the specific parts of the ECG, starting with the P-wave and working my way across, looking for patterns that would indicate a problem.  This is where it’s important to know what a normal ECG looks like.


Are they present in a 1:1 ratio with each QRS?

Do the precede the QRS?

Do they all look the same?

Are they proper morphology

Peaked = RAE >2.5mm and peaked

M shaped = LAE


Is the PRI within normal parameters? (0.12-0.20)

Remember, the PRI will change depending on the heart rate, as will the QT.

Shortened PRI can indicate…

Accessory pathway like WPW, or an alternative supraventricular origin.

Junctional Rhythm



<110ms (0.04 - 0.11 or <3 small boxes)


>120ms (>.12 or 3 small boxes)

Compare this with the deflection in V1 to determine BBB

This is important to recognize early because it will alter your interpretation of axis and ST-T wave changes

Look at J-point



Is the direction of the QRS deflection normal for each lead?

  • Upright in all chest leads except aVR, and in V4-6

  • Inverted

    • Normal in V1-V2

Normal R wave transition occurs between V3 and V4


QT is the measurement from the beginning of the Q wave to the end of the T wave and changes with heart rate.  Because of this variation in QT due to heart rate, clinicians should use the QTc, or corrected QT, to assess the QT interval.

Prolonged if >440 in men / >460 in women


>500 = risk for TdP

Should be < ½ the preceding R-R


Do you see anything extra that you don’t see in most ECG’s

Delta Wave

J-wave (Osborne Wave)

U wave

Pacer spikes

De Winter’s T-wave

Step 4: Determine Axis

There are two primary ways to determine axis.

Look at Lead I, II and III


Read the QRS or R axis printed on the strip.


Typical causes of Right Axis Deviation

Left posterior fascicular block

Lateral MI

RV Hypertrophy

Pulmonary Embolism

Normal finding in children

Typical causes of LAD

Left Anterior fascicular block

Left ventricular hypertrophy

Left bundle branch block

Inferior MI

Wolff-Parkinson-White Syndrome

Typical causes of Extreme Right Axis Deviation

Ventricular rhythms


Step 5: Look for Blocks

Look for Left and Right BBB


Look for Hemiblocks


Left Anterior Hemiblock

Pathological Left Axis

Small Q waves with tall R waves in leads I and aVL

Small R waves with deep S waves in leads II, III, aVF

QRS narrow or wide

Small think bundle of cardiac cells

Single Blood Supply from LAD


Left Posterior Hemiblock

Right Axis Deviation

Small R waves with deep S waves in leads I and aVL

Small Q waves with tall R waves in leads II, III and aVF

QRS narrow or wide

Larger/Thicker bundle of cardiac cells

Two arteries supply blood

LPHB far less common and much more critical

 Bifascicular Blocks

A bifascicular Block is anytime there are 2 out of the 3 FASCICLES of the bundle brand network blocked.

Right BBB + left anterior hemiblock

Right BBB + left posterior hemiblock

Complete Left BBB

Patients are at risk for developing Complete Heart Block

Look for AV Blocks

Anytime you say the word "block" when reading an ECG counts towards the patients risk stratification for complete heart block and hemodynamic collapse.

Example - Right Bundle Branch Block + Left Anterior fascicular block + 1st Degree AV Block

The problem here is both the AV node has been damages plus the Purkinje Fibers have been damaged too.

Step 6: Evaluate the ST segment & T-wave

Remember, the ST segment represents the time from Ventricular Depolarization until complete Repolarization

ST Segment


Measured from the J-point (where the QRS “turns” towards the T-wave.


Could be infarct or not.

Think back to the previous section.  Is there LBBB?  The delayed depolarization of left ventricle “drags” the ST segment up.


Must use Sgarbossa criteria to assess for STEMI in the presence of LBBB

It could also be Benign Early Repolarization or Pericarditis.

If you do see ST Elevation

  • Are there reciprical changes?

  • Where do you see changes?

  • Are the leads contiguous?

  • Does the patient have ACS symptoms?

  • Does the pattern make sense?


  • Could be ischemia or maybe not.

    • Sometimes ST depression indicates posterior elevation, and sometimes it’s ischemia, and sometimes is may be associated with proximal LAD occlusion with De Winter's T-wave


  • Upright

  • Inverted

    • Wellens’ Type 1

      • LAD Stenosis

      • Deeply Inverted in V2-V3

    • Ischemia

    • Strain Pattern

    • BBB

    • Pulmonary embolism

    • Hypertrophic cardiomyopathy

    • Can be normal in children

  •  Concave

    • Benign Early Repolarization

  • Convex

  •  Peaked

    • Hyper K

  • Flat / Rounded

    • Hypo K

  • Size relative to QRS

    • >⅔ the R-wave = abnormal

Remember, if there is a depolarization abnormality there will be a repolarization abnormality too.

Tip #4 - Ask yourself, “Does this make sense?”


Make sure that what you see on the ECG makes sense given the clinical presentation of the patient.

Tip #5 - Follow up with the Cardiologists


Follow up with the cardiologist who cared for your patient, especially if the patient goes to the cath lab.


Or even better, see if you can accompany the patient to the cath lab.

This way you can compare your diagnosis to what the Cardiologists actually finds, and you can ask where you may have been wrong in your reading the the ECG.

Additional Resources

The guys over at Life In The Fast Lane have such an amazing resources for ECG's I highly encourage you to check it out. Check out their entire ECG Library of resources.  Here are some of my favorites:

ECG Clinical Cases

100 ECG Quiz

Other Great Resources

Here's a blog post from Dr. Grauer, ECG Guru and host of the ECG Interpretation Blog, sharing his Systematic Approach to ECG Interpretation

Here's a post by Tom Bouthillet, Paramedic, ECG Master, and Founder of EMS12Lead.com sharing his 6 Step Method for 12-Lead ECG Interpretation

Want more, here are 30 practices ECG's, with diagnosis from Jones and Barttlet

A special thanks Kris for her question this week.  

If you have a question you'd like featured on the podcast, head on over to AskFlightCrit.com, or use the tool at the bottom of this post, and drop me a message.


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