A Systematic Approach to Interpreting a 12 Lead ECG

If you want to watch the YouTube video on this post check out the video below.

Before we get into how to read a 12 lead ECG, we recommend that you review the normal anatomy and conduction pathways of the heart.

When you see a 12-lead do you freeze and get overwhelmed? It happens to everyone especially when you first start interpreting 12 leads. We’ll go into how to break down a 12-lead to help you get out of a pickle. But first, the leads and what part of the heart they show.

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Is this kind of what you think of when you look at a 12 lead? Well let me make it easier.

12 Lead ECG Reference Chart – Cardiovascular Nursing Education Associates

There’s not much to note on the previous images. They simply reference the anatomical region that each lead views. But because we are SCOPE, we thought we would add a mnemonic in there to help you guys remember that.


The mnemonic is “I See All Leads” and you can add in “on a 12 lead” at the end if you wish.

I- Inferior

See- Septal

All- Anterior

Leads- Lateral

Below is 6 step approach to interpreting a 12 lead ECG.

  1. Rate

What is the rate of the rhythm? Find a QRS complex that is on the line of a big box. If the next QRS complex occurs at the next big box, the rate is around 300, if occurs two big boxes away the rate is 150, and so on and so on (picture above will provide the rest of the rates). This is called the “big box method” and is a fairly easy way to quickly get the rate.

A rate greater than 100 beats per minute= tachycardia

A rate between 60-100 beats per minute= normal

A rate less than 60 beats per minute= bradycardia

For patient’s with an irregular heart beat, you may need to count the whole rhythm strip (normally 10 seconds) and multiply by the amount of QRS complexes.

2. Rhythm/Regularity

Next, determine if the rhythm is sinus in nature (determine if the impulses occur from the SA node). You can do this by looking for a positive P wave in leads I and II and a negative P wave in aVR. There should be 1:1 ratio of P waves to QRS complexes. Determine if the rhythm is regular, regularly irregular (sinus with ectopy/ sinus arrhythmia), or irregularly irregular rhythm (atrial fibrillation, WAP, MAT).

3. Axis


Many people glance over and don’t even look at axis because they think its to complex and hard. Above is a picture of how to determine the axis of a 12-lead. You look at the QRS complex and determine if it’s positive (or mostly positive), negative (or mostly negative), or indeterminate (equally positive and negative). Why is this important? The axis can point to underlying health issues and can help you come up with a differential diagnosis. Here are some examples of what each axis can show.

Normal Axis: As the name suggests, this is normal.

Left Axis Deviation (LAD):

  1. Can be normal due to age
  2. Wolff Parkinson White Syndrome
  3. LVH
  4. Hyperkalemia
  5. Multiple types of myocardial infarctions
  6. Left bundle branch block
  7. Left anterior fasicular block

Right Axis Deviation (RAD):

  1. Limb lead reversal (please check your leads)
  2. Pulmonary embolism
  3. Ventricular rhythms (ventricular tachycardia, idioventricular)
  4. RVH
  5. Pulmonary hypertension
  6. Chronic lung disease (COPD)
  7. Right bundle branch block
  8. Left posterior fasicular block
  9. Congenital heart disease

Extreme Axis Deviation:

  1. Limb reversal
  2. Ventricular rhyhms
  3. Hyperkalemia

4. The Waveforms/Intervals


Here are the intervals and the normal ranges:

P wave: This shows the atria depolarizing. The amplitude is normally 0.05 to 0.25mV (0.5 to 2.5 small boxes). Have you ever had a super tachy rhyhtm and couldn’t tell if it is sinus tachycardia or SVT? Try doing a Lewis lead on your stable patients. This is mostly done in electrophysiology and in hospital settings, not much in a prehospital setting, but it is a useful tool to have. This focuses on the right atrium when you view lead I which allows you to more easily see atrial activity.

Lewis lead • LITFL • Medical Eponym Library

Normal duration is 0.06-0.11 seconds (1.5 to 2.75 small boxes). Below is a picture of the various types of P waves you might see on a 12 lead.

RAE- Right atrial enlargment

LAE- Left atrial enlargment


PR interval: 0.12 to 0.20 seconds or 120 to 200 milliseconds. Elongation of this can be caused by a first degree heart block. A short PRI can be caused by a preexcitation syndrome (Wolff Parkinson White syndrome or Lown-Ganong-Levine syndrome). Click the “ECG Rhythm Review” link below to learn about the different heart blocks and look at our WPW post for more information.

Q wave: A q wave is not always noted on every 12 lead ECG. But if it does occur, its the first negative deflection before the R wave in the QRS complex. You can sometimes see them in the lateral leads (I, aVL, V5 and V6).

Borderline Q-waves in individuals without overt cardiovascular disease:  Relations with adiposity, subclinical atherosclerosis and vascular  stiffness - ScienceDirect

Now Q waves are somewhat interesting. The normal Q wave is supposed to be small and narrow. There are a few Q waves you should be aware of:

  1. Pathological Q waves: 1 small box wide or greater than 25% of the QRS complex
Pathologic Q Waves - ECGpedia

2. Dagger Q waves: Deep narrow Q waves that can easily be greater than 25% of the QRS complex but are not wide.

Hypertrophic Obstructive Cardiomyopathy (HOCM) ECG Review - Criteria and  Examples | LearntheHeart.com

Look over leads I, aVL, V4, V5, and V6 for deep narrow Q waves. Deep narrow Q waves and high voltage could indicate that your patient has hypertrophic obstructive cardiomyopathy.

QT interval: This is the distance from the Q wave to the end of the T wave. The normal QT interval is between 400-450ms. At the end of the day, you really only need to be concerned if its over 500ms because that predisposes patients to Torsades de Pointes.

QRS: The normal QRS duration is between 0.08 and 0.10 seconds. This represents the ventricles depolarizing. Some factors that can widen the QRS complex include:

  1. Left and right bundle branch blocks.

2. Accessory pathway (such as the bundle of Kent in Wolff-Parkinson-White Syndrome (yes of course we have a post and a YouTube video on this topic).

3. Non-specific conduction delay

4. Electrolyte imbalances/drug overdoses

5. Pacemaker

T wave: The T wave depicts ventricular repolarization. The size of the T waves vary based on age, sex, and lead placement. Generally they’re asymmetric in nature. T waves should be upright in every lead except for aVR and V1.


Some conditions that can change the T wave appearance include:

  1. Ischemia
  2. Cardiomyopathies
  3. Electrolyte imbalances
  4. Wellen’s Syndrome
  5. Pericarditis/myocarditis
  6. Cerebrovascular injury (cerebral T waves)
  7. Brugada syndrome
  8. Juvenile T wave inversion
  9. Bundle branch blocks
  10. Left/right ventricular strain
  11. Pulmonary embolisms

5. Hypertrophy

Hypertrophy is a term that basically means the ventricles are becoming enlarged and thickening. There are two types:

  1. Right Ventricular Hypertrophy
  2. Left Ventricular Hypertrophy
Right ventricular hypertrophy (RVH): ECG criteria & clinical  characteristics – ECG & ECHO

Hypertrophy really only applies to patients over 35-40 years old (depending on the literature you read). Hypertrophy causes massive QRS complexes.


You need to have:

  1. Right axis deviation
  2. A dominant R wave noted in V1 which is greater than 7mm
  3. Dominant S wave in V5 or V6 which is greater than 7mm
Right Ventricular Hypertrophy (RVH) ECG Review - Criteria and Examples |  LearntheHeart.com

Some additional changes include right bundle branch block with a “strain pattern”. A strain pattern is slight ST depression and T wave inversions which you can see in the example above.


There are a ton of criteria for diagnosing left ventricular hypertrophy on a 12 lead but one of the easiest ones is the Sokolov-Lyon criteria which is the  S wave depth in V1 + tallest R wave height in V5-V6. You add up the number and if its over 35, you got your answer. Another criteria is the Cornell criteria which states you add the R wave in aVL to the S wave in V3. If it is over 28mm in men or greater than 20mm in women its LVH. Now remembering all of the criteria is giving you a headache right? How about you look for just two things…

  1. High voltage
  2. Strain pattern
ECG LVH ST elevation not MI

Look in the precordial leads (V1-V6).. See the massive QRS complexes? That’s your high voltage. Look over in V5, V6, I, II, and III. See the flipped T wave? That’s your strain pattern.

6. Infarction

Infarction is pretty simple to see on a 12 lead. So simple we shouldn’t really pat ourselves on the back for seeing it. If you are interested in picking up subtle STEMIs, check out our Occlusion MI post here:

But if you must know, STEMI criteria includes:

1mm of elevation in two contiguous leads with reciprocal depression

Contiguous and Reciprocal Lead Charts | EMS 12 Lead

And that’s it! Well I lied, there are two circumstances you need to remember as well…

  1. For an anterior STEMI, you need 2mm of ST segment elevation in V2 and V3 to call it a STEMI for patients older than 40… Or greater than 2.5 mm for men less than the age of 40. And for the ladies, you only need 1.5 mm of elevation.

2. Due to a normal 12 lead not being able to see the posterior (back) of the heart, how will we know if the posterior wall is infarcting? Well look at the nifty image I have below. You look for ST depression in V1-V4. You should know that V2 is more sensitive. And then what? You perform a 15 lead. You basically move V4-V6 to the patient’s back. V4 becomes V7, V5 becomes V8, and V6 becomes V9. You only need 0.5 mm of elevation in V7-V9 to confirm a STEMI though. The reason why, is because there’s a lot more fat and muscle to look through to see the posterior wall of the heart.


Here are the steps you need to do on every single 12 lead you interpret:

  1. Look at the rate
  2. Look at the regularity of the rhythm
  3. What is the axis?
  4. Look at all out your intervals! (Is there an AV block, hemi-block, or a bundle branch block?)
  5. Is there hypertrophy?
  6. Infarction

And that’s it! The best advice I can suggest to people new at 12 lead interpretation is to fully look at one and just do the step by step approach. As time goes on it will become more automatic. The more you look at, the easier it will come to you. Now hopefully a lot of our ECG posts will make more sense to you. Medicine is all about baby steps. So take learning one step at a time.

This site is meant to be used for educational use only. We strive to push evidence based medicine with no bias to help you obtain all the important information. You should always follow your protocols that have been set in place.

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-Scopeducation Team (Matt and Ryan)


All of the pictures obtained have the links to them underneath each picture

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