ECG interpretation

How to Read an ECG

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This guide demonstrates how to read an ECG using a systematic approach. If you want to put your ECG interpretation knowledge to the test, check out our ECG quiz on the Geeky Medics quiz platform.

Confirm details

Before beginning ECG interpretation, you should check the following details:

  • Confirm the name and date of birth of the patient matches the details on the ECG.
  • Check the date and time that the ECG was performed.
  • Check the calibration of the ECG (usually 25mm/s and 10mm/1mV).
You might also be interested in our OSCE Flashcard Collection which contains over 2000 flashcards that cover clinical examination, procedures, communication skills and data interpretation.

Heart rate

What’s a normal adult heart rate?

  • Normal: 60-100 bpm
  • Tachycardia: > 100 bpm
  • Bradycardia: < 60 bpm

Regular heart rhythm

If a patient has a regular heart rhythm their heart rate can be calculated using the following method:

  • Count the number of large squares present within one R-R interval.
  • Divide 300 by this number to calculate heart rate.
Heart rate calculation example
  • 4 large squares in an R-R interval
  • 300/4 = 75 beats per minute
ECG - Heart Rate (normal)
How to calculate a heart rate on a normal ECG

Irregular heart rhythm

If a patient’s heart rhythm is irregular the first method of heart rate calculation doesn’t work (as the R-R interval differs significantly throughout the ECG). As a result, you need to apply a different method:

  • Count the number of complexes on the rhythm strip (each rhythm strip is typically 10 seconds long).
  • Multiply the number of complexes by 6 (giving you the average number of complexes in 1 minute).
Heart rate calculation example
  • 10 complexes on a rhythm strip
  • 10 x 6 = 60 beats per minute

Heart rhythm

A patient’s heart rhythm can be regular or irregular.

Irregular rhythms can be either:

  • Regularly irregular (i.e. a recurrent pattern of irregularity)
  • Irregularly irregular (i.e. completely disorganised)

Mark out several consecutive R-R intervals on a piece of paper, then move them along the rhythm strip to check if the subsequent intervals are similar.


If you are suspicious that there is some atrioventricular block (AV block), map out the atrial rate and the ventricular rhythm separately (i.e. mark the P waves and R waves). As you move along the rhythm strip, you can then see if the PR interval changes, if QRS complexes are missing or if there is complete dissociation between the two.

ECG Heart Rhythm
Measure the R-R intervals to assess if the rhythm is regular or irregular 1

Cardiac axis

Cardiac axis describes the overall direction of electrical spread within the heart.

In a healthy individual, the axis should spread from 11 o’clock to 5 o’clock.

To determine the cardiac axis you need to look at leads I, II and III.

Read our cardiac axis guide to learn more.

Normal cardiac axis

Typical ECG findings for normal cardiac axis:

  • Lead II has the most positive deflection compared to leads I and III.
Normal cardiac axis
Normal cardiac axis

Right axis deviation

Typical ECG findings for right axis deviation:

  • Lead III has the most positive deflection and lead I should be negative.
  • Right axis deviation is associated with right ventricular hypertrophy.
ECG - Right axis deviation
Right axis deviation 2

Left axis deviation

Typical ECG findings for left axis deviation:

  • Lead I has the most positive deflection.
  • Leads II and III are negative.
  • Left axis deviation is associated with heart conduction abnormalities.
ECG - Left axis deviation
Left axis deviation 2

P waves

The next step is to look at the P waves and answer the following questions:

1. Are P waves present?

2. If so, is each P wave followed by a QRS complex?

3. Do the P waves look normal? – check duration, direction and shape

4. If  P waves are absent, is there any atrial activity?

  • Sawtooth baseline → flutter waves
  • Chaotic baseline → fibrillation waves
  • Flat line → no atrial activity at all

If P waves are absent and there is an irregular rhythm it may suggest a diagnosis of atrial fibrillation.

ECG - atrial fibrillation - p waves
P waves 1

PR interval

The PR interval should be between 120-200 ms (3-5 small squares).

Prolonged PR interval (>0.2 seconds)

A prolonged PR interval suggests the presence of atrioventricular delay (AV block).

First-degree heart block (AV block)

First-degree heart block involves a fixed prolonged PR interval (>200 ms).

ECG - First-degree AV block
First-degree heart block (AV block)

Second-degree heart block (type 1)

Second-degree AV block (type 1) is also known as Mobitz type 1 AV block or Wenckebach phenomenon.

Typical ECG findings in Mobitz type 1 AV block include progressive prolongation of the PR interval until eventually the atrial impulse is not conducted and the QRS complex is dropped.

AV nodal conduction resumes with the next beat and the sequence of progressive PR interval prolongation and the eventual dropping of a QRS complex repeats itself.

ECG - Second-degree AV block (Mobitz Type 1 - Wenckebach)
Second-degree AV block (Mobitz Type 1 – Wenckebach)

Second-degree heart block (type 2)

Second-degree AV block (type 2) is also known as Mobitz type 2 AV block.

Typical ECG findings in Mobitz type 2 AV block include a consistent PR interval duration with intermittently dropped QRS complexes due to a failure of conduction.

The intermittent dropping of the QRS complexes typically follows a repeating cycle of every 3rd (3:1 block) or 4th (4:1 block) P wave.

ECG - Second-degree AV block (Mobitz type 2 AV block) <sup>3</sup>
Second-degree AV block (Mobitz type 2 AV block) 3

Third-degree heart block (complete heart block)

Third-degree (complete) AV block occurs when there is no electrical communication between the atria and ventricles due to a complete failure of conduction.

Typical ECG findings include the presence of P waves and QRS complexes that have no association with each other, due to the atria and ventricles functioning independently.

Cardiac function is maintained by a junctional or ventricular pacemaker.

Narrow-complex escape rhythms (QRS complexes of <0.12 seconds duration) originate above the bifurcation of the bundle of His.

Broad-complex escape rhythms (QRS complexes >0.12 seconds duration) originate from below the bifurcation of the bundle of His.

Complete heart block (3rd degree)
Complete heart block (3rd degree) 4
Tips for remembering types of heart block

To help remember the various types of AV block, it is useful to know the anatomical location of the block within the conducting system.

First-degree AV block:

  • Occurs between the SA node and the AV node (i.e. within the atrium).

Second-degree AV block:

  • Mobitz I AV block (Wenckebach) occurs IN the AV node (this is the only piece of conductive tissue in the heart which exhibits the ability to conduct at different speeds).
  • Mobitz II AV block occurs AFTER the AV node in the bundle of His or Purkinje fibres.

Third-degree AV block:

  • Occurs at or after the AV node resulting in a complete blockade of distal conduction.

Shortened PR interval

If the PR interval is shortened, this can mean one of two things:

  • Simply, the P wave is originating from somewhere closer to the AV node so the conduction takes less time (the SA node is not in a fixed place and some people’s atria are smaller than others).
  • The atrial impulse is getting to the ventricle by a faster shortcut instead of conducting slowly across the atrial wall. This is an accessory pathway and can be associated with a delta wave (see below which demonstrates an ECG of a patient with Wolff Parkinson White syndrome).
ECG - Delta wave (Wolff Parkinson White Syndrome)
Delta wave 5

QRS complex

When assessing a QRS complex, you need to pay attention to the following characteristics:

  • Width
  • Height
  • Morphology
Components of the ecg
Various components of an ECG


Width can be described as NARROW (< 0.12 seconds) or BROAD (> 0.12 seconds):

  • A narrow QRS complex occurs when the impulse is conducted down the bundle of His and the Purkinje fibre to the ventricles. This results in well organised synchronised ventricular depolarisation.
  • A broad QRS complex occurs if there is an abnormal depolarisation sequence – for example, a ventricular ectopic where the impulse spreads slowly across the myocardium from the focus in the ventricle. In contrast, an atrial ectopic would result in a narrow QRS complex because it would conduct down the normal conduction system of the heart. Similarly, a bundle branch block results in a broad QRS complex because the impulse gets to one ventricle rapidly down the intrinsic conduction system then has to spread slowly across the myocardium to the other ventricle.


Height can be described as either SMALL or TALL:

  • Small complexes are defined as < 5mm in the limb leads or < 10 mm in the chest leads.
  • Tall complexes imply ventricular hypertrophy (although can be due to body habitus e.g. tall slim people). There are numerous algorithms for measuring LVH, such as the Sokolow-Lyon index or the Cornell index.


To assess morphology, you need to assess the individual waves of the QRS complex.

Delta wave

The mythical ‘delta wave‘ is a sign that the ventricles are being activated earlier than normal from a point distant to the AV node. The early activation then spreads slowly across the myocardium causing the slurred upstroke of the QRS complex.

Note – the presence of a delta wave does NOT diagnose Wolff-Parkinson-White syndrome. This requires evidence of tachyarrhythmias AND a delta wave.

ECG - Delta wave
Delta wave 5


Isolated Q waves can be normal.

A pathological Q wave is > 25% the size of the R wave that follows it or > 2mm in height and > 40ms in width.

A single Q wave is not a cause for concern – look for Q waves in an entire territory (e.g. anterior/inferior) for evidence of previous myocardial infarction.

ECG - Q waves
Q waves (V2-V4), with T wave inversion suggestive of previous anterior MI 6

R and S waves

Assess the R wave progression across the chest leads (from small in V1 to large in V6).

The transition from S > R wave to R > S wave should occur in V3 or V4.

Poor progression (i.e. S > R through to leads V5 and V6) can be a sign of previous MI but can also occur in very large people due to poor lead position.

Poor R wave progression
Poor R wave progression 7

J point segment

The J point is where the S wave joins the ST segment.

This point can be elevated resulting in the ST segment that follows it also being raised (this is known as “high take-off”).

High take-off (or benign early repolarisation to give its full title) is a normal variant that causes a lot of angst and confusion as it LOOKS like ST elevation.

Key points for assessing the J point segment:

  • Benign early repolarisation occurs mostly under the age of 50 (over the age of 50, ischaemia is more common and should be suspected first).
  • Typically, the J point is raised with widespread ST elevation in multiple territories making ischaemia less likely.
  • The T waves are also raised (in contrast to a STEMI where the T wave remains the same size and the ST segment is raised).
  • The ECG abnormalities do not change! During a STEMI, the changes will evolve – in benign early repolarisation, they will remain the same.

ST segment

The ST segment is the part of the ECG between the end of the S wave and the start of the T wave.

In a healthy individual, it should be an isoelectric line (neither elevated nor depressed).

Abnormalities of the ST segment should be investigated to rule out pathology.

Normal ECG
Various components of an ECG


ST-elevation is significant when it is greater than 1 mm (1 small square) in 2 or more contiguous limb leads or >2mm in 2 or more chest leads.

It is most commonly caused by acute full-thickness myocardial infarction.

ECG - ST elevation
ST elevation in the anterior leads

ST depression

ST depression ≥ 0.5 mm in ≥ 2 contiguous leads indicates myocardial ischaemia.

ECG ST depression
ST depression

T waves

T waves represent repolarisation of the ventricles.

Tall T waves

T waves are considered tall if they are:

  • > 5mm in the limb leads AND
  • > 10mm in the chest leads (the same criteria as ‘small’ QRS complexes)

Tall T waves can be associated with:

Tall tented T waves
Tall tented T waves 8

Inverted T waves

T waves are normally inverted in V1 and inversion in lead III is a normal variant.

Inverted T waves in other leads are a nonspecific sign of a wide variety of conditions:

Around 50% of patients admitted to ITU have some evidence of T wave inversion during their stay.

Observe the distribution of the T wave inversion (e.g. anterior/lateral/posterior leads).

You must take this ECG finding and apply it in the context of your patient.

ecg - Inverted T wave
Inverted T wave

Biphasic T waves

Biphasic T waves have two peaks and can be indicative of ischaemia and hypokalaemia.

Biphasic T wave
Biphasic T wave 9

Flattened T waves

Flattened T waves are a non-specific sign, that may represent ischaemia or electrolyte imbalance.

Flattened T wave
Flattened T wave 9

U waves

U waves are not a common finding.

The U wave is a > 0.5mm deflection after the T wave best seen in V2 or V3.

These become larger the slower the bradycardia – classically U waves are seen in various electrolyte imbalances, hypothermia and secondary to antiarrhythmic therapy (such as digoxin, procainamide or amiodarone).

U wave
U wave 10

Document your interpretation

You should document your interpretation of the ECG in the patient’s notes (check out our guide to documenting an ECG).


  1. James Heilman, MD. Fast atrial fibrillation. Licence: CC BY-SA 3.0.
  2. Michael Rosengarten BEng, MD.McGill. Right axis deviation. Licence: CC BY-SA 3.0.
  3. James Heilman, MD. Mobitz type 2 AV block. Licence: CC BY-SA 3.0.
  4. James Heilman, MD. Complete heart block. Licence: CC BY-SA 3.0.
  5. James Heilman, MD. Delta wave. Licence: CC BY-SA 3.0.
  6. Michael Rosengarten BEng, MD.McGill. Q-waves. Licence: CC BY-SA 3.0.
  7. Michael Rosengarten BEng, MD.McGill. Poor R-wave progression. Licence: CC BY-SA 3.0.
  8. Michael Rosengarten BEng, MD.McGill. Tall tented T-waves. Licence: CC BY-SA 3.0.
  9. CardioNetworks. T-wave morphology. Licence: CC BY-SA 3.0.
  10. James Heilman, MD. U-wave. Licence: CC BY-SA 3.0.
  11. Michael Rosengarten BEng, MD.McGill. Left axis deviation. Licence: CC BY-SA 3.0.


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