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This guide demonstrates how to read an ECG in a systematic and effective manner.

Always start by confirming the name and date of birth of the patient to confirm the ECG belongs to the right person. Also, confirm the date and time the ECG was performed. It’s also important to document your findings once you’ve read an ECG, so check out our guide to documenting an ECG here.

Step 1 – Heart rate

Heart rate can be calculated simply with the following method:

  • Work out the number of large squares in one R-R interval
  • Then divide 300 by this number and you have your answer

e.g. if there are 4 squares in an R-R interval 300/4 = 75 beats per minute


If the rhythm is irregular:

  • Count the number of complexes on the rhythm strip (each rhythm strip is 10 seconds long)
  • Multiply the number of complexes by 6 (giving you the average number of complexes in 1 minute)

What’s a normal heart rate?

  • Normal = 60 – 100 bpm
  • Tachycardia > 100 bpm
  • Bradycardia < 60 bpm

Hint: If there are obviously P-waves present, check the ventricular rate and the atrial rate. The rates will be the same if there is 1:1 AV conduction.

Step 2 – Heart rhythm

The heart rhythm can be regular or irregular.
Irregular rhythms are regularly irregular (i.e. a recurrent pattern of irregularity) or 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 the same.

Hint – if you are suspicious that there is some atrioventricular 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.

Step 3 – 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 figure out the cardiac axis you need to look at leads I,II and III.

To get a better understanding of cardiac axis read this article.

Normal cardiac axis

In normal cardiac axis Lead II has the most positive deflection compared to Leads I and III.


Right axis deviation

In right axis deviation Lead III has the most positive deflection and Lead I should be negative.

This is commonly seen in individuals with right ventricular hypertrophy.

Left axis deviation

In left axis deviation Lead I has the most positive deflection and Leads II and III are negative.

Left axis deviation is seen in individuals with heart conduction defects.

Step 4 – P-waves

Next we look at the P-waves and answer the following questions:

  • Are P-waves present?
  • If so, is each P-wave followed by a QRS complex?
  • Do the P-waves look normal? (check duration, direction and shape)
  • If not present, is there any atrial activity e.g. sawtooth baseline → flutter waves / chaotic baseline → fibrillation waves / flat line → no atrial activity at all?

Hint – If P-waves are absent and there is an irregular rhythm it may suggest atrial fibrillation.


Step 5 – P-R interval 

The P-R interval should be between 0.12-0.2 seconds (3-5 small squares).

Prolonged PR interval (>0.2 seconds)

A prolonged PR interval suggests there is atrioventricular delay.

Is the prolonged PR interval fixed or does it vary across the ECG?

  • A fixed prolonged PR interval is a FIRST DEGREE AV BLOCK

    First degree heart block


  • If the PR interval slowly increases then there is a dropped beat, this is MOBITZ TYPE I SECOND DEGREE AV BLOCK (Wenckebach)
2nd degree AV block (Mobitz Type 1 - Wenckebach)1

2nd degree AV block (Mobitz Type 1 – Wenckebach)1 


  • If the PR interval is fixed but there are dropped beats, this is MOBITZ TYPE 2 SECOND DEGREE HEART BLOCK (clarify that by the frequency of dropped beats e.g 2:1, 3:1, 4:1)
Mobitz type 2

Mobitz type 2 (3:1)  2


  • If the P waves and QRS complexes are completely unrelated, this is THIRD DEGREE AV BLOCK (complete heart block)
Complete heart block (3rd degree)

Complete heart block (3rd degree)


To help remember these degrees of AV block, it is useful to remember the anatomical location of the block in 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 (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 – occurs AFTER the AV node in the bundle of His or Purkinje fibres.
  • Third degree AV block: 
    • Occurs anywhere from the AV node down causing complete blockage

Shortened PR interval

If the PR interval is short, this means 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)

Delta wave

Step 6 – QRS complex

There are several aspects of the QRS complex to assess.


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 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.


Describe this as 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.


This is where you 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.

Delta wave


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 (anterior / inferior) for evidence of previous MI.

Inferior Q waves (II, III, aVF) with T-wave inversion due to previous MI -

Inferior Q waves (II, III, aVF) with T-wave inversion due to previous MI – 3


R and S waves

Look for R wave progression across the chest leads (from small in V1 to large in V6) with 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 lead position.


Poor R wave progression (previous anterior MI) - Image sourced from

Poor R wave progression (previous anterior MI) 4


J point segment

The J point is where the S wave joins the ST segment. This point can be elevated or the ST segment that follows is then raised…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.

  • It occurs mostly under the age of 50 (over age of 50, ischaemia is more common and should be suspected first) and rarely over 70.
  • 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 changes do not change! During a STEMI, the changes will evolve – in benign early repolarisation, they will remain the same.
Benign Early Repolarisation (High take off) -

Benign Early Repolarisation (High take off) 4

Step 7 – ST segment

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

In a healthy individual it should be an isoelectric line (neither elevated or depressed).

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


ST elevation

ST elevation is significant when it is greater than 1mm (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.



ST depression

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


Step 8 – T waves

The T waves represent repolarisation of the ventricles.

Tall T waves

T waves are 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:

  • Hyperkalaemia (“Tall tented T waves”)
  • Hyper-acute STEMI

Peaked T

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:

  • Ischaemia
  • Bundle branch blocks (V4 – 6 in LBBB and V1 – V3 in RBBB)
  • PE
  • LVH (in the lateral leads)
  • HCM (widespread)
  • General illness

Around 50% of ITU admissions have some evidence of T wave inversion during their stay.

Comment on 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.


Biphasic T waves

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


Flattened T waves

Another non-specific sign, this may represent ischaemia or electrolyte imbalance.

U waves

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 or hypothermia, or antiarrhythmic therapy (such as digoxin, procainamide or amiodarone).

Prominent U waves in a patient with Hypokalaemia

Prominent U waves in a patient with Hypokalaemia 5


Click to show
  1. 2nd degree AV block (Mobitz Type 1 – Wenckebach)1 –  Image from
  2. Mobitz type 2 (3:1) Image sourced from
  3. Inferior Q waves (II, III, aVF) with T-wave inversion due to previous MI –
  4. Poor R wave progression (previous anterior MI) – Image sourced from
  5. Benign Early Repolarisation (High take off)
  6. Prominent U waves in a patient with Hypokalaemia –


Normal axis – wikipedia commons

Left Axis Deviation ECG –

1st Degree Heart Block –

Delta Wave – By James Heilman, MD (Own work) [CC BY-SA 3.0 ( or GFDL (], via Wikimedia Commons

Wolf Parkinson White Syndrome –

Normal QRS –

ST Elevation ECG –

ST Depression –

T wave inversion –

Tall T waves –

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