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What is an ECG?

ECG is short for electrocardiogram.

It is used to record the electrical activity of the heart from different angles to identify and locate pathology.

Electrodes are placed on different parts of a patient’s limbs and chest to record the signal.

Parts of the ECG explained

SinusRhythmLabels.svgP-waves

P-waves represent atrial depolarisation.

In sinus rhythm, there should be a P-wave preceding each QRS complex.

 

PR interval

The PR-interval is from the start of the P-wave to the start of the Q wave.

It represents the time taken for electrical activity to move between the atria and ventricles.

 

QRS complex

The QRS-complex represents depolarisation of the ventricles.

It is seen as three closely related waves on the ECG  (Q,R and S wave).

ST segment

The ST-segment starts at the end of the S-wave and finishes at the start of the T-wave.

The ST segment is an isoelectric line that represents the time between depolarization and repolarization of the ventricles (i.e. contraction).

T-wave

The T-wave represents ventricular repolarisation.

It is seen as a small wave after the QRS complex.

 

RR-interval

The RR-interval starts at the peak of one R wave and ends at the peak of the next R wave.

It represents the time between two QRS complexes.

 

QT-interval

The QT-interval starts at the beginning of the QRS complex and finishes at the end of the T-wave.

It represents the time taken for the ventricles to depolarise and then repolarise.

The 12 lead ECG: how it all works

The first thing to clear up is the definition of the word “lead” in an ECG context.

Lead refers to an imaginary line between two ECG electrodes.

The electrical activity of this lead is measured and recorded as part of the ECG.

A 12-lead ECG records 12 of these “leads” producing 12 separate graphs on the ECG paper.

However you only actually attach 10 physical electrodes to the patient.

 

Electrodes

The electrodes are wires that you attach to the patient to record the ECG.

These electrodes allow leads to be calculated.

For example Lead I is calculated using the electrodes on the right and left arm.

Below are the electrodes used in a 12 lead ECG.

 

Chest electrodes positions 1

V1 – 4th intercostal space right sternal edge

V2 4th intercostal spaceleft sternal edge

V3 – midway between V2 and V4

V4 – 5th intercostal space midclavicular line

V5 – left anterior axillary line same horizontal level as V4

V6 – left mid-axillary line same horizontal level as V4 & V5

 

Limb electrodes

LA – left arm

RA right arm

LL – left leg

RL – right leg – neutral – not used in measurements

 

Leads

Lead refers to an imaginary line between two ECG electrodes.

There are 12 leads measured in a 12-lead ECG.

 

Chest leads

V1Septal view of heart

V2Septal view of heart

V3 Anterior view of heart

V4 Anterior view of heart

V5 Lateral view of heart

V6 Lateral view of heart

Precordial_leads_in_ECG

Placement of chest electrodes


Other leads

Lead I Lateral view (RA-LA)

Lead IIInferior view (RA-LL)

Lead III Inferior view (LA-LL)

aVR Lateral view (LA+LL – RA)

aVLLateral view (RA+LL – LA)

aVF Inferior view (RA+LA – LL )

 

This diagram is a useful way of understanding the relationships between the leads

Hexaxial

Lead viewpoints

Viewpoints of the heart

It’s important to understand which leads represent which part of the heart.

This allows you to localise pathology to a particular heart region.

For example if there is ST elevation in leads V3 and V4 it suggests an anterior MI.

You can then combine this with some anatomical knowledge of the heart’s blood supply.

This would allow you to work out which artery is likely to be affected (e.g left anterior descending artery).

How to read ECG paper

The paper which ECGs are recorded upon is standardised across all hospitals (usually).

  • Each small square represents 0.04 seconds
  • Each large square on the paper represents 0.2 seconds
  • 5 large squares therefore = 1 second
  • 300 large squares = 1 minute

The shape of the ECG waveform

Each individual leads ECG recording is slightly different in shape.

This is due to each lead recording the electrical activity from different directions.

When the electrical activity of the heart travels towards a lead you get a positive deflection.

When the electrical activity travels away from a lead you get a negative deflection.

 

Electrical activity in the heart flows in many directions at once.

The wave seen represents the average direction.

The height of the deflection also represents the amount of electricity flowing in that direction.

The lead with the most positive deflection is closest to the direction the hearts electricity is flowing.

 

If the R-wave is greater than the S-wave it suggests depolarisation is moving towards that lead.

If the S-wave is greater than the R-waves it  suggests depolarisation is moving away from that lead.

If the R and S-waves are of equal size it means depolarisation is travelling at exactly 90° to that lead.

Cardiac axis explained

The electrical activity of the heart starts at the sinoatrial node then spreads to the AV node.

It then spreads down the bundle of His and then Purkinje fibres to cause ventricular contraction.

Whenever the direction of electrical activity is towards a lead you get a positive deflection in that lead.

Whenever the direction of electrical activity is away from a lead you get a negative deflection in that lead.

The cardiac axis gives us an idea of the overall direction of electrical activity when the ventricles are contracting.

 

Normal cardiac axis

In healthy individuals you would expect the axis to lie between -30° and +90º.

The overall direction of electrical activity is towards leads I,II and III (the yellow arrow below).

As a result you see a positive deflection in all these leads, with lead II showing the most positive deflection as it is the most closely aligned to the overall direction of electrical spread.

Normal cardiac axis

You would expect to see the most negative deflection in aVR. This is due to aVR looking at the heart in the opposite direction to the overall electrical activity.

 

NORMAL AXIS

Normal cardiac axis

 

 

 

Right axis deviation

Right axis deviation (RAD) is usually caused by right ventricular hypertrophy.

In right axis deviation the overall direction of electrical activity is distorted to the right (between +90º and +180º).

Extra heart muscle causes a stronger positive signal to be be picked up by leads looking at the right side of the heart.

This causes the deflection in lead I to become more negative and the deflection in III to be more positive.

RAD is associated with pulmonary conditions as they put strain on the right side of the heart.

It can also be a normal finding in very tall individuals

Right axis deviation

 

Right Axis Deviation

Right axis deviation

 

 

Left axis deviation

In left axis deviation (LAD) the direction of overall electrical activity becomes distorted to the left (between -30° and -90°).

This causes the deflection in lead I to become more positive and the deflection in III to be more negative.

LAD is usually caused by conduction defects and not by increased mass of the left ventricle.

 

 

Left axis deviation


Left axis deviation

Left axis deviation

Reading ECGs  – a structured guide

It’s a good idea to have a structure when reading ECGs.

The mnemonic RRAW can help you remember what you should be looking for and in what order:

  • Rate
  • Rhythm
  • Axis
  • Waveform (the various parts of the ECG mentioned above)

 

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

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Rhythm

Look at the R-R intervals again – if they are equally spaced from each other the rhythm is regular. If not the rhythm is irregular.

You can use the ‘card method’ to mark out the distance between each R wave to check the spacing.

An irregular rhythm with no distinct P-waves suggests atrial fibrillation.

 

 

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.

NORMAL AXIS

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 can suggest underlying heart conduction system defects.

 

Waveform

P-waves

P-waves represent atrial depolarisation.

In sinus rhythm, there should be a P-wave preceding each QRS complex.

Look at the p waves and comment on a number of things:

  • Are P-waves present?
  • Do they occur regularly?
  • Is there sinus rhythm (does a P-wave precede each QRS complex?)
  • Do the P-waves look normal? (smooth, rounded and upright)

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

 

P-R interval 

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

Are the P-R intervals consistent or do they change throughout the ECG?

A prolonged P-R interval may suggest the presence of heart block.

First degree heart block

A shortened P-R interval may suggest the presence of  Wolf Parkinson White syndrome.

Wolf Parkinson White syndrome

 

 

QRS complex

The QRS-complex represents depolarisation of the ventricles.

It is seen as 3 closely related waves on the ECG (Q / R / S wave):

  • The first downward deflection is the Q-wave
  • Any upward deflection is an R-wave
  • A downward deflections after an R-wave is called the S-wave

 

Check the width of the QRS complexes:

  • The QRS complexes should be approximately 0.12 seconds (3 small squares)

If longer than 0.12 seconds it suggests the complex originated in the ventricles.

If shorter than 0.12 seconds it suggests the complex is supraventricular in origin.

 

 

ST segment

The ST-segment starts at the end of the S-wave and finishes at the start of the T-wave.

It represents the interval between ventricular depolarisation and repolarisation.

It should be level with the PR-segment and the T-P segment in healthy individuals.

 

 

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.

stemi

ST elevation

 

ST depression

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

 

ST depression

ST depression

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T waves

The T-wave represents ventricular repolarisation.

It is seen as a small wave after the QRS complex.

 

Are the T waves inverted?

Inverted T-waves are one of the most common abnormalities found on an ECG.

T-wave inversion has a large number of potential causes.

Inverted T-waves in V1 and V2 are not significant and are often seen in healthy individuals.

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Some of the causes of inverted T-waves include:

  • Hypokalaemia, Pericarditis, myocardial infarction (acute and previous)
  • Bundle branch block
  • Wolff Parkinson White syndrome

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

T-wave inversion

T-wave inversion

 

Are the T-waves Tall?

A T-wave is considered tall when it is greater than:

  • 5mm in the standard leads
  • 10mm in the precordial leads

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Tall T-waves can be caused by:

  • Hyperkalaemia
  • Myocardial ischaemia (hyper-acute MI)

In hyperkalaemia the T-waves are described as “Tall Tented T-waves“.

This is because alongside been tall they are also very narrow, with a sharp apex.

In hyper-acute MI you also get tall T-waves however they are not as narrow as in hyperkalaemia.

Peaked T

Tall T- waves

RR-interval

The RR-interval starts at the peak of one R wave and ends at the peak of the next R wave.

It represents the time between two QRS complexes.

It can be used to calculate heart rate.

 

QT-interval

The QT-interval starts at the beginning of the QRS complex and finishes at the end of the T-wave.

It represents the time taken for the ventricles to depolarise and then repolarise.

It is heart rate dependent and corrected QT (QTc) is calculated by most modern ECG machines.

A lengthened QT interval increases the risk of ventricular tachyarrhythmias e.g. torsades de pointes

References

Click to show

1. Chest electrode positions – SCST guidelines – Recording a 12-lead ECG v2.0 – http://www.scst.org.uk/resources/CAC_SCST_Recording_a_12-lead_ECG_final_version_2014_CS2v2.0.pdf

Normal axis – wikipedia commons

Left Axis Deviation ECG – http://lifeinthefastlane.com/wp-content/uploads/2011/02/LAD.jpg

1st Degree Heart Block – http://humanbodydisease.com/abnormal-rhythms-block-heart-signals-712.html

Wolf Parkinson White Syndrome – http://www.ask.com/wiki/Bundle_of_Kent

Normal QRS – http://www.ambulancetechnicianstudy.co.uk/rules.html

ST Elevation ECG – http://lifeinthefastlane.com/ecg-library/

ST Depression – http://lifeinthefastlane.com/ecg-library/

T wave inversion – http://www.wikidoc.org/index.php/File:T_wave_morphology.png

Tall T waves – http://lifeinthefastlane.com/ecg-library/

Documenting an ECG – https://geekymedics.com/how-to-document-an-ecg/

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