Understanding an ECG


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 electrical activity.
  • Check out our ECG quiz on the new Geeky Medics quiz platform to put your knowledge to the test.

Parts of the ECG explained

ECG different wave types
The components of an ECG

P waves

  • P waves represent atrial depolarisation.
  • In healthy individuals, there should be a P wave preceding each QRS complex.

PR interval

  • The PR interval begins at the start of the P wave and ends at the beginning 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 appears as three closely related waves on the ECG (the Q, R and S wave).

ST segment

  • The ST segment starts at the end of the S wave and ends at the beginning of the T wave.
  • The ST segment is an isoelectric line that represents the time between depolarisation and repolarisation of the ventricles (i.e. ventricular contraction).

T wave

  • The T wave represents ventricular repolarisation.
  • It appears as a small wave after the QRS complex.

RR interval

  • The RR interval begins 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 begins at the start 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.

How to read ECG paper

The paper which ECGs are recorded on is standardised across most hospitals:

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


How the 12 Lead ECG Works

  • Lead” refers to an imaginary line between two ECG electrodes.
  • The electrical activity of a lead is measured and recorded as part of an ECG.
  • A 12-lead ECG records 12 of these leads, producing 12 separate graphs on a piece of ECG paper.
  • Only 10 physical electrodes are attached to the patient, to generate the 12 leads.

Electrodes

  • Electrodes are wires that you attach to a patient to record an ECG.
  • The data gathered from these electrodes allows the 12 leads of the ECG to be calculated.
  • For example, lead I is calculated using data from the electrodes on both the right and left arm.
  • Below are the electrodes used to generate a 12 lead ECG.

Chest electrodes 

  • V1 – 4th intercostal space – right sternal edge
  • V2 – 4th intercostal space – left 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
Chest electrode positions
Chest electrode positions

Limb electrodes

  • LA – left arm
  • RA – right arm
  • LL – left leg
  • RL – right leg – neutral – not used in measurements

 

Leads

Chest leads

  • V1 – Septal view of the heart
  • V2 – Septal view of the heart
  • V3 – Anterior view of the heart
  • V4 – Anterior view of the heart
  • V5 – Lateral view of the heart
  • V6 – Lateral view of the heart

Other leads

  • Lead I – Lateral view (RA-LA)
  • Lead II – Inferior view (RA-LL)
  • Lead III – Inferior view (LA-LL)
  • aVR – Lateral view (LA+LL – RA)
  • aVL – Lateral view (RA+LL – LA)
  • aVF – Inferior view (RA+LA – LL )

The Shape of the ECG Waveform

  • Each individual lead’s ECG recording is slightly different in shape.
  • This is because each lead is recording the electrical activity of the heart from a different direction (a.k.a viewpoint).
  • When the electrical activity within the heart travels towards a lead you get a positive deflection.
  • When the electrical activity within the heart travels away from a lead you get a negative deflection.
  • In reality, electrical activity in the heart flows in many directions simultaneously.
  • Each deflection (a.k.a. wave) on the ECG represents the average direction of electrical travel (which is calculated using mathematical formulae by the ECG machine)
  • The height of the deflection represents the amount of electrical activity flowing in that direction (i.e. the higher the deflection, the greater the amount of electrical activity flowing towards the lead).
  • The lead with the most positive deflection is the most aligned with the direction the heart’s electrical activity 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.
QRS complex
ECG waves


Localising Pathology on the ECG

  • It’s important to understand which leads represent which anatomical territory of the heart, as 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 myocardial infarction (MI).
  • You can then combine this with some anatomical knowledge of the heart’s blood supply, to allow you to work out which artery is likely to be affected (e.g. left anterior descending artery).

Views of the heart
Views of the heart

Cardiac Axis

  • The electrical activity of the heart starts at the sinoatrial node then spreads to the atrioventricular (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

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.
  • You would expect to see the most negative deflection in aVR. This is due to aVR looking at the heart in the opposite direction.
Normal cardiac axis
Normal cardiac axis

Right axis deviation

  • Right axis deviation (RAD) involves the direction of depolarisation being distorted to the right (between +90º and +180º).
  • The most common cause of RAD is right ventricular hypertrophy.
  • Extra right ventricular tissue results in a stronger electrical signal being generated by the right side of the heart.
  • This causes the deflection in lead I to become negative and the deflection in lead aVF/III to be more positive.
  • RAD is commonly associated with conditions such as pulmonary hypertension, as they cause right ventricular hypertrophy.
  • RAD can, however, be a normal finding in very tall individuals.
Right axis deviation
Right axis deviation 1

Left axis deviation

  • Left axis deviation (LAD) involves the direction of depolarisation being distorted to the left (between -30° and -90°).
  • This results in the deflection of lead III becoming negative (this is only considered significant if the deflection of lead II also becomes negative).
  • LAD is usually caused by conduction abnormalities.

Want to Learn More About ECGs?

Check out our other ECG focused articles:


References


Reviewer

Dr Matthew Jackson

Consultant Interventional Cardiologist


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