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.

Parts of the ECG explained


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).


The T-wave represents ventricular repolarisation.

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



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.


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.


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 

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


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

Chest electrode positions


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

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 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).

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 on the ECG paper 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 heart’s 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 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 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. 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 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 1

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 2

Want to learn more about ECGs?

We’ve also got some other articles to help you gain a better understanding of ECGs:

  • How to read an ECG – a step by step guide to reading an ECG – [LINK]
  • Cardiac axis explained – an article focusing on purely on cardiac axis – [LINK]
  • How to document an ECG – a guide to documenting your interpretation on an ECG in the notes – [LINK]


1. Author: Michael Rosengarten BEng, MD.McGill – Right axis deviation – via Wikimedia Commons – Licence: CC BY-SA 3.0

2. Author: Michael Rosengarten BEng, MD.McGill  Left axis deviation – via Wikimedia Commons – Licence: CC BY-SA 3.0 .jpg


Dr Matthew Jackson – Interventional Cardiology Registrar

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