Understanding ECGs
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This article “Understanding ECGs” is part of the almostadoctor ECG series. It provides a basic introduction to ECG waveform, and the practicalities of performing an ECG.  Once you understand the basics of ECG Interpretation, you can move onto ECG Abnormalities and Summary of ECG Abnormalities.

The Basics

  • ECG stands for electrocardiogram 
    • Often in American English written as EKG – electrocardiogram – from the original German
  • It is a method of measuring the efficiency of the conducting system of the heart
ECG Interpretation - basic complexes
ECG Waveform

ECG Interpretation – basic complexes

  • P waves represent depolarisation of the atria
  • QRS complexes represent depolarisation of the ventricles
  • T waves represent repolarisation of the ventricles
  • There is no wave for repolarisation of the atria because this process is masked by the depolarisation of the ventricles. It is quite normal for no q wave to be present.

ECG paper

On standard ECG paper each large square represents 0.2s (200milliseconds) and each small square represents 40ms (5 small squares per large square).

  • 1 big square is 0.5cm
  • 1 small square is 1mm

Five large squares represent 1 second

There are 300 large squares per minute. So, if a QRS occurs once every large square, the HR is 300/min. However, if they occur once only 4 squares, the heart rate is 300/4 = 75bpm. When you work this out, you say you are doing the R-R interval.

Be aware that in very fast heart rates, this scale can be altered to aid the ECG interpretation. An ECG printout should confirm the scale in the corner of the paper

ECG Segments and Intervals

ECG segments and Intervals


This term is used to refer to a part of the ECG between the end of one wave and the start of the next. The two main segments are the PR segment and the ST segment.


This term is more ambiguous and refers to various stretches of the ECG:

  • PR Interval – this is from the beginning of the P wave, until the beginning of the QRS complex. It measures the time taken for excitation to spread from the SA node (all the way through the AV node, and various bundles) to the ventricular muscle.
  • Normal interval – 0.12-0.2s – i.e. it should be between 3-5 little squares.
  • QT interval – this is from the beginnings of the QRS complex until the end of the t wave.
  • QRS duration – this tells us the amount of time it took the signal to spread throughout the ventricles.
  • Normal interval – 0.12s (i.e. about 3 little squares) – however various conditions can cause a lengthened QRS duration.


Normally you take a ‘12-lead ECG’. This term is confusing because the word ‘lead’ has two meanings.

  • It can mean the physical electrical cable that you use to connect the reader with the patient, or, more correctly it refers to an ‘imaginary line’ between two ECG electrodes, along which electrical conductivity is measured.
  • So a ’12-lead ECG’ has 12 of these imaginary lines along which conductivity is measured, and thus 12 graphs can be produced. However, there are only 10 physical electrodes and ‘leads’ that you attach to the patient’s body.
  • To try and avoid confusion here I will refer to the physical connections as ‘electrodes’ 

It is important to attach your electrodes in the right place so they you get signals that can be properly interpreted.

Calibrating the machine

The height of the complexes can be important in defining certain conditions. Thus it is important that the reader is calibrated correctly so we can interpret the wave height. A signal of 1mV should cause the graph to rise 1cm (2 large squares). At the start of each ECG trace, this calibration signal should be seen on the print out.

Taking a reading

  • The patient should be laid down and relaxed (to prevent muscle tremors which may cause interference)
  • Now connect the electrodes
  • Calibrate the machine
  • Make the recording!

Placing your electrodes

You should connect the electrodes in the following order:

  • V1 – 4th IC space on the right sternal edge
  • V2– 4th IC space on the left sternal edge
  • V4– 5th intercostal space at the midclavicular line- at the apex beat.
  • V3 – place this half way between II and IV so that it sits on the 5th rib.
  • V6–mid axillary line at the 5th intercostals space
  • V5 – anterior axillary line at the 5th intercostal space.
  • LA – should be connected to the left arm – it can be done anywhere on the arm, but use your common sense – for example if they have a tremor don’t place it at the extremity. If they don’t have an upper limb, you should try to still place the electrode distally to the shoulder joint on the stub (if they have one – they will pretty much always have something there!).
  • The electrode is yellow – “lemon left”
  • RA – right arm. The electrode is Red – “red right”
  • RL – right leg – the electrode is black.
  • LL – left leg – the electrode is green (spleen! – just means put it on the left ankle – put it over a bony prominence!)
  • LA – left arm – the lead is yellow – lemon left!

Note that the colouring scheme of leads varies (usually by country).

ECG Lead Placement
ECG Lead Placement

In cases of massive posterior MI there will be marked anterior ST depression, it is possible to take electrodes V4-6 off and put them on the back. This is to look if there is anterior ischaemia (don’t thrombolise) or if there is posterior MI. If there is just anterior ischaemia, there won’t be ST elevation in 7-9. If there is posterior MI, there will be ST elevation in the posterior leads.

  • 7- posterior axillary line, 5th IC space, left hand side
  • 8 – between 7 & 9
  • 9 – left spinal edge at the 5th IC space


There are three types of lead; bipolar (limb) leads, chest unipolar leads and augmented unipolar leads. Each type of lead measures the conductivity of the heart in a different plane, and thus this allows us to see the activity of different parts of the heart. Bipolar limb leads – these comprise of a positive electrode and a single negative electrode, through which the conductivity of the heart are measured. Unipolar leads – these utilise a single positive electrode, and then use a combination of other electrodes to represent a negative electrode.

Bipolar limb leads

LeadFrom ToPlaneViewing the…
Lead IRA (-)LA(+)LateralLateral wall of left ventricle
Lead IIRA (-)LL (+)InferiorDiaphragmatic surface
Lead IIILA (-)LF (+)InferiorDiaphragmatic surface

You might wonder how the LA can operate as both a negative and a positive electrode simultaneously. Well so do I! I guess these voltages are relative, so it is relatively more positive than the Right arm, whilst still being more negative than the left leg. Also note that the ‘inferior’ plane leads are viewing the heart from below, and thus looking at the diaphragmatic surface, which consists mainly of the left ventricle.

Augmented Unipolar Limb leads

LeadFromToPlaneViewing the…
aVRLA+LL (-)RA (+)Lateral (reversed)Right atrium*
aVLRA+LL (-)LA (+)LateralLateral wall of left ventricle
aVFRA+ LA (-)LL (+)InferiorDiaphragmatic surface
Einthoven’s triangle
Einthoven’s triangle

*although the view is pretty non-specific Some people find Einthoven’s triangle helpful in understanding the leads:  

Just thought of a little trick to remember which goes where: aVFFoot aVR Right aVL Left

Unipolar chest leads

These account for the other 6 leads of the 12-lead ECG. Each of the electrodes is positive.

LeadPlaneViewing the…
V1SeptalSeptal wall of ventricles
V2SeptalSeptal wall of ventricles
V3AnteriorAnterior surface*
V4AnteriorAnterior surface*
V5LateralLateral wall of left ventricle
V6LateralLateral wall of left ventricle

*note that this consists mainly of the right ventricle You may notice that the Right Leg electrode is not mentioned, and this is because it is a NEUTRAL electrode, and thus not utilised in the measurements themselves.

Graph Production

When the ECG graph is drawn, there is a positive line when the signal travels towards a particular lead, and a negative one, when the signal travels away from a lead. Depolarisation spreads throughout the heart in many directions at once, but the wave produced is the average of this. When the R wave is greater than the S wave, this means the signal is generally positive, and therefore moving towards the lead. When the S wave is greater than the R wave, it means the signal is negative and moving away from the lead. When the depolarisation wave is moving at right angles to the lead, then R and S waves will be of equal size. Imagine looking at the heart from the front. From here, the waves of depolarisation generally spread through the heart from 11 o’clock to 5 o’clock. This direction of spread of the signal is known as the cardiac axis. Therefore, they head along lead II. This creates a nice positive R wave in lead II. aVR however has its positive and negative electrodes the other way around, and so the signal travels against this lead, and thus there is a negative signal produced in this lead. You can determine the direction of the cardiac axis (to check if it is normal or not) by looking at leads I-III. A normal cardiac axis will cause a positive signal in all 3 of these leads – because between them these leads measure the lateral (right to left) and inferior dimensions – and the signal is travelling laterally and inferiorly! The deflection is greatest in lead II because this lead measure both laterally and inferiorly. However, in right ventricular hypertrophy the cardiac axis becomes displaced. The overall direction of charge is now from 1-7 o’clock, because there is extra muscle, and therefore extra signal strength on the right side of the heart. This alters the signal, such that lead I will display a negative QRS, and lead III will now have the tallest QRS (not lead II). This axis shift is called right axis deviation. It is associated mainly with pulmonary conditions that put a strain on the heart. Left ventricular hypertrophy can also cause an axis deviation. We call this left axis deviation. In this, the lead I signal will become very weak, whilst leads II and III will become negative. This is often a result of a conduction defect rather than left ventricular hypertrophy. Basically, the cardiac axis points to any where the R is larger than the S, and points away from any lead where the S is larger than the R. however, it is unlikely that it will point directly towards any of the leads, and thus you only have an approximation – thus this is why it is useful to have several leads!  Where R and S are of equal size, the cardiac axis is at 90’ to that particular lead. Deviations in the cardiac axis may not themselves be significant – they can occur in normal people (often if they are very tall, or very thin or fat). However, they prompt you to look for other things. Also remember; right axis deviation is often a result of right ventricular hypertrophy (due to pulmonary disease) but in left axis deviation the most likely cause is NOT hypertrophy, but a conduction blockage.

QRS in the V leads

  • V1 and V2 – look at the right ventricle
  • V3 and V4 – look at the septum
  • V5 and V6 – look at the left ventricle

There is more muscle on the left than the right – therefore the ECG trace is biased towards the left of the heart. This means that for leads looking at the right of the heart, and to some extent at the septum, the overall flow of charge, is away from these leads – thus leading to a negative QRS. The general rule is that V1 and V2 have a negative QRS, V3 and V4 have roughly equal sized R and S waves, and V5 and V6 have a positive QRS The point where R and S waves are equal indicates the site of the septum – it is called the transition point. Looking at these allows you to determine of there is right ventricular hypertrophy – because if there is, then the transition point will not appear in lead V3/V4, but instead may appear in V5/V6, as the enlarged ventricle shifts the septum to the left.

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Dr Tom Leach

Dr Tom Leach MBChB DCH EMCert(ACEM) FRACGP currently works as a GP and an Emergency Department CMO in Australia. He is also a Clinical Associate Lecturer at the Australian National University, and is studying for a Masters of Sports Medicine at the University of Queensland. After graduating from his medical degree at the University of Manchester in 2011, Tom completed his Foundation Training at Bolton Royal Hospital, before moving to Australia in 2013. He started almostadoctor whilst a third year medical student in 2009. Read full bio

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