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Understanding ECG's

Edit this Article | Original article by Tom Leach | Last updated on 21/11/2013 | View Revisions

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 THE BASICS

  • ECG stands for electrocardiogram.
  • It is a method of measuring the efficiency of the conducting system of the heart.
 
P represents depolarisation of the atria
QRS represents depolarisation of the ventricles
 
T represents repolarisation of the ventricles
 
There is no wave for repolarisation of the atria because this is covered up by the depolarisation of the ventricles.
 
It is quite normal for no q wave to be present.
 
 
 
 
 
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.
 
 
Segments and intervals
 
Segment
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.
 
Intervals
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.
 
Practicalities
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 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 mmse-mini-mental-state-exam" class="alinks-link" title="Confusion: AMTS and MMSE">amts-and-mmse-mini-mental-state-exam" class="alinks-link" title="Confusion: AMTS and MMSE">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 waves 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.
 
In cases of massive posterior MI there will be marked anterior ST depression, you have to take 4-6 off and put them on the back. this is to look if there is just 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
 
Leads
There are three types of lead; bipolar (limb) leads, chest unipolar leads and augmented unipolar leads. Each type of lead basically 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
Lead
From
To
Plane
Viewing the…
Lead I
RA (-)
LA(+)
Lateral
Lateral wall of left ventricle
Lead II
RA (-)
LL (+)
Inferior
Diaphragmatic surface
Lead III
LA (-)
LF (+)
Inferior
Diaphragmatic 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
Lead
From
To
Plane
Viewing the…
aVR
LA+LL (-)
RA (+)
Lateral (reversed)
Right atrium*
aVL
RA+LL (-)
LA (+)
Lateral
Lateral wall of left ventricle
aVF
RA+ LA (-)
LL (+)
Inferior
Diaphragmatic surface
*although the view is pretty non-specific
 
Some people find Einthoven’s triangle helpful in understanding the leads: (sorry about the captions in Dutch but I’m sure you can understand!)
 
Just thought of a little trick to remember which goes where:
aVF - Foot
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.
Lead
Plane
Viewing the…
V1
Septal
Septal wall of ventricles
V2
Septal
Septal wall of ventricles
V3
Anterior
Anterior surface*
V4
Anterior
Anterior surface*
V5
Lateral
Lateral wall of left ventricle
V6
Lateral
Lateral 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 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.