A Student’s Guide to the
Interpretation of ECGs
By Richard McKearney
Unit 3 - Medicine and Surgery iSSC
Supervisor: Dr Townsend
Word Count: 1,943
• To produce a short guide to the interpretation of ECGs (electrocardiograms) aimed
at medical students enabling them to:
a) determine features of a normal ECG
b) assess rate and rhythm
c) Identify a clear myocardial infarction
• To reflect upon what I have learnt from producing this educational material.
ECG interpretation is a vital skill for all medical students and doctors as ECGs are the most
commonly used and widely available investigation used to diagnose heart disease.1
ability to interpret ECGs correctly means that the correct management can be chosen for the
patient and avoids otherwise preventable adverse events.2
Training in ECG interpretation
often varies a lot between medical students so I felt that it, for these reasons, was important
to produce a guide.
Both when interpreting ECGs and presenting your findings, it is important to do so in a
logical and structured manner to avoid misinterpretation. When presenting an ECG, one
should first say the patients name, age and the date the recording was done. After
introducing the basics of ECG electrophysiology I will go through ECG interpretation in the
following logical order as recommended by the book “the ECG made easy”:3
1. rate & rhythm
2. conduction intervals
3. cardiac axis
4. QRS complexes
5. ST segments and T waves
This guide relies on a previous understanding of the conduction system of the heart which
the ECG looks at.
What is the 12-lead ECG
ECG stands for electrocardiogram. It involves the measurement of the electrical signals of
the heart and is used in conjunction with patient history and examination to diagnose heart
disease and decide how to manage it.
Fig. 1A Diagram of the position of the 6 chest recording electrodes (V1-V6).4
1B Placement of the four limb electrodes.5
The RL electrode is an earth.
The 9 recording electrodes are placed as shown in fig. 1A and 1B. The 3 limb electrodes are
used by the ECG monitor to produce 6 limb leads. A lead is a pictorial representation of the
hearts electrical activity (fig. 2).
Fig. 2 A normal 12-lead ECG.6
Key: Yellow – The 6 “standard leads”.
Red – The 6 chest leads
Green – The rhythm strip used for determining heart rate
Purple – The rate at which the ECG is recorded. At the standard rate of 25mm/s one
large square = 0.2 seconds.
The 12 leads look at the electrical activity of the heart from different angles depending on
which recording electrodes they are derived from and where these electrodes are
placed (fig. 3). This allows the exact location of cardiac abnormalities to be detected.
3A (horizontal plane)
3B (vertical plane)
Fig. 3A The 6 chest leads which look at the heart in the horizontal plane.7
Therefore: V1, V2 – look at the right ventricle
V3, V4 – look at the interventricular septum and the anterior part of the left ventricle
V5, V6 – look at the anterior and lateral sides of the left ventricle
3B The 6 “standard leads” which look at the heart in the vertical plane.7
1. Rate and Rhythm
Depolarisation usually originates from the sinoatrial node. This is known as sinus rhythm. If
depolarisation originates from a different part of the heart, the rhythm is named after it and
it is known as an arrhythmia. The heart rate is usually between 60 and 100 beats per minute
(bpm) and is usually calculated from the rhythm strip (see fig. 2). Because of the rate at
which the ECG is recorded, one big square = 0.2 seconds and therefore there are 300 big
squares per minute. If a QRS complex occurred once per big square the heart rate would
therefore be 300 bpm.
Heart rate = 300 0
R-R interval (in big squares)
It is beyond the scope of this SSC to look at all of the cardiac arrhythmias that can appear on
an ECG. It is however important to recognise if one is present. Check for abnormal rhythm by
lining up a piece of card with the rhythm strip and making a mark on it at the peak of 3 or 4 R
waves. Then slide this further along the rhythm strip and see if the R-R interval remains the
same for the duration of the recording. Check that each P wave is followed by a QRS
complex of normal length and that the P-R interval is normal.
2. Conduction Intervals
The P-R interval should be between 120-200ms long or 3-5 small squares and represents the
time between the onset of atrial and ventricular contraction (fig. 4). It is representative of
the delay in conduction at the atrioventricular (AV) node to prevent the atria and ventricles
contracting at the same time. If the P-R interval is <120ms either the atria are depolarised
from a source close to the AV node or the conduction from the atria to the ventricles in
abnormally quick. The P-R interval is lengthened if there is a conduction defect such as first-
degree heart block.
Fig. 4 The P-R interval should be between 120-200ms.8
The Q-T interval should be <450ms in duration.
The Q-T interval is prolonged in the presence of electrolyte abnormalities and can also be
caused by certain drugs. Prolongation of the Q-T interval can develop into ventricular
3. Cardiac Axis
The cardiac axis refers to the average direction of the electrical wave travelling through the
ventricles in the vertical plane. The normal cardiac axis is between 90° and -30° using lead I
as the 0° reference point (fig. 5).9
A normal cardiac axis means that the wave of
depolarisation is spreading towards leads I, II and III resulting in a predominantly upward
deflection in all three leads. Positive deflection should be greatest in lead II as this lead is
closest to the axis as shown in fig. 5. Refer to fig. 3B to see how the limb leads relate to the
anatomical position of the heart, especially note the direction which the interventricular
septum, containing the bundle if His, is facing.
Fig. 5 This hexaxial diagram shows the position of the six limb leads in the vertical plane.9
Deviation of the axis past -30° is known as left axis deviation. Deviation of the axis in the
other direction past 90° is right axis deviation.
A simple method for determining the cardiac axis is by looking at the level of deflection in
leads I, II and III (fig. 6).
Fig. 6 As previously mentioned, leads I, II and III
should all be positive with lead II being more
positive than leads I and III. In this example, the
complexes in leads II and III are negative. Lead one
is predominantly positively deflected. Using fig. 5
and our knowledge that an electrical wave
travelling towards a lead will cause a positive
deflection, we can work out that the cardiac axis
has swung away from leads II and III and towards
lead I. This is therefore left axis deviation.9
As a simple rule of thumb:
• If leads II and III are negative and I is positive; there is left axis deviation
• If lead I is negative and leads II and III are positive; there is right axis deviation
Small amounts of axis deviation are rarely significant and can occur in both extremes of BMI.
If you do see axis deviation you should also look for other signs of heart disease. Causes of
cardiac axis deviation are shown in fig. 7.
Left axis deviation Right axis deviation
Left ventricular hypertrophy Right ventricular hypertrophy
Conduction defects Pulmonary embolus
Congenital heart disease
Fig. 7 Some causes of cardiac axis deviation.9
4. QRS Complexes
The duration of the QRS complex should be < 120ms (3 small squares). A QRS complex width
of >120ms is indicative of either a bundle branch block or a depolarisation initiated in the
The increased width is explained by the depolarisation being spread throughout
the ventricles in an abnormal fashion and therefore not via the usual conduction
mechanisms which are faster.
Ventricular hypertrophy leads to an increase in electrical activity on that side of the heart
due to the increased muscle bulk. This is reflected in the ECG by an increased height of the
QRS complexes in the chest leads looking at the electrical activity of the hypertrophied
Q waves in leads V1 – V3 (left leads) arise due to the left to right depolarisation of the
interventricular septum as an impulse is conducted down it. Q waves with a width of > 1mm
and depth of > 2mm are pathological and diagnostic of a myocardial infarction in the areas
of the leads which have pathological Q waves (fig. 8). The transmural death of myocardium
due to an infarct leads to the creation of an electrical window which measures the potential
of the cavity of the ventricle. Because the ventricles depolarise from the inside out, this is
recorded as a large (negative) deflection known as a Q wave (Fig. 9).
Anterior V3, V4
Anteroseptal V1, V2
Inferior II, III, aVF
Lateral I, aVL, V5, V6
Fig. 8 The lead location of Q waves in relation to the area of myocardium that has
N.B. Posterior infarctions manifest themselves in the ECG as tall R waves in V1
Fig. 9 Q waves in leads V1 – V4 indicative of an anteroseptal transmural infarction. This
could have been from a long time ago as Q waves never disappear.11
5. ST Segments & T waves
The ST segment should be isoelectric i.e. at the same height as the part between the next T
and P waves (fig. 4). Abnormalities include either elevation or depression (fig. 10).
Fig. 10A Normal ECG complex
10B ST elevation is indicative of an acute
myocardial infarction in the parts of the
heart where the leads are affected or
pericarditis (generalised ST elevation).
10C ST depression. ST depression with an
upright T wave is indicative of ischaemia. ST
depression may be a reciprocal mirroring of
an infarction elsewhere.
T wave inversion can be pathological, but is normal in leads III, aVR and V1. In any other
lead, T wave inversion can be caused by:
1. Bundle branch block
3. ventricular hypertrophy (inversion in the leads looking at that particular ventricle)
T wave peaking or flattening or peaking with an abnormal QT interval are indicative of
electrolyte abnormalities e.g. hyperkalaemia can cause a peaked T wave with a shortened
QT interval. Hypokalaemia causes flattening of the T wave and an extra hump on the T wave
Correct ECG interpretation is necessary to diagnose heart disease in patients and decide
what treatment should be given. Studies have shown that in the emergency department, up
to 5% of ECGs from patients with chest pain are not interpreted correctly.12
However; half of
these cases could have been diagnosed correctly if ECG interpretation was improved.13
correct ECG interpretation it is important to have a systematic way of looking at the various
aspects of the ECG trace. I recommend the 5-step system as outlined above. This ensures
that abnormalities are not missed, avoiding misdiagnosis and therefore incorrect
In reflection, I have learnt a lot from producing this educational guide. Not just about ECG
interpretation, but about how to use information and graphics to teach colleagues about
important subjects. I have spoken to my colleagues about what they have found difficult
about understanding ECG interpretation and then through testing different approaches to
teaching certain aspects (e.g. determining cardiac axis) have determined which images and
descriptions have worked best at conveying a complex idea. I do however appreciate that
different students learn better in different ways and that any one single approach will not
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3. Hampton JR. The ECG made easy. London: Churchill Livingstone; 2008.
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& Common Cardiac Arrhythmias. Available at:
Accessed Feb 16, 2010.
5. 12 Lead ECG Lead Placement Diagrams. Available at:
Accessed Feb 16, 2010.
6. Learn the Heart. Available at:
Accessed Feb 17, 2010.
7. National Center for Biotechnology Information. Available at:
Accessed Feb 17, 2010.
8. Long QT Syndrome Part II. Published Oct 04 2009. Available at:
Accessed Feb 21, 2010.
9. Cardiology Explained – Arrhythmia. Published 2004. Available at:
Accessed Mar08, 2010.
10. Breen D. EKG Interpretation. Available at:
Accessed Feb 21, 2010.
11. Cardiology Explained. Conquering the ECG. Published 2004. Available at:
Accessed Feb 21, 2010.
12. Berger A, Meier JM, Stauffer JC, Eckert P, Schlaepfer J, Gillis D, Cornuz J, Yersin B,
Schaller MD, Kappenberger L, Wasserfallen JB. ECG interpretation during the acute phase of
coronary syndromes: in need of improvement? Swiss Medical Weekly 2004;134:695-699.
13. Brady WJ, Perron A, Ullman E. Errors in emergency physician interpretation of ST-
segment elevation in emergency department chest pain patients. Academic Emergency