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

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Aims 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.

IntroductionECG 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 The

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.

OverviewBoth 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

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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.

1A 1B

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

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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 RhythmDepolarisation 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

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(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 IntervalsThe 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.

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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

tachycardia.3

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.

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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

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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

ventricles.3 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

ventricle.

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

infarcted.10 N.B. Posterior infarctions manifest themselves in the ECG as tall R waves in V1

and V2.

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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 wavesThe 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.

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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

2. ischaemia

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

(U wave).3

ConclusionCorrect 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 For

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

management.

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

suit everyone.

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References

1. Fisch C. Evolution of the clinical electrocardiogram. Journal of the American College of

Cardiologists 1989;14:1127-38.

2. Salerno SM, Alguire PC, Waxman HS. Competency in interpretation of 12-lead

electrocardiograms: a summary and appraisal of published evidence. Annals of internal

Medicine 2003;138:751-60.

3. Hampton JR. The ECG made easy. London: Churchill Livingstone; 2008.

4. The University of Nottingham. A Beginners Guide to Normal Heart Function, Sinus Rhythm

& Common Cardiac Arrhythmias. Available at:

http://www.nottingham.ac.uk/nursing/practice/resources/cardiology/function/

chest_leads.php

Accessed Feb 16, 2010.

5. 12 Lead ECG Lead Placement Diagrams. Available at:

http://ems12lead.blogspot.com/2008/10/12-lead-ecg-lead-placement-diagrams.html

Accessed Feb 16, 2010.

6. Learn the Heart. Available at:

http://www.learntheheart.com

Accessed Feb 17, 2010.

7. National Center for Biotechnology Information. Available at:

http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?

book=cardio&part=A39&rendertype=figure&id=A83

Accessed Feb 17, 2010.

8. Long QT Syndrome Part II. Published Oct 04 2009. Available at:

http://stanford.wellsphere.com/health-education-article/long-qt-syndrome-part-ii/823531

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Accessed Feb 21, 2010.

9. Cardiology Explained – Arrhythmia. Published 2004. Available at:

http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=cardio&part=A548

Accessed Mar08, 2010.

10. Breen D. EKG Interpretation. Available at:

http://www.fammed.wisc.edu/files/webfm-uploads/documents/med-student/ekg-

interpretation.pdf

Accessed Feb 21, 2010.

11. Cardiology Explained. Conquering the ECG. Published 2004. Available at:

http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=cardio&part=A39

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

Medicine 2000;7:1256–60.

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