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ECG Interpretation: The Basics
This short introduction to ECGs uses a systematic approach to aid pattern recognition. It does not
cover cardiac arrest. It outlines a basic framework for recognising some common findings. There is
intentionally no reference to the Wolf Parkinson White syndrome. In eleven pages, a simple approach
is presented which is inevitably not comprehensive and which assumes some prior knowledge of the
ECG (eg orientation of leads and the relationship of normal complexes to the conducting system).
After you have read this document, you should be familiar with the rate-rhythm-axis-p wave-qrs-t wave
approach to looking at ECGs. Along the way you will have seen examples of:
• Sinus rhythm
• Atrial fibrillation
• Atrial flutter
• Type 2 heart block (Wenckebach type)
• Ventricular tachycardia
• Left ventricular hypertrophy
• Left bundle branch block
• Right bundle branch block
• Anteroseptal myocardial infarction
• Anterior myocardial infarction
• Inferior myocardial infarction
• Inferolateral myocardial infarction
1) Rate
Divide 300 by the approximate number of big squares between complexes to get the rate.
Now test yourself
What is the rate in each of the ECG strips which follow?
a)
b)
c)
d)
e)
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Answers
Example
Number of big squares
between complexes (approximately)
a
b
c
d
e
Rate (beats per minute)
Approximately
4.5
7
1.5
2
6
60-70
40
200
150
50
2) Rhythm
•
•
•
•
•
•
•
? P wave before each QRS- sinus rhythm
If long PR interval, this is first degree heart block (see later).
If progressively increasing PR interval and then a dropped beat, this is a form of
second degree heart block (Wenckebach phenomenon)
? evidence of complete heart block with P waves and QRS complexes completely
dissociated and very slow rate
irregular with no clear P wave- atrial fibrillation
saw tooth pattern – atrial flutter (the atria flutter at 300/min, so each saw tooth P
wave occupies approximately one large square)
beware the patient with a heart rate of exactly 150 (2 large squares between
complexes): this is most likely atrial flutter with 2:1 block where every other saw tooth
wave is masked by a QRS complex.
extrasystoles- narrow QRS complex- atrial ectopics;
broad QRS complex - ventricular ectopics
if tachycardia, ?broad complexes or narrow (broad complex tachycardia can be due to
ventricular tachycardia or atrial tachycardia with aberrant conduction- bundle branch
block pattern)
Now test yourself
What is the rhythm in each of the ECG samples which follow?
a)
b)
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c)
d)
e)
f)
g)
h)
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Answers
Example
a)
Rhythm
Second degree block
(Wenckebach)
b)
Broad complex tachycardia
c)
Atrial fibrillation with fast
ventricular rate
Atrial flutter with 9:1 block
Sinus rhythm
Paced rhythm
d)
e)
f)
g)
h)
Atrial flutter with 2:1 block
Rate exactly 150
Probably sinus rhythm with
ventricular ectopic
Comment
Progressive prolongation of
P wave leading to a missed
beat with a P wave and no
subsequent QRS
Most likely ventricular
tachycardia; could be atrial
tachycardia with bundle
branch block
Irregularly irregular
complexes
Saw tooth appearance
Normal pattern
Sharp upstroke of
pacemaker potential before
each broad complex
Saw tooth appearance of
baseline
Broad complex ectopic
suggests ventricular origin
3) Axis
This is the mean direction of the wave of ventricular depolarization (QRS) in the vertical
plane.
Look at I, II, III and see which has the maximal upward deflection (R wave).
Normally either l or ll has the biggest upward deflection.
Right axis deviation (RAD) is relatively rare- here III has a bigger R than II.
Left axis deviation (LAD) is commoner than RAD.
One-up-two-down
A quick eyeball test is "one-up, two-down" ie the dominant deflection in I is an upward R
wave, bigger than the downward S, and at the same time the dominant deflection in II is
downwards ie dominant S wave bigger than the R wave. Think of an odd house advert- one
up two down.
Now test yourself
Which of the following show left axis deviation?
Only leads l, ll and lll are shown from each of four ECGs
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a)
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c)
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d)
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Answers
The axis in a) and c) is normal, but in b) and d) shows the one-up-two down appearance of LAD
4) Now check each wave
Check P wave
? normal shape
(may be bifid in some patients with mitral valve disease, best seen in V1 and V2)
? flat or absent in hyperkalaemia (associated with tenting of T waves)
? regular in relation to QRS (if not, consider heart block)
Check QRS duration
Left bundle branch block
Wider than 3 small squares with RSR pattern in I, avL and v5-6
This is always abnormal; often due to ischaemic heart disease, hypertension or
cardiomyopathy. Its presence means that it is hard to interpret the ST segment and T wavesthese are often abnormal as part of LBBB and so cannot be used to diagnose ischaemia,
infarction or "strain". Serial ECGs can be very helpful here- if someone has a normal ECG
and then develops LBBB as part of an infarct, this carries a worse prognosis.
RBBB- wider than 3 small squares with RSR pattern in v1, v2; may well be within normal
limits as common finding; 1 in 20 people have RBBB. May be associated with an atrial septal
defect and may also be caused by ischaemia in older patients.
(Partial RBBB/ LBBB means RSR pattern without prolongation of QRS to 3 small squares- not usually
of major significance.)
Check R wave size
Left ventricular hypertrophy by electrical criteria exists when the R wave height in V6 is
added to the S wave depth in V2 and comes to 7 big squares (35mm)
(There are other criteria, but this is a reasonable rule of thumb)
Q wave
Small Q wave normal, but a deep Q may be a sign of full thickness infarct
ST segment
ST elevation of >2mm is a cardinal sign of myocardial infarction, though there are other rarer
causes (pericarditis, ventricular aneurysm etc)
ST depression
This often indicates ischaemia.
May be present with left ventricular hypertrophy; sometimes called LVH with strain; more
precisely LVH with repolarisation abnormality
T wave inversion
Is normal in V1 and sometimes present in lead 111.
T wave inversion may be part of left ventricular hypertrophy (LVH)- when it is called LVH with
strain or (more accurately) LVH with repolarisation abnormality (as above).
T waves often invert after myocardial infarction.
When found alone, without other changes, they are a non-specific finding.
Very tall tented T waves may suggest hyperkalaemia.
Now test yourself
What are the main abnormalities in these ECGs?
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a)
b)
c)
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Answers
a) LVH with strain pattern
LVH: R in V6 plus S in V2= more than 7 large squares
“Strain”: inverted T waves and ST depression in l, ll and V4-V6
b) LBBB (broad complexes with M shape of RSR in l, V5 and V6)
c) RBBB (with RSR- ie M shape- in V1)
Chest Pain and Myocardial Infarction (MI)
There are three main sources of evidence for an infarct :
• History
• Serial ECGs
• Cardiac enzymes including troponins
It is important to remember that the ECG may be normal initially in about a third of those with
MI. In a few people, no clear changes develop. This is sometimes called a non-ST elevation
myocardial infarct (NSTEMI). The usual pattern is for ST elevation to occur early (usually
within 24 hours) and then Q waves if the infarct is full thickness.
Myocardial infarction
ST elevation
• Left main stem
Large anterior
V1-V6
• Left ant. descending
Anteroseptal
V1-V4
• Left circumflex
Anterolateral
1, aVL,
V4-6
• Right coronary
Inferior
2, 3,
aVF
Q wave
The affected leads can give a clue to the likely vascular territory of the coronary thrombosis.
Left main stem
Circumflex
Right coronary artery
Left anterior descending
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Now test yourself
What are the main abnormalities in these ECGs?
All these patients presented with chest pain.
a)
b)
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c)
d)
e)
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Answers
Example
a)
b)
c)
d)
e)
Comments
Q waves and ST elevation in V2-V4: anteroseptal infarct
Q waves and ST elevation in V2-V5: anterior infarct
Q wave in 111 and ST elevation in ll, lll and aVF inferior infarct
Q waves and ST elevation in ll, lll, aVf with ST elevation in V5 and V6:
inferolateral infarct. The circumflex artery is sometimes dominant in
supplying the base of the heart and thrombosis then leads to this pattern,
with both inferior and lateral changes.
V5 strip showing ST depression; similar changes in V4 and V6 suggested
lateral ischaemia
Finally “clever things”
….if you've time on your hands or you're an enthusiast!
Check PR interval- should be more than 3 small squares- if shorter, consider pre-excitation only relevant in a patient with palpitations or funny turns where a tachyarrhythmia might be
occurring. That is the only hint of Wolf Parksinson White in this document.
Check PR again-? prolonged should be less than or equal to 5 small squares- if longer,
?beta blockers, ?first degree heart block , ? digoxin toxicity, ? hypokalaemia
Check QT interval- the "ESR of the heart"- can increase with almost any condition, metabolic
abnormality or drug. The upper limit normal at heart rate 60-70 is 420ms ie just over 2 big
squares; should be less than half the R-R interval; main risk is when gets very long- over
600ms= 3 big squares, there is a risk of ventricular fibrillation (torsade de pointes). If very
high consider hypocalcaemia, drugs such as antidepressants, phenothiazines, quinidine and
cardiac disease eg ischaemia, myocarditis, left ventricular hypertrophy etc
Overview
Rate:
300 divided by number of large squares between QRS complexes
Rhythm:
?SR with P wave before each QRS ? atrial fibrillation with irregularly irregular
complexes ?saw tooth pattern of atrial flutter ?heart block
Axis:
? one-up-two down appearance of LAD
P wave:
flat in hyperkalaemia, long PR is first degree block
QRS:
broad complexes suggests BBB
ST:
? elevation suggesting MI
?depression suggesting repolarisation abnormality if associated with BBB or
LVH. Suggests ischaemia without these.
Q:
deep Q waves suggest full thickness infarct.
T:
inverted T normal in V1: elsewhere abnormal
If the only abnormality is a non-specific finding
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