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Number 234
Electrocardiograms (ECGs)
Exam questions frequently ask candidates to explain electrical activity (and blood pressure changes) during the cardiac cycle.
This Factsheet:
• Summarises the electrical activity of the cardiac cycle
• Describes how electrocardiogram (ECG) traces can be used to identify heart defects
• Illustrates common exam questions on this topic
What happens during a heart beat?
Fig. 1 shows the pathway of electrical activity through the heart .
aorta
LA = Left Atrium
LV = Left Ventricle
RA = Right Atrium
RV = Right Ventricle
sino-atrial
node (SAN/
pacemaker) in
wall of right
atrium
2
2
LA
1
bundle of His
3
2
4
5
LV
RA
artri-ventricular node
(AVN) in the
atrioventricular septum
1. wave of excitation that starts each
heart beat is sent from the SAN
2. spreads over atrial walls causing
contraction
3. delayed at non-conducting
atrioventricular septum
4. wave of excitation is sent from AVN
5. passes along bundles of His/Purkine
fibres to the base of the ventricles
which then contract
6. before spreads over the walls of the
ventricles causing them to contract
Purkine fibres
6
RV
6
5
5
intraventricular septum
Typical Exam Question
Read through all of the following passage and then fill in the spaces with the most appropriate word or words.
The cardiac cycle is initiated and controlled by the heart itself. Cardiac muscle is said to be ......................... since it will contract and
relax of its own accord. The beat is initiated by the .........................which is situated in the wall of the ............... Waves of depolarisation
travel through the atria causing atrial .............. The waves of depolarisation can only travel to the ventricles via the ................... situated
at the top of the ventricular septum. From here the waves travel to the apex of the heart through the ..................... which is made of
specialised conducting cells called ........................... These then carry the waves of depolarisation through the ventricle walls causing
both ventricles to contract simultaneously. At this stage the ..................... are open and the ........................... are shut so that blood can
be forced into the arches.
myogenic; sinoatrial node/pacemaker; right atrium; systole/contraction; atrioventricular node; bundle of His/left + rightbundle branches;
Purkinje fibres; semilunar valves; atrioventricular valves;
Markscheme
Extract from Chief Examiner’s Markscheme
A surprising number of candidates were confused about the roles of the SAN, AVN and bundle of His. Many thought that the only role
of the SAN was to relay impulses from the CNS. Few candidates realized that the AVN produces the electrical activity that causes the
ventricles to contract and a quarter of the candidates thought that the bundle of His was responsible for the contraction of the
ventricles from their bases.
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Bio Factsheet
234. Electrocardiograms (ECGs)
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Electrocardiograms (ECGs)
Extract from Chief Examiner’s Markscheme
Few candidates realised that ECGs shows electrical activity
rather than blood flow. Only about half the candidates knew
that the P wave corresponded to electrical activity prior to
atrial systole and that the QRS corresponded to electrical
activity prior to ventricular systole.
ECGs are a record of the electrical currents that cause the heart
muscle to contract. Electrodes placed on the skin detect the electrical
activity. Electrode jelly ensures good electrical contact and the
patient is required to keep still because movement causes interference
to the trace due to the electrical impulses from the muscles (Fig 2).
Fig. 2 Typical ECG for a normal cardiac cycle
Key
P: atrial systole (contraction)
the wave of depolarisation in the walls of the atria i.e. atrial
systole. The atria contract and blood flows into the ventricles.
The heart rate can be calculated from the interval between
successive P waves
R
electrical
potential /mV
QRS: ventricular systole (contraction)
the wave of depolarisation in the walls of the ventricles i.e.
ventricular systole. The ventricles contract and the valve
between atrium and ventricle closes
T
P
Q
T : ventricular diastole
The ventricles relax
• The PR interval = the time taken for an electrical impulse to
travel from the atria to the ventricles
S
0
0.2
0.4
time (s)
0.6
0.8
•
The QT interval = the contraction time because the
ventricles are contracting
•
The interval between T of one cardiac cycle and Q of the
following cycle = the filling time because blood is firstly
filling the atria, then the ventricles
Fig. 3 shows an abnormal ECG trace from a patient who has suffered
a heart block. The ECG trace shows a much longer than normal time
interval between the P and R waves. This could be a result of damage
to the Purkine tissue or improper functioning of the AV node.
Atrial fibrillation causes blood flow to slow down and it may even
stop. This can cause ‘pooling’ of the blood in certain places in the
circulatory system.
Fig.3 ECG of patient with heart block
Fig.5 shows an ECG trace from a patient suffering tachycardia in
which the heart beat much faster than normal - exceeding 100bpm
even when the patient is at rest. Greater voltages are also generated.
Fig. 5 ECG showing tachycardia
1 mv
1 sec
Fig.4 shows an abnormal ECG trace from a patient suffering who has
suffered ventricular fibrillation caused either by an acute heart attack
or electrical shock.
Fig. 4 ECG showing ventricular fibrillation
Typical Exam Question
Suggest what effects tachycardia could have on cardiac output.
Cardiac output could increase if ventricles have time to fill
sufficiently;
But it could decrease if there was insufficient time to fill the
ventricles between contractions;
Net effect depends on whether the decrease in stroke volume
is compensated by the increase in heart rate;
Markscheme
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234. Electrocardiograms (ECGs)
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Fig.6 shows an ECG trace from a patient suffering from hypothermia
– the heart beat is much slower than normal but retains the shape of
that of a normal individual
However, because even patients with heart disease can display a
normal ECG trace modifications of the ECG procedure are routinely
used:
Fig.6 ECG trace showing hypothermia
1. 24 hour ‘ambulatory’ monitoring of the ECG.
The patient wears the heart monitor and takes readings about every
20 minutes. The data are stored and the patient keeps a diary of how
they are feeling, enabling feelings of breathlessness etc to be linked
to changes in their ECG trace.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
2. Exercise tolerance test or stress test
The ECG electrodes are attached to the patient who then walks on
a slowly – moving treadmill set at a slight incline. Both the incline
and the speed of the treadmill incline are gradually increased so
that the patient’s heart rate increases.
1.8
Typical Exam Question
(a) The electrocardiogram (ECG) trace below was recorded over a
period of 5 seconds from a healthy person.
Exam Hint:- Do not confuse electrical activity with blood flow.
ECGs show electrical activity, NOT blood flow
Fig.7 shows an ECG trace from a patient suffering from a damaged
AVN. Because it is the AVN that produces electrical activity, there is
a much lower spike and the low AVN spike results in less electrical
activity in the rest of the heart.
Fig. 7 ECG comparing healthy heart to one with damaged
AVN
Count the number of complete cardiac cycles, and then
calculate the heart rate in beats per minute (2).
(b) The following ECG trace was taken from a different person.
healthy
damaged
AVN
Identify one irregularity shown in the trace (1).
Typical Exam Question
The diagram shows ECGs from a healthy heart and from a diseased
heart.
(c) A woman was found to have collapsed in the street. A
paramedic obtained the following trace using a portable ECG
monitor.
healthy heart
Explain why the person collapsed (4).
diseased
heart
(c) Heart attack / myocardial infarction/ cardiac arrest;
Branch of coronary artery blocked with blood clot/ atheroma;
Reduced oxygen supply (to heart muscle;
Ventricular fibrillation / ventricles quiver;
Random / haphazard depolarisation of cells in ventricle;
Uncoordinated waves /No coordinated heart beat;
Atria and ventricles not contracting simultaneously;
Rapid drop in blood pressure;
electrical
activity in
muscles of atria
electrical activity in
muscles of ventricles
Looking at the ECGs, a consulting cardiologist was reasonably
certain that the damage caused to the diseased heart had not
affected the sinoatrial node. Explain why (2)
(b) frequency/distance between peaks varies/ arrhythmia;
Markschemes
SAN is in the (right) atrium;
ECG from the healthy person is identical to the trace for the
diseased heart in the region of the atria;
(a) 5 complete cardiac cycles;
5 × 12 = 60 (beats per minute;
Markscheme
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Bio Factsheet
234. Electrocardiograms (ECGs)
www.curriculum-press.co.uk
Linking electrical activity to blood flow
Exam Hint:- Don’t go into the exam hall if you can’t explain
Fig.8!
Fig. 8 shows the pressure, volume and electrical changes that occur
during the cardiac cycle.
Fig.8 An electrocardiogram (ECG) and associated pressure changes
semilunar
valve closes
120
semilunar
valve opens
100
pressure
(mm Hg)
4
aortic
pressure
3
80
60
40
atrioventricular
valve open
atrioventricular
valve closed
20
atrial
pressure
5
1
6
2
0
ventricular
pressure
R
ECG trace
T
P
Q
0
0.1
atrial
systole
S
0.2
0.3
0.4
time (s)
ventricular systole
0.5
0.6
0.7
0.8
Aorta
Atrial and ventricular diasole
P: atrial systole (contraction)
The wave of depolarisation in the walls of the atria i.e. atrial
systole. The atria contract and blood flows into the ventricles.
semilunar
(aortic) valve
QRS: ventricle systole (contraction)
the wave of depolarisation in the walls of the ventricles i.e.
ventricular systole. The ventricles contract and the valve
between atrium and ventricle closes
bicuspid
(atrioventricular)
valve
Left atrium
Left ventricle
T : ventricular diastole. The ventricles relax
1. Atria contracting so blood flowing into ventricle
2. Ventricles start to contract so ventricular pressure > atrial pressure and the atrioventricular valve closes
3. Ventricular pressure > aortic pressure forcing open aortic valve so blood flows from the ventricle into the aorta
4. Ventricular pressure falls below aortic pressure, so aortic valve closes
5. Ventricular pressure falls below atrial pressure so that blood flows from the atria to the ventricles and ventricular volume rises
rapidly
6 Atrium filling with blood from pulmonary vein until atrial pressure> ventricular pressure and blood flows from atrium to ventricle
Acknowledgements:
This Factsheet was researched and written by Kevin Byrne and Martin Griffin.
Curriculum Press, Bank House, 105 King Street, Wellington, Shropshire, TF1 1NU.
Bio Factsheets may be copied free of charge by teaching staff or students, provided that their school is a registered subscriber. No part of these Factsheets may be reproduced, stored in a retrieval system, or transmitted, in any other form or
by any other means, without the prior permission of the publisher. ISSN 1351-5136
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