Intro to ECG

1. 01/22/16 © Clinical Skills Resource Centre, University of Liverpool, UK1
Basic electrocardiogram
(ECG)

2. 01/22/16 © Clinical Skills Resource Centre, University of Liverpool, UK2
Basic electrophysiology of conduction of electrical
impulse - sequence of events
Sino-atrial node
depolarisation
Atrial depolarisation
Atrio-ventricular node
depolarisation
Bundle of His
Right and left bundles
Ventricular depolarisation
Ventricular repolarisation

3. 01/22/16 © Clinical Skills Resource Centre, University of Liverpool, UK3
The Heart
R
As a result of
Atrial & Ventricular
depolarisation a
visual
representation is
produced on the
12 lead ECG or on
a cardiac monitor.
This is one
complete cardiac
cycle.

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Relationship of electrical events to ECG
SA node
Atrial depolarisation (P
wave)
AV node (main component
of PR interval)
Bundles of His and
ventricular depolarisation
(QRS)
Ventricular repolarisation
(T wave)

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The Iso-electrical Line
This represents the resting potential of the heart.
The electrical events of the cardiac cycle will be
represented by deflections away from this line.

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Sino-atrial node depolarisation
The events of the
cardiac cycle are
initiated by
depolarisation of the
sino-atrial node

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Atrial Depolarisation (P Wave)
The wave of electrical
depolarisation is
conducted through the
cardiac muscle of both
atria

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Atrial Contraction (P Wave)
The depolarising
wave causes
contraction of the
atria, pushing blood
into the ventricles

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AVN depolarisation (PR Interval)
The wave of depolarisation
reaches the atrio-
ventricular node which
depolarises and conducts,
but slows the wave

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Ventricular depolarisation (QRS
Complex)
The AVN conducts the
depolarisation to the
Bundle of His
The wave of
depolarisation quickly
moves through the
specialised conducting
tissue

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Ventricular contraction
(QRS Complex)
The co-ordinated,
synchronised depolarisation is
depicted below
This produces an effective
contraction of both ventricles

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Ventricular Repolarisation
(T Wave)
After depolarisation
and contraction the
ventricles repolarise,
returning to the resting
potential.

13. 01/22/16 © Clinical Skills Resource Centre, University of Liverpool, UK13
Taking a 12 lead ECG

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12 Lead ECG
12 views of the heart
 6 chest leads
6 limb leads
Only 10 wires

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The limb leads
Positioning the limb
leads


AVR AVL
RL AVF
Black
Green
Red
Yellow
Position of the
electrodes for limb
leads are just above:
Right wristº AVR
Left wrist º AVL
Left ankleº AVF
Right ankle (earth)
Right Left
Please note if placed elsewhere this must
be clearly documented on the ECG to
avoid potential misinterpretation of the
recording

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The chest leads
V1 - 4th ICS RSE
V2 - 4th ICS LSE
V3 - Midway between V2 & V4
V4 - 5th ICS MCL
V5 – Horizontal with V4 AAL
V6 – Horizontal with V4 MAL
V1 V2 V3 V4 V5
V6
Sternomanubrial joint - Angle of Louis
ICS = Intercostal space
SE = Sternal edge
CL = Clavicular Line
AL = Axillary Line

17. The patient
01/22/16 © Clinical Skills Resource Centre, University of Liverpool, UK17
Wash your hands, introduction yourself and check patient
identity
Explain the procedure, warn the patient they will need to expose
their chest (including removing any underwear) as well as their
ankles and wrist.
Gain consent, consider a chaperone.
Patient should lie supine (if unable this should be recorded on the
ECG as it may alter the appearance of the trace.

18. Skin Preparation
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You may need to remove chest hair to ensue adequate contact with
the skin, remember to seek consent and follow Trust policy.
If the electrodes will not fix to the skin, then light exfoliation with a
paper towel, gauze swab or tape designed for the purpose.
Sometimes cleaning the skin helps remove any oils or creams applied
to the skin, please follow Trust policy (ranges from soap and water to
alcohol swabs). Avoid cleaning broken or dry skin.
Once the electrodes are in place, cover the patient with a gown to
maintain dignity.

19. Recording an ECG
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The patient should be as relaxed as possible, with arms at the side of them
and supported by the bed
Many machines require you to enter the patients details electronically
before recording, alternatively they must be written on the trace
immediately after.
The patient should be encouraged to stay as still as possible
Press to record (usually start or auto)
Lots of interference record? Perform a second recording with the filter
button selected, if filter selected this must be recorded on the ECG
recording.

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Relationship of limb and chest leads
The chest leads look at the
heart across the horizontal
plane
The limb leads look at the
heart in a vertical plane
Leads aVR, aVL and aVF
look from three separate
directions
The bipolar leads of I, II
and III are summation of
potential differences
between limb leads
I
II III
V1
V2
V3
V4
V6
V5
aVR aVL
aVF

21. Bipolar leads view when myocardial
conduction is normal
Lead I is the sum of the
potentials from the left arm and
right arm electrodes and looks
at the left lateral surface of the
heart
Lead II is the sum of the
potentials between the AVR
and AVF and also looks at the
left lateral surface (and inferior
surface)
Lead III is the sum of AVL and
AVF look at the inferior surface
01/22/16 © Clinical Skills Resource Centre, University of Liverpool, UK21
I
II
III

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12 lead ECG
please see full size ECG at the back of the study guide

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Positive / Negative Deflections
Positive deflections above
the Iso Electrical line
mean the electricity is
flowing towards that lead
Negative deflections below
the Iso Electrical line mean
the electricity is flowing
away from that lead

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ECG Changes in Relation to Lead
LEAD AVR
AVR
Lead AVR is the view from the
right superior aspect of the heart.
The electrical impulse’s is
moving away from the electrode
and therefore the deflections are
away from the isoelectric line
(and look upside down). This is
normal.

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ECG Changes in Relation to Lead
LEAD AVF
AVF
Lead AVF is the veiw from the
inferior aspect of the heart. The
electrical impulse is moving directly
to the electrode and therefore the
deflections are above the isoelectrical
line

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ECG Changes in Relation to Leads
 Look at the chest leads V1 – V6 .
The electrical impulse in V1 is moving away
from the electrode, the resulting complex is
below the isoelectric line.
V2 is less negative as the impulse is moving
more towards the electrode than V1.
This continues across the chest leads.
Therefore V6 is the most positive.

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ECG paper
ECG paper runs at a
standard speed of 25
mm/second
Standard calibrated paper
is used:
·each large square is
equivalent to 0.2
seconds
·each small square is
equivalent to 0.04
seconds
·the vertical scale is
standardised at 1
millivolt per cm

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12 Lead ECG (normal)
PR Interval ( 3-5 small
squares 0.12 – 0.20 secs)
QRS Complex (2-3 small
squares 0.08 – 0.12 secs)
ST Segment < 3 small
squares deflection from Iso
electrical line in health
Occasionally a small
negative deflection is seen
after the T wave in health
and (no significance)
R

29. 12 Lead ECG
QRS
Complex
RR
Interval
ST
Segment
T
Wave
QR
S
QT
P
R
P
Wave

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Basic rhythm assessment

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How to read a rhythm strip
1.How is the patient?
Always treat the patient not the monitor.
1.Is there any electrical activity?
If you can see deflections above and/or below the
isoelectric line then there is electrical activity.
1.What is the ventricular (QRS) rate?
4.Is the QRS rhythm regular or irregular?
Measure 2 consecutive R waves and then
transpose that measure onto the next 2 R waves
and see if they are the same. If they are the same
the then rhythm is regular.

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What is the ventricular rate?
Normal 60 -100 per minute
<60 = bradycardia
>100 = tachycardia
To calculate rate
Count number of QRS complexes in a given number of seconds
(e.g. 5 sec. = 25 large squares), then calculate rate per min
(multiply by 12 for a 5 second period), or
Count number of large squares between consecutive R waves and
divide into 300, or
Count number of small squares between consecutive R waves and
divide into 1500
NB last two methods apply if rhythm is regular

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RR interval
Paper speed =
25 mm/second
Each small square
= 0.04 seconds
(= 1/25 sec)
Five small squares =
0.2 seconds
(= 1/5 sec)
Five large squares
= 1 sec
Rate = 300/RR interval (in large squares) or
= 1500/RR interval (in small squares)
ECG paper timings
PP interval

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How to read a rhythm strip
5.Is the QRS width normal or prolonged?
The QRS complex should measure < 3 small squares
<0.12 seconds.
6.Is atrial activity present? (If so, is there a normal
P wave or some other atrial activity)
A normal P wave is rounded.
7.How is atrial activity related to ventricular activity?
Is there a P wave for every QRS complex.
Is the P-R interval within normal limits.

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Basic electrocardiogram
(ECG) interpretation
ECG - common rhythm
abnormalities

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Bradycardia
Rate <60
May be
sinus (normal PR interval)
heart block
first
second
third

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Heart block
1st degree
retains 1:1 relationship (P:QRS)
slowed AVN conduction
prolonged PR interval (>0.2s)
2nd degree
loss of 1:1 relationship
dropped QRS complexes
3rd degree
complete dissociation of P and QRS waves
idio-ventricular rate (~40-50/min)

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2nd degree heart block
Mobitz type 1
PR interval progressively lengthens until a QRS
complex is dropped
Mobitz type 2
PR interval constant, but a QRS complex is
periodically dropped
dropped QRSs may occur in runs

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Tachycardia
Rate >100
May be
Narrow complex (impulses are initiated above
the ventricles and follow normal conductive
pathway)
Broad complex (impulses are initiated at the
ventricles or are aberrantly (abnormally)
conducted through the ventricles)

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Narrow complex tachycardia
Atrial Rate = 140
Ventricular Rate = 140
Rhythm = Regular
QRS complex = 0.08 (2 small squares)
P-R interval =0.16 (4 small squares)

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Atrial fibrillation
Atrial Rate = unable to determine
Ventricular Rate = approx 100
Rhythm = Irregular
QRS complex = 0.08 (2 small squares)
P-R interval = unable to see P waves therefore no P-R
interval

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Broad complex tachycardia
Atrial Rate = No P waves
Ventricular Rate = 220
Rhythm = Regular
QRS complex = Wide (0.20 second, 4 small squares)
P-R interval = No P waves therefore no P-R interval

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1st degree heart block
Atrial Rate = 60
Ventricular Rate = 60
Rhythm = Regular
QRS complex = Normal 0.06 (1.5 small squares)
P-R interval = Prolonged 0.28 seconds (7 small squares)

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2
nd
degree Heart Block Mobitz I
Atrial Rate = 80
Ventricular Rate = 60
Rhythm = irregular
QRS complex = normal (0.08 second, 2 small squares)
P-R interval = Progressively getting longer until QRS complex is
dropped.

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2
nd
degree Heart Block Mobitz II
Atrial Rate = 80
Ventricular Rate = 60
Rhythm = irregular
QRS complex = normal (0.08 seconds, 2 small squares)
P-R interval = constant P-R interval with intermittently
dropped QRS complexes.

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3rd degree heart block
Atrial Rate = 80
Ventricular Rate = 40
Rhythm = P waves are regular, QRS complexes are regular
QRS complex = Wide (0.20 second, 4 small squares)
P-R interval = No measurable P-R interval, The atria and ventricles are
producing impulses independently.

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Ventricular fibrillation
Atrial Rate = unable to determine
Ventricular Rate = Unable to determine
Rhythm = irregular (erratic)
QRS complex = Wide bizarre
P-R interval = none

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Ventricular asystole
Atrial Rate = None
Ventricular Rate = None
Rhythm = No rhythm
QRS complex = None
P-R interval = None