Basic electrocardiogram
(ECG)
01/22/16
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Basic electrophysiology of conduction of electrical
impulse - sequence of events
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Sino-atrial node
depolarisation
Atrial depolarisation
Atrio-ventricular node
depolarisation
Bundle of His
Right and left bundles
Ventricular depolarisation
Ventricular repolarisation
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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 atrioventricular 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)
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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.
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Taking a 12 lead ECG
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12 Lead ECG
 12
views of the heart
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6 chest leads
6
limb leads
 Only
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10 wires
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The limb leads
Positioning the limb
leads
Right
Left
AVR
AVL
Yellow
Red
RL
Black
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AVF
Green
 Position
of the
electrodes for limb
leads are just above:
 Right wrist
≡ AVR
 Left wrist
≡ AVL
 Left ankle
≡ AVF
 Right ankle (earth)
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
Sternomanubrial joint - Angle of Louis
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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
ICS = Intercostal space
SE = Sternal edge
CL = Clavicular Line
AL = Axillary Line
V1
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V2
V3
V4
V5 V6
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The patient
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.
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Skin Preparation
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.
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Recording an ECG
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
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I
aVR
aVL
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V6
V5
II
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III
V1
V2
V3
aVF
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V4
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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
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Bipolar leads view when myocardial
conduction is normal
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I
III
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II
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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
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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
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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
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
LEAD AVF
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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
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ECG paper runs at a
standard speed of 25
mm/second
Standard calibrated paper
is used:
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•
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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)
R
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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)
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12 Lead ECG
RR
Interval
QRS
Complex
P
Wave
P
R
ST
Segment
QR
S
QT
T
Wave
Basic rhythm assessment
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How to read a rhythm strip
1.
How is the patient?
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1.
Is there any electrical activity?
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1.
4.
If you can see deflections above and/or below the
isoelectric line then there is electrical activity.
What is the ventricular (QRS) rate?
Is the QRS rhythm regular or irregular?
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Always treat the patient not the monitor.
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?
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Normal 60 -100 per minute
<60 = bradycardia; >100 = tachycardia
1. Count the number of QRS complexes over a 6 second
interval. Multiply by 10 to determine heart rate. This method
works well for both regular and irregular rhythms.
2. Count number of small squares between consecutive R
waves and divide into 1500
• In the second image, the number of small boxes for the
R-R interval is 21.5. The heart rate is 1500/21.5, which
is 69.8.
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ECG paper timings
Paper speed =
25 mm/second
RR interval
PP interval
Rate
Each small square
= 0.04 seconds
(= 1/25 sec)
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Five small squares =
0.2 seconds
(= 1/5 sec)
= 300/RR interval (in large squares) or
= 1500/RR interval (in small squares)
Five large squares
= 1 sec
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How to read a rhythm strip
Is the QRS width normal or prolonged?
5.
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The QRS complex should measure < 3 small squares
<0.12 seconds.
Is atrial activity present? (If so, is there a normal
P wave or some other atrial activity)
6.
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A normal P wave is rounded.
How is atrial activity related to ventricular activity?
7.
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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
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sinus (normal PR interval)
heart block
 first
 second
 third
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Heart block
 1st
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degree
retains 1:1 relationship (P:QRS)
slowed AVN conduction
prolonged PR interval (>0.2s)
 2nd
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loss of 1:1 relationship
dropped QRS complexes
 3rd
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degree
degree
complete dissociation of P and QRS waves
idio-ventricular rate (~40-50/min)
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2nd degree heart block
 Mobitz
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PR interval progressively lengthens until a QRS
complex is dropped
 Mobitz
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type 1
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
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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
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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
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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
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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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2nd degree Heart Block Mobitz I
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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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2nd degree Heart Block Mobitz II
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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
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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
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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
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Atrial Rate = None
Ventricular Rate = None
Rhythm = No rhythm
QRS complex = None
P-R interval = None
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