A Basic approach to interpretation of the 12 lead ECG

1. Clinical Skills
School of undergraduate medical education UKZN
March 2012
Dr RM Abraham

2. PRACTICAL APPROACH TO A 12
LEAD ECG
OBJECTIVES
 INTRODUCTION
 USES
 ELECTRICAL CONDUCTION SYSTEM OF THE
HEART
 RECORDING AN ECG
 THE NORMAL ECG AND INTERPRETATION
 REPORTING AN ECG

3. ECG
 Stands for Electrocardiogram or Electrocardiograph.
 Diagnostic tool that measures and records the
electrical activity of the heart during the cardiac cycle.
 Term ECG introduced by Willem Einthoven in 1893.In
1924 Einthoven received the Nobel prize for his life's
work in developing the ECG.

4. USES
Extremely useful, easy, non- invasive, and relatively
cheap to carry out.
ECG used as an adjunct to Hx and clinical examination.
In the hands of an experienced practitioner, can be used
to detect a wide range of cardiac pathologies.

5. USES
 Essential for Dx and Mx of abnormal cardiac rhythms.
 Assist in the Dx of chest pain.
 Assist in the Mx of Myocardial infarction.
 Assist in the Dx of the cause of breathlessness.
 Pre-operatively-surgery done under GA, done to detect
unsuspected cardiac pathologies that might worsen with
the stress of surgery and anesthesia.
 Routinely done to people in occupations that-
1) stress the heart e.g. professional athletes or firefighters.
2) involve public safety e.g. commercial airplane pilots,
train drivers and bus drivers.

6. ELECTRICAL CONDUCTION SYSTEM
OF THE HEART
To fully understand how an ECG reveals useful information, a basic
understanding of the anatomy and physiology of the heart is essential.
The heart has its own electrical system to keep it running independently of
the rest of the body's nervous system.
All 4 chambers have an extensive network of nerves, electrical impulses
travelling through them trigger the chambers to contract with perfectly
synchronized timing.
Revise:
Electrical discharge initiated in SA node Atrium AV node Bundle of
His right and left bundle branches Purkinje fibres within the
Ventricles

7. Electrical conduction system

8. RECORDING AN ECG
 As the heart undergoes depolarization(contraction) and
repolarization(relaxation),electrical currents are generated and
spread not only within the heart but throughout the body,
because the body acts as a volume conductor.
 This electrical currents/activity generated by the heart can be
measured by an array of "electrodes" placed on the body surface.
 The electrodes are connected by wires to an ECG recorder that
measures potential differences btw selected electrodes and the
electrical picture obtained is called a "Lead".
 The recorded tracings is called an electrocardiogram(ECG).

9. RECORDING AN ECG
 ELECTRODES: Detect the electrical signals of the heart from the
surface of the body.
 4 Limb electrodes- placed on each arm and leg.
 6 Chest electrodes- placed at defined locations on the chest.
 V1right 4th ICS parasternally.
 V2left 4th ICS parasternally.
 V3midway btw V2 and V4
 V4left 5th ICS mid- clavicular line(the imaginary line that extends
down from the midpoint of the clavicle).
 V5left 5th ICS ant- axillary line (the imaginary line that runs down
from the point midway between the middle of the clavicle and the
lateral end of the clavicle)
 V6left 5th ICS mid- axillary line (the imaginary line that extends
down from the middle of the patient's armpit.)

10. RECORDING AN ECG
Limb and chest electrodes

11. RECORDING AN ECG
 LEADS: The views or the
electrical picture of the
heart.
 There are 12 view points of
the heart:
 6 Standard
Leads(I,II,III,AVR,AVL,
AVF)
 6 chest Leads(V1-V6)

12. RECORDING AN ECG
 Standard leads: recorded from
the electrodes attached to the
limbs, look at the heart in a
vertical plane(i.e from the sides
or the feet):
 Leads I,II and AVLlooks at the
lat.surface of the heart.
 Leads III and AVFLooks at the
inferior surface of the heart.
 Leads AVRlooks at the right
atrium.

13. RECORDING AN ECG
 Chest leads looks at the
heart in a horizontal plane
(i.e from the front and the
left side)
 Lead V1&V2look at the
R. ventricle.
 Lead V3&V4look at the
interventricular septum.
 Lead V5&V6look at the
ant.&lat.walls of the L.
ventricle.

14. RECORDING AN ECG
 Steps when recording an ECG:  ECG machines records changes in
1)The pt. must be supine and relaxed(to electrical activity by drawing a trace
prevent muscle tremor, as contraction of on a moving paper strip.
skeletal muscles will be detected by the
electrode).
2)Connect the limb and chest electrodes  All ECG machines run at a standard
correctly. Good electrical contact btw the rate (25mm/sec) and use paper with
electrodes and skin is essential. May be standard-sized squares.
necessary to shave the chest in a male pt.
3)The ECG machine/recorder must be  Each small square represents
calibrated to a std signal of 1 millivolt,
0.04secs,each large square(5mm)
this should move the stylus vertically
1cm or 2 large squares. represents 0.2secs,so there are 5
4)Record the 6 standard leads- 3 or 4
large squares per second and
complexes are sufficient for each lead. therefore 300 large squares per
5)Record the 6 chest(V) leads. minute.

15. THE NORMAL ECG
(Basic shape of the normal ECG)
 The letters P,Q,R,S,T were chosen
arbitrarily in the early days.
 The P,Q,R,S and T deflections are all
called waves.
 The Q,R and S waves together make
up a complex.
 Interval btw the beginning of P
wave and beginning of QRS
complex is called the PR interval.
 Interval btw end of the S wave and
beginning of the T wave is called the
ST 'segment'.

16. COMPONENTS OF THE ECG
COMPLEX
 P Wave
 first upward deflection
 represents atrial
depolarization
 usually 0.10 seconds or less
( less that 3 small squares)
 usually followed by QRS
complex

17. COMPONENTS OF THE ECG
COMPLEX
 QRS Complex
 Composition of 3 Waves
 Q, R & S
 represents ventricular
depolarization
 usually < 0.12 sec(less than
3 small squares)

18. COMPONENTS OF THE ECG
COMPLEX
 Q Wave
 first negative deflection
after P wave
 depolarization of
interventricular septum
from left to right
 not always seen

19. COMPONENTS OF THE ECG
COMPLEX
 R Wave
 first positive deflection
following P or Q waves
 Depolarisation of the
main mass of the
ventricles

20. COMPONENTS OF THE ECG
COMPLEX
 S Wave
 Negative deflection
following R wave
 Depolarisation of the area
of the heart near the base

21. COMPONENTS OF THE ECG
COMPLEX
 PR Interval
 time impulse takes to spread from
the SA node through the atrial
muscle and AV node, down the
Bundle of His and into the
ventricular muscle
 The PR interval is therefore a good
estimate of AV node function
 from beginning of P wave to
beginning of QRS complex
 normally 0.12 - 0.2 sec(less than 1
large square)
 may be shorter with faster rates

22. COMPONENTS OF THE ECG
COMPLEX
 QRS Interval
 time impulse takes to
depolarize ventricles
 (shows how long excitation
takes to spread through the
ventricles)
 Atrial repolarisation hidden
by ventricular depolarisation
 Represents normal
conduction through AV node
and bundle of His
 from beginning of Q wave to
beginning of ST segment
 usually < 0.12 sec(less than 3
small squares)

23. COMPONENTS OF THE ECG
COMPLEX
 ST Segment
 early repolarization of
ventricles
 measured from end of
QRS complex to the onset
of T wave
 Usually <0.32sec(less than
8 small squares)
 Is an isoelectric line
 elevation or depression
may indicate abnormality

24. COMPONENTS OF THE ECG
COMPLEX
 QT interval
-ventricular depolarisation
and ventricular
repolarisation
 Measured from the onset
of the QRS complex to the
end of the T wave

25. COMPONENTS OF THE ECG
COMPLEX
 T Wave
 repolarization of
ventricles
 concurrent with end of
ventricular systole

26. COMPONENTS OF THE ECG
COMPLEX
 U wave
- Inconstant finding due to slow
repolarization of the Purkinje
fibres and papillary muscles

27. HOW TO REPORT/ANALYSE AN ECG
This takes the form of a description followed by an
interpretation of an ECG.
Description should always be given in the same sequence.
 Patient and ECG details(Name, date, time)
 Rhythm
 Rate
 Conduction intervals
 Cardiac axis
 Description of QRS complex
 Description of ST segments and T waves

28. RHYTHM
RHYTHM
 Measure R-R intervals across strip
 Should find regular distance between R waves
 Classification
 Regular
 Irregular
 Regularly irregular
 Irregularly irregular
 Regular rhythm-A P wave should precede every QRS complex with
consistent PR-intervalSinus rhythm
 Irregular rhythm-No P wave preceding each QRS complex with an irregular
rateAtrial fibrillation

29. RATE
RATE- Regular rhythm  RATE- Irregular rhythm
R-R method  Count RR or PP intervals over
 divide 300 by # of large a six-second period (i.e. 30 x
squares between 5mm blocks) and multiply
consecutive R waves this figure by 10.
 E.g. 3 large squares btw
consecutive R waves
 E.g 30 large squares contain
Hrt rate=300/3 10 QRS complexes
=100beats/min (30 large squares correspond to
Hrt rate>100bpm (Sinus 6 secs)
tachycardia)  Rate = 10 x 10 = 100 beats/min
Hrt rate<60bpm (Sinus
bradycardia)

30. CONDUCTION INTERVAL
Conduction interval
 PR Interval
 Constant?
 Less than 0.20 seconds (1
large box)
 Short PR intervalRapid
conduction through AV
node
 Long PR interval1st
degree AV block

31. CARDIAC AXIS
Cardiac axis
 Normal Cardiac axis
(-30 and +90 degrees)
 The direction of the axis can be derived most easily
from the QRS complex in leads I,II,and III
 With a normal cardiac axis, the wave of depolarization
is spreading towards leads I,II,and III
predominantly upward deflection in all these leads
 RVH(Right ventricular hypertrophy)Axis swings
towards the right
(btw +90 and -150 degrees) deflection in lead I is
downwards Right axis deviation
RVH secondary to COPD (Pulm. condition putting a strain
on the right side of the heart)
 LVH(Left ventricular hypertrophy)Axis swings to
the left (btw -30 and -150 degrees) deflection in lead
III is predominantly downwardsLeft axis deviation
LVH secondary to systemic hypertension

32. DESCRIPTION OF QRS COMPLEX
AND ST SEGMENT
QRS complex ST segment and T wave
 Duration of QRS  Depressed ST segment
complex<0.12sec(less Ischaemic heart disease
than 3 small squares)
 Elevated ST segment and T
 Broad/wide QRS wave inversion Acute
complexbundle branch Myocardial infarction
block

33. ECG TRACINGS
Normal sinus rhythm
 Rate- 60-100bpm
 Rhythm-regular
 P waves-normal
 PR interval-0.12-0.20sec
 QRS duration-0.04-0.12sec
 Any deviation from above is
sinus tachycardia, sinus
bradycardia or arrhythmias
1st Degree AVB
 Prolonged PR
interval(>0.20sec)

34. ECG TRACINGS
Atrial tachycardia
 Rate->150-250bpm
 Rhythm-regular
 P wave-upright/normal
 PR interval-0.12-0.20sec
 QRS complex-0.04-0.12sec
Atrial flutter
 Rate-250-300bpm
 Rhythm-Atrial: regular; Vent: varies
 P waves- Big F waves-Saw tooth pattern
 PR interval-normally constant, may vary
 QRS complex-0.04-0.12sec
Atrial Fibrillation
 Rate-Atrial:350-750bpm,Vent:varies
 Rhythm-irregularly irregular ventricular
 P waves-little F waves, no pattern
 PR interval-no discernable P wave
 QRS complex-0.04-0.12sec

35. ECG TRACINGS
Ventricular tachycardia
 Rate-100-250bpm
 Rhythm-usually regular
 P waves-If present, not
associated
 PR interval-none
 QRS complex- >0.12sec
Ventricular fibrillation
 Rate-none
 Rhythm- Chaotic, no set rhythm
 P waves-absent
 PR interval-absent
 QRS complex-not discernable

36. ECG TRACINGS
Depressed ST segment
 Myocardial ischaemia
Elevated ST segment
 Myocardial infarction
Asystole
 Rate-no electrical activity
 Rhythm-no electrical rhythm
 P waves- absent
 PR interval- absent
 QRS complex- absent

37. ECG Analysis
 A monitoring lead can tell you:  A monitoring lead can not tell
you:
 How often the myocardium is  Presence or absence of a
depolarizing myocardial infarction
 How regular the  Axis deviation
depolarization is  Chamber enlargement
 How long conduction takes in  Right vs. Left bundle branch
various areas of the heart blocks
 The origin of the impulses that  Quality of pumping action
are depolarizing the  Whether the heart is
myocardium beating!!! It is possible to be
in cardiac arrest with a normal
ECG signal (a condition known
as pulseless electrical activity
also known by the older term
Electromechanical
Dissociation ).

38. ECG Analysis
 An ECG is a diagnostic tool, NOT a treatment!
 No one was ever cured by an ECG!!
 Treat the patient not the monitor!!!

39. REFERENCES
 The ECG Made Easy by John R.Hampton
 Medical Science Naish, Revest, Syndercombe Court
(2009) Elsevier
 ECG protocol
 Review of Medical Physiology by William F. Ganong
 Dr Matthews for all protocols and copy of Naish
chapter on ECG
To the whole team (Drs Gan, Ntando,& Motala) at
clinical skills unit, ukzn for input and advice