2. ORIENTATION OF
THE 12-LEAD ECG
Types of Leads
1.Bipolar limb
leads
2.Augmented limb
leads
3.“Unipolar” (+)
chest leads
3. 1. Bipolar limb leads (frontal
plane)
(RA = right arm; LA = left arm, LL = left
leg)
- Lead I: RA (- pole) to LA (+ pole) (Right -to- Left
direction)
-Lead II: RA (-) to LL (+) (mostly Superior -to- Inferior
direction)
-Lead III: LA (-) to LL (+) (mostly Superior -to-
Inferior
direction)
4. 1. Bipolar limb leads (frontal
plane)
(RA = right arm; LA = left arm, LL = left
leg)
5. 2. Augmented limb leads (frontal plane)
(RA = right arm; LA = left arm, LL = left leg)
Homeobook.com
-Lead aVR: RA (+) to [LA & LL] (-) (mostly
Rightward direction)
-Lead aVL: LA (+) to [RA & LL] (-) (mostly
Leftward direction)
-Lead aVF: LL (+) to [RA & LA] (-) (Inferior
direction)
6. 2. Augmented limb leads (frontal plane)
(RA = right arm; LAHo
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arm, LL = left
10. CHEST LEAD PLACEMENT
Precordial lead placement V1: 4th
intercostal space (IS)
adjacent to right sternal border
V2: 4th IS adjacent to left sternal
border
V3: Halfway between V2 and V4
V4: 5th IS, midclavicular line V5:
horizontal to V4; anterior axillary
line
V6: horizontal to V4-5; midaxillary
line
(Note: in women, the precordial
leads should be placed on the breast
surface not under the breast to
insure proper lead placement)
15. P -
WAVE
P wave: sequential depolarization of
the right and left atria
16. P -
WAVE•The impulse is originating at the SA node
•It spreads over the atria in an usual
direction.
•There is no defect of conduction.
•The strength of conduction, mass of atrial
musculature, nutritions are normal.
•Duration 0.12 sec(3 small squares)
•Amplitude 0.25 mv(2.5 small square)
•Upright in all leads except VR
•Inverted – dextro cardia
17. P – WAVE
MORPHOLOGY
The P wave in general should not be more than 1 box wide or 1
box tall. If it exceeds these, it generally means that either or
both atria is enlarged (hypertrophied). The best lead to look at
the P wave is V1. In lead V1, the following characteristics
indicate pathology:
•Positive deflection greater than 1 box wide or 1 box in height
--> right atrial hypertrophy
•If P wave is inverted SA node fails to initiate the impulse.
•If P wave is absent in atrial fibrillation – nodal rhythm, sino
atrial block and hyperkalaemia
•If P wave is hidden – in rapid tachycardia
19. Negative deflection which preceeds R wave
•Caused by the activity of septum.
•Small negative wave & often inconspicious detection.
•Absent in infants suffering from congenital patency
of the septum.
•Prominent Q wave indicates old infarction.
•Q wave in lead III and aVR should be ignore
•Abnormal Q wave- MI, ischemia, bundle branch
block, left ventricular hypertrophy
•A Q wave is significant if it is greater than 1 box
wide or greater than 1/3 the amplitude of the QRS
complex.
Q –
WAVE
20. Negative deflection which preceeds R wave
•Caused by the activity of septum.
•Small negative wave & often inconspicious detection.
•Absent in infants suffering from congenital patency
of the septum.
•Prominent Q wave indicates old infarction.
•Q wave in lead III and aVR should be ignore
•Abnormal Q wave- MI, ischemia, bundle branch
block, left ventricular hypertrophy
•A Q wave is significant if it is greater than 1 box
wide or greater than 1/3 the amplitude of the QRS
complex.
Q – WAVE MORPHOLOGY
21. R –
WAVE
R - waves indicate the changing direction of the
electrical stimulus as it passes through the
heart's conduction system.
22. R –
WAVER is most constant & conspicuous wave having the
tallest amplitude.
•First positive deflection during ventricular
depolarisation.
•Follows immediately upon Q wave.
•In abnormal conditions of ventricles, the shape, size,
duration of R & S altered.
•R wave in aVL must not be > 13mm
•R wave in aVF must not be > 20mm
•Minimum voltage: Atleast one R wave should be >
8mm
•Maximum voltage : The tallest R should not be > 27mm
23. S –
WAVE
S wave represents depolarization in the Purkinje
fibres.
24. S is next downward deflection to R.
•In lead I, R is mainly caused by right ventricle and S due
to left ventricle.
•In lead III, it is just reverse.
•For instance, in bundle branch heart block their
duration is prolonged beyond 0.1sec. & their relative
amplitude varies.
•Deepest S wave shouldnot be > 30mm
•Sum of tallest R wave and deepest S wave should not be
> 40mm
•Abnormal R and S wave suggest ventricular
hypertrophy, posterior MI, WPW syndrome,
dextrocardia+
S –
WAVE
25. •Steady progression from V1 to V6. (R wave).
•At V1 it is predominantly downwards.(S wave)
•From V2 to V6 it progresses to upwards.
•Duration: should be 0.08 - 0.10 sec (2 - 2.5 boxes).
LOW VOLTAGE:
•QRS complex < 5 small square in most of the leads:
pericardial effusion, myxoedema, emphysema, obesity,
HIGH VOLTAGE:
•Ventricular hypertrophy
•Abnormal QRS suggest: incomplete budle branch block
and WPW syndrome
QRS - MORPHOLOGY
27. T –
WAVE
Represents: ventricular repolarization.
•Always upright in leads I and II
•Always inverted in aVR
•Lead V1 normally upright (may be inverted in
20%)
•Lead V2 normally upright( may be inverted in
5
%)
•If upright in V1 and inverted in V2 : abnormal
•Must be upright in leads V3 and V6
28. T – WAVE MORPHOLOGY
T wave Morphology
*Abnormal T wave in shape, size, direction, duration,
reaction to exercise in leads I & II are of great prognostic
significance – myocardial damage with cardiac hypoxia.
*Ischemia: when T waves are in an opposite direction
(inverted), it may indicate that ischemia is present,
especially when it occurs in a pattern as previously
described for ST segment changes.
*Hyperkalemia: associated with tall peaked T waves, flat P
waves, and wide QRS complexes
*Hypokalemia: associated with flat T waves, U waves, U
waves taller than T waves
29. U –
WAVE
The U wave occurs when the ECG machine picks up
repolarisation of the Purkinje fibres.
electrolyte imbalances (potassium) but, again, this is
not very common.
30. This upright wave, when present, follows
the T wave. What it represents is not
certain.
U Wave Morphology
•The presence of U waves may indicate
hypokalemia.
U –
WAVE
31. This is the junction between the QRS complex
and the ST segment.
J – Point
32. •Sinus rhythm PP- RR are equal.
•They are used to calculate the heart
rate.
•PP is atrial rate& RR is the
ventricular rate.
CALCULATION
•Y = 1500/X beats/min.
•X = PP or RR interval.
P-P & R-R
INTERVAL
33. Measures from the beginning of P wave to the
beginning of QRS complex. It represents the total
amount of time required for depolarization of atria
( p wave) as well as the time required for the impulse
to travel slowly through the AV junction, through the
bundle branches, & just upto the point of QRS.
•Normally 0.13 to 0.16 sec.should not exceed 0.2 sec.
P-R
INTERVAL
34. NORMAL
INTERVAL
P-R interval = 0.12 - 0.20 sec (3 - 5 small squares)
QRS width = 0.08 - 0.12 sec (2 - 3 small squares)
Q-T interval 0.35 - 0.43 sec
* The PR interval should really be referred to as the PQ
interval; however it is commonly refeH
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the PR
97. Implantable Cardioverter
Defibrillator
Implantable cardioverter defibrillator
Most of this 12-lead recording is polymorphic ventricular tachycardia but, in
the rhythm strip, the large deflection (arrowed) is the defibrillator
discharging.
Following the defibrillation a dual chamber pacemaker can be seen.
OK so I cheated a little with this one as the odds of catching this on a 12-
lead ECG recording are very slim indeed. This is a reconstructed 12-lead
recording from an electrophysiology study testing the device after
placement.
A 36 year old lady with recurrent blackouts.
