16. Leads: Definitions
• A lead is a recording electrode or a
pair of recording electrodes at a
• A lead can also refer to an electrical
17. Bipolar and Unipolar Leads
• A bipolar lead has a distinctly positive pole and a
distinctly negative pole. These leads include the frontal
leads in an ECG: I, II, and III.
• A unipolar lead has a pole with a distinct positive pole
but does not have a distinct negative pole. These leads
include aVL, aVR, and aVL. The chest leads are also
unipolar: leads V1 through V6.
Planes of the Body
Some 12-lead ECG leads
are on the frontal plane and
some are on the horizontal
The frontal leads scan the
top surface of the body.
The horizontal leads sense
electrical forces from front
to back of the body.
19. Frontal Plane Leads: Lead I
Lead I: RA (-) to LA (+) (Right Left, or lateral)
• Lead I has a positive electrode on
the left arm and a negative
electrode on the right arm.
• Lead I is a bipolar, indirect lead.
• field of observation on the frontal
plane of the body.
20. Frontal Plane Leads: Lead II
RA (-) to LF (+) (Superior Inferior)
• Lead II has a positive electrode on
the left foot and a negative
electrode on the right arm.
• Lead II is a bipolar, indirect lead.
• As a frontal leads, Lead II is a
field of sensing on the frontal
plane of the body.
21. Frontal Plane Leads: Lead III
Lead III: LA (-) to LF (+) (Superior Inferior)
• Lead III has a positive pole
on the left foot and a
negative pole on the left
• Lead III is a bipolar,
• As a frontal leads, Lead III is
a field of sensing on the
frontal plane of the body.
22. Leads I, II, and III:
• These three leads together
compose Einthoven’s Triangle.
• They are bipolar leads. Each
lead has a positive and a
• These leads are called indirect
because they are more than two
cardiac diameters from the heart.
• Leads I, II, and III are indirect
26. Einthoven's Triangle
• If leads I, II and II are then criss-crossed
over the heart, we begin to build a
system of leads that will cover the
surface of the chest.
• At this point, we have a triaxial system.
• Note Leads II and III appear to have
• We have room for more coverage in the
spaces between I, II, and III.
27. Einthoven’s law
• Einthoven’s triangle.
• if we know the voltage
of two bipolar leads, the
voltage of 3rd lead can
be found by adding the
remaining two leads.
• i.e. II = I + III
28. Augmented Limb Leads:
aVL, aVF, and aVL
• Without adding any more physical
leads on the body a number of
physicians over time designed the
virtual leads aVL, aVF and aVR.
• These virtual leads use the existing
limb leads, I, II, and III, and
mathematical formulas to create three
additional frontal-plane leads.
Augmented Limb Leads:
aVL, aVF, and aVL
• aVL, aVF, and aVR are unipolar leads. They use a positive
pole on the surface of the body.
• They use the heart as a negative reference point.
• The “a’ stands for augmented because the signal is boosted
for our vision by the ECG machine.
• The augmented limb leads are unipolar, indirect, frontal
31. • Adding the leads aVL,
aVR, and aVF between
the open spaces
completes the frontal
leads of the 12-Lead
• We now have six limb
• Now we have an hexaxial
In Summary: Six Limb Leads
32. Precordial Leads
• The three standard limb leads (I, II, and III) and the three virtual leads
(aVL, aVF and aVR) compose six of the twelve leads in a 12-lead ECG.
• The other six leads are the precordial leads.
• The precordial leads are unipolar leads. They have a positive electrode on
the chest wall and use the heart as a general negative reference point.
• The precordial leads record the heart’s electrical forces in a transverse (or
• The precordial leads are semi-direct leads because they are close to the
heart but not directly on the muscle.
33. Precordial (Chest) Leads
• Chest leads are placed in a circular
pattern around the heart:
• V1: 4th intercostal space to the
immediate right of the sternum.
• V2: 4th intercostal space to the
immediate left of the sternum.
• V3: midway between V2 and V4.
• V4: in the midclavcular line, in the 5th
• V5: in the anterior axillary, in the 5th
• V6: in the midaxillary line, in the 5th
34. Precordial Leads
• Why six anterior leads?
– V1 and V2 reflect the
right side of the heart
– V3 and V4 reflect the
septum (location of
His Bundle and Right
and Left Bundle
– V5 and V6 reflect the
left side of the heart
36. Summary of Leads
• There are six limb leads. They are indirect leads.
• Three of the limb leads are bipolar: I, II, and III. Three of the limb leads
are unipolar: aVL, aVR and aVF.
• The limb leads are in the frontal plane.
• There are six precordial (chest) leads. The precordial leads lie in the
horizontal plane. They are unipolar, semi-direct leads.
• Bipolar leads have a negative and positive pole. Unipolar leads have a
positive pole and a negative reference in the area of the heart.
• Indirect leads lie more than the distance of two heart widths from the
• Semi-direct leads are in close proximity to the heart.
• Direct leads are in direct contact with the heart.
41. Certain conventions
to record ECG.
• For 1mV input there
must be 1 cm deflection
• 1 ss on Y-axis = 0.1 mV
• Speed of the ECG paper
is 25 mm/sec.
• 1 ss on x-axis = 0.04sec.
we can double the
speed in tachycardia.
43. Components of normal ECG
• Waves of ECG:
2. QRS complex
• Segments of ECG:
1. PQ or PR segment
2. ST segment
• Intervals of ECG:
1. PR interval
2. QT interval
3. RR interval
44. P Wave
• it represents atrial depolarization
which occurs just before atrial
• The peak of the P wave
coincides with time when
cardiac impulse reaches the AV
• Amplitude of P wave is 0.1
– 0.2 mV and duration is 0.1
45. QRS Complex
• Due to ventricular depolarization which
occurs just before ventricular contraction.
• Voltage is variable in different leads. We
take the voltage from the peak of R wave
to the bottom of S wave.
• Duration: 0.08 to 0.1 sec
• Q wave is due to depolarization of upper
part of ventricular septum,
• R wave is due to depolarization of lower
part of septum and apex of heart.
• S wave is due to depolarization of wall
and base of the ventricle.
• The peak of the R wave coincides with the
beginning of the ventricular systole.
47. T Wave
• a positive wave, due to
• Voltage = 0.2
– 0.3 mV
• Duration =
0.16 - 0.20 sec.
• End of the T wave
approximately coincides with
the end of ventricular systole.
48. U WAVE
• Sometimes there is U wave after
T wave which is due to slow
repolarization of papillary muscle
• U wave becomes prominent in
• Note: Wave of atrial
repolarization is masked by QRS
49. SEGMENTS OF ECG
1. PR OR PQ segment: End of P wave to
beginning of Q wave. 0.05-0.12 sec
2. ST Segment: End of S WAVE to beginning
of T wave. 0.08-0.12 sec
• These two segments are isoelectric because
there is no current flow in heart.
• In PR segment, atria are completely
• In ST segment ventricles are completely
CLINICAL: In acute myocardial infarction (MI)
there is elevation of ST segment due to flow
of injury current which flows between the
area of infarction and normal myocardium.
52. PR Interval
• between beginning of P wave and beginning of
• It represents the atrial depolarization plus
conduction through AV node. AV conduction is
also included in PR interval.
• Duration: On average it is 0.16 sec. range is 0.12
to 0.2 sec.
• It is prolonged in rheumatic fever and different
types of AV blocks and also in hypokalemia.
• PR interval is shortened in accelerated AV
conductions and in WPW syndrome.
• It is approximately equal to interval between the
beginning of atrial contraction and beginning of
53. QRS Interval
• duration of QRS
complex: 0.08 to
• Prolonged in bundle
branch blocks and
54. QT Interval
• From beginning of Q wave to end
of T wave.
• It represents ventricular
depolarization and ventricular
• duration = 0.36 to 0.4 sec.
• it is approximately equal to the
duration of ventricular systole.
• Shortened in Hypercalcemia.
• Prolonged in hypocalcaemia and
also in ventricular extra systole.
55. RR INTERVAL
• interval between two
successive R waves.
• It is equal to duration
of one cardiac cycle i.e.
56. Calculation of heart rate from ECG
• If the heart rate is regular then by RR interval, we can find
out the heart rate by the following formulae:
1. 60 / RR interval or
2. 300 / no. of large squares b/w two successive R waves. or
3. 1500 / no. of smallest squares between two successive R
• In case of irregular heart rate we can’t use this formula. In
that case we count the number of heart beats in 6 seconds and
multiply it with ten.
57. Table 28-2. ECG intervals.
Normal Duration(s) Events in the Heart
Intervals Average Range During Interval
PR interval1 0.182 0.12-0.20 Atrial depolarization and
conduction through AV node
QRS duration 0.08 to 0.10 Ventricular depolarization
and atrial repolarization
QT interval 0.40 to 0.43 Ventricular depolarization
ST interval (QT
0.32 . . . Ventricular repolarization
1 Measured from the beginning of the P wave to the beginning of the QRS
2 Shortens as heart rate increases from average of 0.18 at a rate of 70
beats/min to 0.14 at a rate of 130 beats/min.
67. ECG CHANGES in MI
• ST elevation=>
• Convexity upwards and elevated segment merged
with T wave.
• T wave inversion=>
• Pathological Q wave=>
• Wider and deeper. Depth more than one third the
height of R wave of that QRS complex.
68. SYSTEMATIC INFARCT RECOGNITION
• Infarct location:
• ST Elevation Found In:
• Anterior – Septal :- V1, V2, V3 and V4
• Posterior :- V1 and V2
• Inferior :- II, III and aVF
• High Lateral :- I and aVL
• Low Lateral :- V5 and V6
71. HYPERKALEMIA (K+ > 5 meq / L)
• Increase in potassium in ECF
• Prolonged Depolarisation
• Initially tall T wave
• Later wide QRS complex,
Diminished or absent P wave,
reduced QT interval.
• arrhythmia and
72. HYPOKALEMIA (K+ < 3.5 meq / L)
• Decrease in potassium ion in ECF
• Delayed ventricular repolarization
• (1) prolonged PR interval
• (2) ST segment depression
• (3) T wave inversion
• (4) U wave present
• (5) Reduced amplitude of QRS complex
• Increase calcium in ECF
• Increased myocardial contractility
• Heart relaxes less during diastole
• Short ST segment & short QT interval
• Eventually stops in systole
• CALCIUM RIGOR