ECG basics, Lead placement,

1. Electrocardiogram(ECG)
Dr. Deepesh Sharma(PT)
Assistant Professor
JNUCPT

2. Introduction
The electrocardiogram (ECG or EKG) is a diagnostic tool
that measures and records the electrical activity of the heart
in exquisite detail.
The term electrocardiogram was introduced by Willem
Einthoven in 1893 at a meeting of the Dutch Medical
Society.
In 1924, Einthoven received the Nobel Prize for his life's
work in developing the ECG.

3. Historical perspective
invention
Animal electricity
Electrical current
accompanies each heart
beat(frog)
Capillary electrometer
String galvanometer
Attempted ECG in humans
Recorded first human ECG
Naming of deflection in
ECG as “PQRST”
Scientist
Bancroft(1769)& Walsh(1773)
Muller(1856)
Gabriel Lippmann(1872)
Willem einthoven (1901)
Alexander muirhead(1869)
Augustus D waller(1887)
Willem einthoven (1901)

4. Cardiac conduction system

6. How ECG Works
Heart muscle cells are polarized at rest. This means the
cells have slightly unequal concentrations of ions across
their cell membranes.
An excess of positive sodium ions on the outside of the
membrane causes the outside of the membrane to have a
positive charge relative to the inside of the membrane.
The inside of the cell is at a potential of about 90 millivolts
(mV) less than the outside of the cell membrane. The 90
mV difference is called the resting potential.
The typical cell membrane is relatively impermeable to the
entry ofsodium.

7. However, stimulation of a muscle cell causes an increase
in its permeability to sodium.
Sodium ions migrate into the cell through the opening of
voltage-gated sodium channels. This causes a change
(depolarization) in the electrical field around the cell.
This change in cell potential from negative to positive and
back is a voltage pulse called the action potential.
In muscle cells, the action potential causes a muscle
contraction.
The sum action potential generated during the
depolarization and repolarization of the cardiac muscle can
be recorded by electrodes at the surface of the skin

8. Pacemakers of the Heart
SA Node - Dominant pacemaker with an intrinsic rate of
60 - 100 beats/minute.
AV Node - Back-up pacemaker with an intrinsic rate of 40
- 60 beats/minute.
Ventricular cells - Back-up pacemaker with an intrinsic
rate of 20 - 45 bpm.

9. Normal Impulse Conduction
Sinoatrial node
AV node
Bundle of His
Bundle Branches
Purkinje fibers

10. Impulse Conduction & the ECG
Sinoatrial node
AV node
Bundle of His
Bundle Branches
Purkinje fibers

11. Electrical Cardiac Cycle

12. ECG Waves , Complexes & Segments

13. The “PQRST”
P wave - Atrial
depolarization
• T wave - Ventricular
repolarization
• QRS - Ventricular
depolarization

14. Start of EKG Cycle

15. P-Wave
Depolarization of atrial muscle
Low voltage (2-3mm in amplitude)
Duration <.11 seconds
Shape and duration of P may indicate atrial
enlargement

16. Early P Wave

17. Later in P Wave

18. PR interval
• from onset of P wave to onset of QRS
• Normal duration = 0.12-2.0 sec (120-200 ms)
(3-4 horizontal boxes)
• Represents atria to ventricular conduction time
(through His bundle)
• Prolonged PR interval may indicate a 1st
degree heart block

19. QRS Complex
Depolarization of ventricles
Larger Muscle Mass
Amplitude as high as 25mm
Duration with Normal Conduction <.10
Amplitudes >25mm can mean chamber
enlargement as in ventricular hypertrophy
The width of the QRS complex should not
exceed 110 ms, less than 3 little squares
The QRS complex should be dominantly
upright in leads I and II

20. QRS and T waves tend to have the
same general direction in the limb
leads

21. Early QRS

22. Later in QRS

23. ST Segment
Time between completion of depolarization and
onset of repolarization
• Normally isoelectric & gently blends into upslope
of T wave
• Point where ST takes off from QRS= J point
Plays important role in diagnosis of ischemic heart
disease.
• Duration of 0.08-0.12 sec (80-120 msec

24. S-T Segment

25. T wave:
Represents repolarization or recovery of
ventricles
• Interval from beginning of QRS to apex of T is
referred to as the absolute refractory period

26. Early T Wave

27. Later in T-Wave

28. QT Interval
Measured from beginning of QRS to the end
of the T wave
• Normal QT is usually about 0.40 sec
• QT interval varies based on heart rate

29. 12 lead ECG

30. 12 Lead EKG
Standard 12-lead EKGs are designed to look at the left
ventricle.
Give multiple views of heart does not give three
dimensional view of heart.
Shows myocardial damage by the disruption of normal
electrical activity.
Inference is made about which anatomical region of the
heart is injured

31. Planes of Heart
Frontal Plane
Divides front and back
Limb leads show frontal plane views
Horizontal Plane
Divides top and bottom
Chest leads show horizontal plane views

32. Lead Placement
The person acquiring the 12-lead EKG must pay close
attention to the application of the EKG leads.
Misplacement of the leads can alter the EKG machine’s
representation of electrical flow and increase the chances
for misinterpretation

33. 12 Lead Basics
Standard 12 lead
4 limb leads
6 chest leads
Also called vector leads

34. Limb Leads
Color coded
Left leg – Red
Left arm – Black
Right arm – White
Right leg – Green

35. Chest Lead Placement
Chest leads are generally marked by vector
abbreviations and color coded:
Chest lead 1 – V1 – Red
Chest lead 2 – V2 – Yellow
Chest lead 3 – V3 – Green
Chest lead 4 – V4 – Blue
Chest lead 5 – V5 – Orange
Chest lead 6 – V6 – Purple

36. Chest Lead Placement
V1 - 4th intercostal R of sternum
V2 - 4th intercostal L of sternum
V4 – 5th intercostal midclavicular
V6 – 5th intercostal midaxillary
V3 – between V2 and V4
V5 – 5th intercostal between V4 and V6
Try not to place lead directly over rib
Hint: Angle of Louis at 2nd rib

37. 12 Leads but only 10 electrodes?
Limb Leads
I
II
III
aVR
aVL
aVF
Chest Leads
V1
V2
V3
V4
V5
V6

38. Limb Leads
Limb leads are used to view the heart from more than one
angle on frontal plane.

39. Limb Leads
Looks at inferior wall of left ventricle
Lead I
RA to LA
Lead II
RA to LL
Lead III
LA to LL

40. Augmented Limb Leads
Voltage is so low it has to be augmented by the machine
aVR
Heart to RA
augmented voltage right arm
aVL
Heart to LA
augmented voltage left arm
aVF
Heart to LL
augmented voltage left foot

41. Chest Leads
Look at heart from horizontal plane
Leads are positive
Heart is theoretical negative electrode
Also called precordial or vector leads

42. What Each Lead “Sees”
LEAD VIEW
II, III, aVF Inferior
V1, V2 Septal
V3, V4 Anterior
V5, V6, I, aVL Lateral

43. The ECG Paper
Horizontally
One small box - 0.04 s
One large box - 0.20 s
Vertically
One large box - 0.5 mV

44. The ECG Paper (cont)
Every 3 seconds (15 large boxes) is marked
by a vertical line.
This helps when calculating the heart rate.
3 sec 3 sec

45. ECG Basics
ECG graphs:
1 mm squares
5 mm squares
Paper Speed:
25 mm/sec standard
Voltage Calibration:
10 mm/mV standard

46. ECG Paper: Dimensions
5 mm
1 mm
0.1 mV
0.04 sec
0.2 sec
Speed = rate
Voltage
~Mass

47. Heart Rate Determination
• Grid Methods
• Scan Method

48. Grid Methods
1500/No. of small squares between 2
consecutive R
300/No. of large squares between 2
consecutive R

49. Scan Method
Check for 3 sec/ 6 sec markers
Count the number of QRS complexes
between markers
Multiply by 20 / 10 for 3 & 6 sec markers
respectively

50. Normal Sinus Rhythm
Regular narrow-complex rhythm Rate
60-100 bpm
Each QRS complex is proceeded by a P
wave
P wave is upright in lead II & downgoing
in lead aVR

51. Normal Sinus Rhythm

52. Sinus Bradycardia
HR< 60 bpm;
Every QRS narrow, preceded by p
wave Normal in well-conditioned athletes
HR can be<30 bpm in children, young
adults during sleep, with up to 2 sec
pauses

53. Sinus bradycardia

54. Sinus tachycardia

55. Sinus Techycardia
HR > 100 bpm, regular
Often difficult to distinguish P and T
waves

56. Sinus Arrhythmia
Variations in the cycle lengths between
P waves/ QRS complexes
Will often look irregular on exam
Normal P waves, PR interval, normal,
narrow QRS

57. Sinus Arrhythmia

58. Atrial Fibrillation
Caused by a large reentrant circuit in
the wall of the right atrium
Caused by numerous wavelets of
depolarization spreading throughout the
atria simultaneously, leading to an
absence of coordinated atrial contraction

59. Atrial Fibrillation

60. Ventricular Tachycardia
Ventricular tachycardia is usually
caused by reentry, and most commonly
seen in patients following myocardial
infarction