A brief introduction to mechanism of ECG, interpretation of ECG, features of a normal ECG, and ECG seen with limb lead reversal.

3. OBJECTIVES
 What is an ECG and why it is used?
What is the normal calibration of ECG?
 What is the conductive system of the heart?
 Where the electrodes are placed?
 What are the polarities of the 12 leads?
 What are parts of an ECG complex?
 How to estimate heart rate from ECG?
 What are features of normal sinus rhythm?
 What is axis of heart and axis deviation?
 What is normal P-QRS-T morphology in different leads?
 What are changes seen in limb leads reversal?

5. WHAT IS ECG?
It is the graphical recording of electrical conduction in
the heart over a period of time (usually 10 sec), using
electrodes placed over the skin.
In conventional 12 lead ECG, a total 10 electrodes are
placed: 6 over the chest and one on each limb.
The overall magnitude of electrical activity of heart is
measured from 12 different angles or “leads”.
The graph of voltage versus time produced is called
Electrocardiogram.

6. USES OF ECG?
It is used to detect:
 Rate and rhythm disorders
 Conduction problems
 Myocardial ischemia
 Myocardial infarcts
 Chamber dilation
 Chamber hypertrophy
 Inflammation i.e. pericarditis
 Electrolyte disturbances
 Drug toxicity
 Other: pulmonary embolism

8. SPEED CALIBRATION
X-axis  time
– 1 small square
• 40 ms (0.04 sec)
– 1 large square
• 200 ms (0.2 sec)
Speed: 25 mm/sec

9. VOLTAGE CALIBRATION
Y axis  voltage
– 1 large square
 0.5 mV
– 2 large squares
 1.0 mV
– Therefore, 1 mV
produces deviation of 2
large squares (10 mm)

11. CHAMBERS OF HEART
Heart has two electrically functional units: Atria and Ventricles.
Electrically divided by the properties of anulous fibrosus cordis.

12. CONDUCTION PATHWAY

13. CARDIAC CYCLE

14. PACEMAKERS
Sinoatrial “SA” Node
- Dominant pacemaker
- Intrinsic rate of 60 - 100 beats/minute.
Atrioventricular “AV” Node
- Back-up pacemaker
- Intrinsic rate of 40 - 60 beats/minute.
Ventricular cells
- Back-up pacemaker
- Intrinsic rate of 20 - 45 beats/minute.

15. AXIS OF THE HEART
The electrical axis of the heart is the mean direction
of the action potentials traveling through the
conductive system of the heart.

16. WAVEFORMS
Contraction of any muscle is associated with
electrical changes called depolarization.
This is followed by its relaxation, which is
associated with the reversal of these changes
called repolarization.
These electrical changes will produce
waveforms on the ECG. i.e. P-QRS-T waves.
No electrical changes will produce an flat
isoelectric baseline on the ECG.

17. ECG COMPLEX
 Waveforms
– P wave
– QRS complex
– T wave
– U wave
 Segments
– PR segment
– ST segment
 Intervals
– PR interval
– QT interval
– RR interval

18. ECG COMPLEX

19. ELECTROMAGNETICS
1. depolarization toward the
positive electrode produces a
positive deflection
2. depolarization away from the
positive electrode produces a
negative deflection
3. repolarization toward the
positive electrode produces a
negative deflection
4. repolarization away from the
positive electrode produces a
positive deflection

20. ELECTROMAGNETICS
If direction of conduction is at right angle to the
positive electrode it will produce positive
deflection (depolarization) and then negative
deflection (repolarization).

21. ELECTROMAGNETICS

23. LIMB ELECTRODES

24. CHEST ELECTRODES
Chest Electrode Placement
V1 4th ICS, right of sternum
V2 4th ICS, left of sternum
V3 Between V2 and V4
V4 5th ICS, in left mid
clavicular line
V5 Same height as V4, in
left anterior axillary line
V6 Same height as V4, in
left mid axillary line

25. LEADS
 3 limb leads
– I - (+LA and -RA)
– II - (+LL and -RA) also called ‘sinus lead’
– III - (+LL and -LA)
 3 augmented limb leads:
– aVR - (+RA and average of LA & LL)
– aVL - (+LA and average of RA & LL)
– aVF - (+LL and average of RA & LA)
 6 Chest leads:
– V1, V2, V3, V4, V5, V6 (have +ve electrodes of same name)
– Negative electrode is WCT (Wilson’s central terminus), an
average of the three limb electrodes (RA, LA, LL)
 Other Chest leads:
• Posterior chest leads: V7, V8, V9
• Right chest leads: V3R, V4R, V5R

26. PLANE PERSPECTIVES

27. VECTORS OF LIMB LEADS

28. EINTHOVEN’S TRIANGLE

29. CIRCLE OF AXIS

30. LEAD PERSPECTIVES

31. LEAD PERSPECTIVES

32. LEAD PERSPECTIVES

34. REPORTING AN ECG
Estimated heart rate
Comment on the rhythm
Comment on the axis
Comment on:
– P wave morphology
– PR segment
– QRS morphology
– ST segment
– T wave morphology
– QT interval
Compare with a previous ECG
Conclusion

35. ESTIMATING HEART RATE
Square method
– Count large boxes between two adjacent R waves

36. ESTIMATING HEART RATE
Alt. method
– count number of small boxes between two
consecutive R waves
– divide 1500 by that number to est. HR
3 second method
– count number of QRS complexes that fit into 3
seconds (15 large squares)
– multiply this number with 20 to est. HR
– preferred method in irregularly irregular rhythms

37. HEART RATE INTERPRETATION
HR of 60-100 beats/min  Normal range
HR > 100 beats/min  Tachycardia
 Physiologic i.e. exercise
 Inappropriate i.e. fever, anxiety, tachyarrhythmia
HR < 60 beats/min  Bradycardia
 Physiologic i.e. athletes at rest
 Inappropriate i.e. heart blocks, vaso-vagal reflex
Paediatric values
 New born i.e. 110 - 150 b/m
 2 years i.e. 85 - 125 b/m
 4 years i.e. 75 - 115 b/m
 6 years + i.e. 60 - 100 b/m

38. NORMAL SINUS RHYTHM
Normal heart rate
Regular rhythm
P waves should be sinus
P wave is round and upward in lead I & II
Each QRS is preceded by a P wave
The PR interval should remain constant
QRS complexes should be narrow

39. AXIS DEVIATION (THUMB RULE)
QRS IN
LEAD I
QRS IN
LEAD AVF
AXIS
DEVIATION
SEE IN
POSITIVE POSITIVE NORMAL
POSITIVE NEGATIVE LEFT AXIS
DEVIATION
• Elevated diaphragm
(ascites, pregnancy)
• IWMI, hyperkalemia
• LVH alone, LVH with LBBB
• Occassionally LBBB alone
NEGATIVE POSITIVE RIGHT AXIS
DEVIATION
• Young, thin people
• LWMI
• RVH alone, RVH with RBBB
• Occassionally LBBB alone
NEGATIVE NEGATIVE NORTH WEST
AXIS
• Severe RVH
• Severe hyperkalemia

40. AXIS DEVIATION

41. P WAVE MORPHOLOGY
 sinus P wave is round and upward in lead I / II
 always inverted in aVR
 can be biphasic or inverted in lead V1
 maximal height - 2.5 mm in lead II / III
 duration is shorter than 0.12 sec (3 small sq)

42. PR SEGMENT & PR INTERVAL
PR segment
– an iso-electric line, due to conduction delay to AV node
– from end of P wave to start of QRS complex
– diffuse PR segment depression in acute pericarditis
PR interval
– includes P wave + PR segment
– from start of P wave to start of QRS
– normally 0.12 to 0.2 sec (3-5 small sq)
– short PR interval are seen in pre-excitation:
• MAT, WPW, junctional rhythms
– prolonged PR interval are seen in:
• 1st and 2nd degree AV block
• hypokalemia, digitalis toxicity, carditis

43. DELTA WAVE
In WPW syndrome, short PR interval manifests as a
“delta wave”, a slurred upstroke in the QRS complex

44. QRS COMPLEX
Normal duration is < 110 ms or < 3 small squares
– Q wave – 1st downward deflection after P wave
• seen in I, aVL, V5 and V6; usually absent in leads V1-V2
– R wave – 1st upward deflection
• short in V1-V2, long in V5-V6
– S wave – 2nd downward deflection
• long in V1-V2, short in V5-V6
R wave progression
 from V1 to V6, R wave height  but S wave depth 
Q
S
R
TZ Normal Transition Zone is
at V3-V4, when S wave
equals R wave.
Represents apex of heart.

45. QRS ABNORMALITIES
Broad QRS complex (>120 ms)
– LBBB, RBBB, hyperkalemia, VT etc
Increased QRS height
– LVH, RVH
Poor R wave progression
– AWMI, LVH, LBBB, WPW
Dominant R wave in V1
– PWMI, RVH, RBBB, WPW, children
Pathologic Q waves
– Markers of previous MI
– Q wave width > 1 small sq. + Q wave depth > 2 small sq.
– Or, Q wave is ≥ 25% of the R wave
Pathologic Q

46. VENTRICULAR HYPERTROPHY
Sokolow Lyon index for LVH:
• S wave depth in V1/V2 + R wave height in V5/V6 ≥ 35 mm
• R wave height in aVL ≥ 11 mm
Criteria for RVH:
• dominant R in V1 + dominant S waves in V5/V6
• deep S waves in leads I, II, III, aVL, V5, V6

47. ST SEGMENT
 Usually iso-electric (flat)
 Measured from J point to beginning of T wave
 J point is the junction between QRS and ST segment
 ST elevation in STEMI (convex or obliquely straight upwards)
 Diffuse ST elevation with PR depression in acute pericarditis
 ST depression in unstable angina / NSTEMI
 Sagging ST depression in digoxin effect and hypokalemia
ST segment
J point
P T “sagging” ST
depression

48. ST SEGMENT ELEVATION
“saddleback”
pattern of diffuse
ST elevation
& PR depression
in Pericarditis
obliquely straight
ST elevation in
STEMI
upward convex
ST elevation in
STEMI
notched J point
“fish-hook” pat. of
ST elevation seen
in BER

49. T WAVE AND U WAVE
Upwards in most leads
Can be inverted in V1, but always inverted in aVR
Should be less than 2/3rd the height of R wave
Abnormalities of T wave
Tall narrow T waves in hyperkalemia
Tall broad T waves in acute STEMI (hyperacute)
Generalized flat T waves in hypokalemia
Flattened or inverted T waves – early sign of ischemia
Deep T waves in chest leads – Wellen’s sign
Diffuse deep “cerebral” T waves – raised ICP
U wave
Late repolarization of ventricles
Usually seen in V6 after T wave
Prominent in hypokalemia

50. QT INTERVAL
 From beginning of QRS to the ending of T wave
 Duration 0.35 - 0.45 sec ( 9 -12 small sq. )
 QT duration is inversely proportional to Heart Rate
 QTc also  with  HR which can be corrected using Bazzet
formula:
Short QTc (< 340 ms)
• Hypercalcemia
• Digoxin effect
Long QTc (> 460 ms)
• Hypothermia
• Hypokalemia
• Hypomagnesiumia
• Hypocalcemia
• Drugs - amiodarone, quinidine, TCAs, erythromycin

51. Chamberline’s 10 RULES
1. PR interval should be 0.12-0.2 sec (3-5 small sq.)
2. With of QRS should not exceed 0.11 sec (3 small sq.)
3. QRS should be dominantly upwards in lead I and II
4. QRS and T waves have same direction in limb leads
5. All waves are negative in lead aVR
6. R wave must grow from V1 to V4 while S wave must
grow from V1 to at least V3 and disappear in V6
7. ST segment should start iso-electric except in V1 and
V2 where it may be elevated
8. P waves should be upright in I, II, and V2 to V6
9. No pathologic Q wave in I, II, and V2 to V6
10. T wave must be upright in I, II, and V2 to V6

53. LA / RA REVERSAL
 Lead I becomes inverted.
 Leads II and III switch places.
 Leads aVL and aVR switch places.
 Lead aVF remains unchanged.
 What will you see?
• Lead I is completely inverted.
• Lead aVR often becomes positive.
• There may be marked right axis
deviation.

54. LA / LL REVERSAL
 Lead III becomes inverted.
 Leads I and II switch places.
 Leads aVL and aVF switch places.
 Lead aVR remains unchanged.
 What will you see?
• Lead III is completely inverted.
• P wave is unexpectedly larger in
lead I than in lead II.

55. RA / LL REVERSAL
 Lead II becomes inverted.
 Leads I and III become inverted
and switch places.
 Leads aVR and aVF switch places.
 Lead aVL remains unchanged.
 What will you see?
• Lead I, II, III and aVF all are
completely inverted.
• Lead aVR is upright.

56. RA / RL (N) REVERSAL
 Leads I and aVL become inverted.
 Lead II will be flat.
 Lead III is unchanged.
 Lead aVR and aVL become
identical.
 What will you see?
• Lead II is a flat line.

57. LA / RL (N) REVERSAL
 Lead I becomes identical to lead II.
 Lead II is unchanged.
 Lead III is flat.
 Lead aVR is an inverted lead II.
 Lead aVL and aVF become identical.
 What will you see?
• Lead III is a flat line.

58. LL / RL (N) REVERSAL
 Einthoven’s triangle is preserved.
 What will you see?
• ECG is unchanged.

59. LA with LL / RA with RL
 Bilateral arm-leg electrode reversal.
 Lead I is flat.
 Lead II is an inverted lead III.
 Lead III is inverted.
 aVR and aVL become identical.
 aVF looks like negative lead III.
 What will you see?
• Lead I is a flat line.

60. QUICK SPOTTING
OF LEAD REVERSAL
Lead I is flat or completely inverted.
Lead II is flat or completely inverted.
Lead III is flat or completely inverted.
Lead aVR is positive.
P wave is larger in lead I than in lead II.

61. DEXTROCARDIA
What will you see?
Right axis deviation.
Complete inversion of lead I.
All waves in aVR are positive.
Absent R wave progression in chest leads
- S wave is dominant throughout