1. ECG interpretation
Dr Sudhir Dev
House Officer
Dept. Of GP and Emergency Medicine

2. Objectives
• Justify the reasons for performing an ECG
• Develop a structured approach to interpreting
an ECG
• Practice interpreting ECGs

3. The ECG
“The ECG (electrocardiogram) is a transthoracic
interpretation of the electrical activity of the
heart.”

4. The ECG
SA node -> atrial muscle -> AV node -> bundle of His ->
Left and Right Bundle Branches -> Ventricular muscle

6. Lead Placement

7. EKG Distributions
• Anteroseptal: V1, V2, V3, V4
• Anterior: V1–V4
• Anterolateral: V4–V6, I, aVL
• Lateral: I and aVL
• Inferior: II, III, and aVF
• Inferolateral: II, III, aVF, and V5 and V6

8. Why perform an ECG?
• It’s part of the admission bundle
• Indicated by the patient’s symptoms
- symptoms of IHD/MI
- symptoms associated with dysrhythmias
• Indicated by the patient’s examination findings
- cardiac murmur

9. ECG Interpretation
• Quality of ECG?
• Rate
• Rhythm
• Axis
• P wave
• PR interval
• QRS duration
• QRS morphology
• Abnormal Q waves
• ST segment
• T wave
• QT interval

10. Quality of the ECG
• Patient name
• Date of the ECG
• Is there any interference?
• Is there electrical activity from all 12 leads?

11. Calibration
• Standard calibration of the ECG is 10mm/mV and normal speed 25mm/sec.
At this calibration, 1 miliVolt calibration signal is expected to produce a
rectangle of 10 mm height and 5 mm width.
• When ECG waves are tall, the R or S waves may extend into the QRS
complexes above or below them. To prevent this superimposition, the whole
ECG may be calibrated at 5mm/mV .
LOW VOLTAGE
Low voltage of the limb leads is present when the amplitude of the QRS
complex in each of the three standard limb leads (I, II, and III) is <5 mm . Low
voltage of all leads is diagnosed when the average voltage in the limb leads is
<5 mm and the average voltage in the chest leads is <10 mm.

12. An example of correct calibration at 10 mm/mV and speed of 25 mm/sec : The
calibration signal is a rectangle making 90 degrees of angles.

13. ECG interpretation
• Quality of ECG?
• Rate
• Rhythm
• Axis
• P wave
• PR interval
• QRS duration
• QRS morphology
• Abnormal Q waves
• ST segment
• T wave
• QT interval

14. Rate
• The 6 Second Rule: All but very slow heart rates can be determined by counting
the number of cycles that occur within 6 seconds, and then multiplying that
number by 10.
• The rule of 300 : 300/number of big squares between R waves
• Rate is either:
- Normal
– Bradycardia – HR < 40bpm
– Tachycardia HR > 120bpm

15. Rate

16. Rhythm
• Look for:
• Are there P waves?
• Are they regular?
• Does one precede every QRS complex?
Rythm could either be of sinus or non sinus in origin . Having P wave is sinus origin.
Without P wave or buried with preceeding T wave is either Venticular or Supraventricular
origin.

17. Axis
Definition:
the mean direction of electrical forces in the frontal plane ( limb leads) as measured
from the zero reference point (lead 1)
Normal values
P wave: 0 to 75 degrees
QRS complex: -30 to 90 degress
T wave: QRS-T angle <45 degrees frontal or <60 degrees precordial

18. The Quadrant Approach
• QRS up in I and up in aVF = Normal

19. Axis
Positive in I and II
= NORMAL
Positive in I and
negative in II = LAD
Negative in I and
positive in II = RAD

20. D/D of Right Axis Deviation (RAD)
• Differential diagnosis Right Ventricular Hypertrophy (RVH) — most common
• Left Posterior Fascicular Block (LPFB) — diagnosis of exclusion
• Lateral and apical MI
• Acute Right Heart Strain, e.g. acute lung disease such as pulmonary embolus
• Chronic lung disease, e.g. COPD
• Dextrocardia
• Ventricular pre-excitation (WPW) — LV free wall accessory pathway
• Ventricular ectopy
• Hyperkalemia
• Sodium-channel blockade, e.g. tricyclic toxicity
• Normal in infants and children
• Normal young or slender adults with a horizontally positioned heart can also
demonstrate a rightward QRS axis on the ECG.

21. D/D ofLeft Axis Deviation (LAD)
• left ventricular hypertrophy (LVH)
• Left Anterior Fascicular Block (LAFB) — diagnosis
of exclusion
• LBBB
• inferior MI
• ventricular ectopy
• paced beats
• Ventricular pre-excitation (WPW)

22. ECG interpretation
• Quality of ECG?
• Rate
• Rhythm
• Axis
• P wave
• PR interval
• QRS duration
• QRS morphology
• Abnormal Q waves
• ST segment
• T wave
• QT interval

23. P wave
• The P wave is the first positive deflection on the ECG
• It represents atrial depolarisation
Characteristics of the Normal Sinus P Wave
• Morphology
• Smooth contour
• Monophasic in lead II
• Biphasic in V1
• Axis
• Normal P wave axis is between 0° and +75°
• P waves should be upright in leads I and II, inverted in aVR
• Duration
• < 120 ms
• Amplitude
• < 2.5 mm in the limb leads,
• < 1.5 mm in the precordial leads

24. P wave abnormalities
Common P wave abnormalities include:
• P mitrale (bifid P waves), seen with left atrial
enlargement.
• P pulmonale (peaked P waves), seen with right atrial
enlargement.
• P wave inversion, seen with ectopic atrial and
junctional rhythms.
• Variable P wave morphology, seen in multifocal atrial
rhythms.

26. P mitrale

27. P pulmonale

28. PR interval
• The PR interval is the time from the onset of the P wave to the
start of the QRS complex.
• It reflects conduction through the AV node.
• Start of P wave to start of QRS complex
• Normal = 0.12 - 0.2 seconds (3-5 small squares)
• Decreased = can indicate an accessory pathway
• Increased = indicates AV block (1st
/2nd
/3rd
)

29. ECG interpretation
• Quality of ECG?
• Rate
• Rhythm
• Axis
• P wave
• PR interval
• QRS duration
• QRS morphology
• Abnormal Q waves
• ST segment
• T wave
• QT interval

30. QRS complex
Main Features to Consider
• Width of the complexes: Narrow versus broad.
• Voltage (height) of the complexes.
• Spot diagnoses: Specific morphology patterns that are important to
recognise.
• Normal = <0.12 seconds
• Narrow complexes (QRS < 100 ms) are supraventricular in origin.
• Broad complexes (QRS > 100 ms) may be either ventricular in origin, or
may be due to aberrant conduction of supraventricular complexes (e.g.
due to bundle branch block, hyperkalaemia or sodium-channel blockade)

31. Low Voltage QRS
• Low Voltage
• The QRS is said to be low voltage when:
• The amplitudes of all the QRS complexes in
the limb leads are < 5 mm; or
• The amplitudes of all the QRS complexes in
the precordial leads are < 10 mm

32. QRS complex
• Is there LVH?
• Probably the most commonly used are the Sokolov-Lyon criteria (S wave
depth in V1 + tallest R wave height in V5-V6 > 35 mm).
>35mm is suggestive of LVH

33. Q waves
• Q waves are allowed in V1, aVR & III
• Pathological Q waves can indicate previous MI

34. ECG interpretation
• Quality of ECG?
• Rate
• Rhythm
• Axis
• P wave
• PR interval
• QRS duration
• QRS morphology
• Abnormal Q waves
• ST segment
• T wave
• QT interval

35. ST segment
• The ST segment is the flat, isoelectric section of the ECG between the end of the S
wave and the beginning of the T wave.
• It represents the interval between ventricular depolarisation and repolarisation.
• The most important cause of ST segment abnormality (elevation or depression) is
myocardial ischaemia / infarction.
• ST depression (upsloping, horizontal and downsloping)
- downsloping or horizontal = ABNORMAL
• ST elevation
- infarction
- pericarditis (widespread)

36. ST segment depression

37. ST segment elevation

38. T wave
• Small = hypokalaemia
• Tall = hyperkalaemia
• Inverted/biphasic = ischaemia/previous infarct
• Hyperkalemia: Potassium reduces myocardial excitability, with
depression of pacemaking and conducting tissues.
• K+ >5.5 meq/l is a/w repolarization abnormalities causing peaked T waves
(earliest sign of hyperkalemia)
• K+>6.5 a/w progressive paralysis of artia , ECG shows wide and flat P
wave, incerase PR and P wave eventually dissapear.
• K+ more than 7 causes conduction abnormalities and bradycardia and
eventually asystole.

39. T wave

40. T wave

41. T wave

42. QT interval
• Start of QRS to end of T wave
• Needs to be corrected for HR
• Normal QTc = < 400ms
• Long QT can be genetic or iatrogenic

43. QT interval

44. Blocks
AV blocks
First degree block
PR interval fixed and > 0.2 sec
Second degree block, Mobitz type 1
PR gradually lengthened, then drop QRS
Second degree block, Mobitz type 2
PR fixed, but drop QRS randomly
Type 3 block
PR and QRS dissociated

45. Some important ECG examples

46. Lateral MI
Reciprocal changes

47. Inferolateral MI
ST elevation II, III, aVF
ST depression in aVL, V1-V3 are reciprocal changes

48. Anterolateral / Inferior Ischemia
LVH, AV junctional rhythm, bradycardia

49. Left Bundle Branch Block
Monophasic R wave in I and V6, QRS > 0.12 sec
Loss of R wave in precordial leads
QRS T wave discordance I, V1, V6
Consider cardiac ischemia if a new finding

50. Right Bundle Branch Block
V1: RSR prime pattern with inverted T wave
V6: Wide deep slurred S wave

51. First Degree Heart Block, Mobitz Type I (Wenckebach)
PR progressively lengthens until QRS drops

52. Supraventricular Tachycardia
Narrow complex, regular; retrograde P waves, rate <220
Retrograde P waves

53. Right Ventricular Myocardial Infarction
Found in 1/3 of patients with inferior MI
Increased morbidity and mortality
ST elevation in V4-V6 of Right-sided EKG

54. Ventricular Tachycardia

55. Prolonged QT
QT > 450 ms
Inferior and anterolateral ischemia

56. Second Degree Heart Block, Mobitz Type II
PR interval fixed, QRS dropped intermittently

57. Acute Pulmonary Embolism
SIQIIITIII in 10-15%
T-wave inversions, especially occurring in
inferior and anteroseptal simultaneously
RAD

58. Wolff-Parkinson-White Syndrome
Short PR interval <0.12 sec
Prolonged QRS >0.10 sec
Delta wave
Can simulate ventricular hypertrophy, BBB and previous MI

59. Hypokalemia
U waves
Can also see PVCs, ST depression, small T waves

60. THANK YOU