1. Practical Electrocardiography - QRS Axis Determination -
Scott E. Ewing DO
Interventional Cardiology Fellow
Lecture #3

2. Review
ECG Frontal Plane
Rate
Rhythm

3. Normal ECG

4. ECG Frontal Plane

5. Rate

6. Normal Sinus Rhythm

7. Question
Why is the rhythm strip usually Leads II, V1, and sometimes V5?

8. 8-Step Method ECG Interpretation
Rate
Rhythm
QRS Axis
P wave
PR interval
QRS complex
QT interval
ST segment and T wave

9. Why Determine the Axis?
Conduction defects – left anterior and posterior fasicular block
Ventricular enlargement – left and right ventricular hypertrophy
Broad complex tachycardia – bizarre axis suggestive of ventricular origin
Congenital heart disease – atrial septal defects
Pre­excited conduction – Wolff­Parkinson­White syndrome
Pulmonary embolus, lateral / inferior wall MI

10. Left Axis Deviation Artificial cardiac pacing Ascites COPD Expiration
High diaphragm
Hyperkalemia
Inferior myocardial infarction
Left anterior hemiblock
Left atrial hypertrophy
Left Bundle Branch Block
Left ventricular hypertrophy
Normal variations
Ostium primum atrial septal defect (ASD)
Right ventricular ectopic rhythms
Tricuspid atresia
Wolff-Parkinson-White Syndrome

11. Right Axis Deviation Anterolateral myocardial infarction
Atrial Septal Defect
COPD
Dextrocardia
Inspiration
Left posterior hemiblock
Left ventricular ectopic rhythms
Left ventricular failure with right ventricular strain
Normal variation (children, tall thin adults)
Pulmonary Embolus
Pulmonary Hypertension
Right Bundle Branch Block
Right ventricular hypertrophy
Switched electrodes
Ventricular Septal Defect (VSD)
Wolff-Parkinson-White Syndrome

12. Extreme Axis Deviation
Artificial cardiac pacing
COPD
Hyperkalemia
Lead transposition
Ventricular tachycardia

13. Depolarization

14. QRS Complex
QRS Axis
Represents direction of the mean QRS vector in the frontal plane
Determined using hexaxial reference system derived from the Einthoven equilateral triangle

17. ECG Frontal Plane

18. QRS Complex Normal QRS axis is -30° to 100°
Axis is usually shifted leftward with age
In individuals < 30, axis is seldom superior to 0° (normal 0° to 100°)
In individuals > 40, axis is seldom to right of 90° (normal -30° to 90°)

19. QRS Complex There is an association between QRS axis and body weight
Thinner persons tend to have more vertical axes (toward 90°, or rightward)
Obese persons tend to have more horizontal axes (toward 0°, or leftward)
There is no significant gender difference in the axis

20. ECG Axis Frontal plane leads are represented on a hexaxial diagram
Positive pole of each lead axis (solid line) and negative pole (hatched line) are designated by their angular position relative to the positive pole of lead I (0°)
Mean electrical axis of the QRS complex is measured with respect to this display

21. QRS Axis Determination
Lead orientation
Normal axis -30° to 100° I II
aVF
III
aVR
aVL

22. QRS Axis Determination - Approach
1st Method (Quick)
Is it normal?
2nd Method
Use algebraic sum of the deflections in 2 leads, usually I and aVF
Plot out axis
3rd Method
Find lead with isoelectric complex
QRS is perpendicular to this lead, with positive terminus pointing toward lead with largest net positive deflection

23. QRS Axis Determination - Quick Method
Lead I lies at 0°
If the QRS in Lead I is mainly positive (even if only slightly) it means the axis will be anywhere between -90° to 90°
Now, look at Lead II I II
aVF
III
aVR
aVL

24. QRS Axis Determination - Quick Method
Lead II lies at 60°
If the QRS in Lead II is mainly positive (even if only slightly) then the axis can be anywhere between -30° to 150° I II
aVF
III
aVR
aVL

25. QRS Axis Determination - Quick Method
If Lead I is positive the axis is between -90° and 90°
If Lead II is positive the axis is between -30° and 150°
Combined, the axis must lie between -30° and 90° I II
aVF
III
aVR
aVL

26. Normal or Abnormal???

27. QRS Axis Determination - Approach
1st Method (Quick)
Is it normal?
2nd Method
Use algebraic sum of the deflections in 2 leads, usually I and aVF
Plot out axis
3rd Method
Find lead with isoelectric complex
QRS is perpendicular to this lead, with positive terminus pointing toward lead with largest net positive deflection

28. QRS Axis Determination - Using Leads I and aVF
0° - Lead I
90° - Lead aVF
-30°
100°

29. Case #1

30. QRS Axis Determination - Case #1
Lead I corresponds to the x-axis
Sum the total positive and negative deflection
8 mm in this case
Lead I

31. QRS Axis Determination - Case #1
Lead aVF corresponds to the y-axis
Sum the total positive and negative deflection
9 mm in this case
Lead aVF

32. QRS Axis Determination – Case #1 Plot Results
0° - Lead I
90° - Lead aVF
-30°
100°

33. Case #2

34. QRS Axis Determination - Case #2
Lead I corresponds to the x-axis
Sum the total positive and negative deflection
9 – 5 = 4 mm in this case
Lead I

35. QRS Axis Determination - Case #2
Lead aVF corresponds to the y-axis
Sum the total positive and negative deflection
4 - 2 = 2 mm in this case
Lead aVF

36. QRS Axis Determination – Case #2 Plot Results
0° - Lead I
90° - Lead aVF
-30°
100°

37. Case #3

38. QRS Axis Determination - Case #3
Lead I corresponds to the x-axis
Sum the total positive and negative deflection
6 mm in this case
Lead I

39. QRS Axis Determination - Case #3
Lead aVF corresponds to the y-axis
Sum the total positive and negative deflection
0 - 7 = -7 mm in this case
Lead aVF

40. QRS Axis Determination – Case #3 Plot Results
0° - Lead I
90° - Lead aVF
-30°
100°

41. Case #3

42. QRS Axis Determination - Using Leads I and III
-30°
0° - Lead I
120° - Lead III
100°

43. QRS Axis Determination
Lead I corresponds to the x-axis
Sum the total positive and negative deflection
6 mm in this case
Lead I

44. QRS Axis Determination
Lead III corresponds to the y-axis
Sum the total positive and negative deflection
= -11 mm in this case
Lead III

45. QRS Axis Determination - Using Leads I and III
120° - Lead III
0° - Lead I
-30°
100°

46. Same Results?

47. QRS Axis Determination

48. QRS Axis Determination - Approach
1st Method (Quick)
Is it normal?
2nd Method
Use algebraic sum of the deflections in 2 leads, usually I and aVF
Plot out axis
3rd Method
Find lead with isoelectric complex
QRS is perpendicular to this lead, with positive terminus pointing toward lead with largest net positive deflection

49. QRS Axis Determination
Find the isoelectric lead if there is one (i.e. the lead with equal forces in the positive and negative direction)
Often this is the lead with the smallest QRS
QRS axis is perpendicular to that lead's orientation
Since there are two perpendiculars to each isoelectric lead, choose the perpendicular that best fits the direction of the other ECG leads
If there is no isoelectric lead, there are usually two leads that are nearly isoelectric and these are always 30° apart
Find the perpendiculars for each lead and choose an approximate QRS axis within the 30° range

50. QRS Axis Determination - Using Isoelectric LeadII
aVF
III
aVR
aVL

51. Case #1

52. QRS Axis Determination - Case #1
Lead aVL = -30° is the most isoelectric lead
Lead II is perpendicular at 60° (and 240°)
Lead aVL

53. QRS Axis Determination - Case #1
Lead II positive
Axis is close to 60°
What if Lead II had total negative deflection?
Axis closer to 240°
Lead II

54. QRS Axis Determination - Case #1
aVL - isoelectric
II – axis near here

55. QRS Axis Determination - With Original Findings
aVL - isoelectric
II – axis near here

56. Case #3

57. QRS Axis Determination - Using Isoelectric Lead
aVR
aVL I
III II
aVF

58. QRS Axis Determination - Case #3
Lead II = 60° is the most isoelectric lead
Lead aVL is perpendicular at -30° (and 150°)
Lead II

59. QRS Axis Determination - Case #3
Lead aVL positive
Axis is close to -30°
What if Lead aVL had net negative deflection?
Axis closer to 150°
Lead aVL

60. QRS Axis Determination - Case #3
aVL – axis near here
II – isoeletric

61. QRS Axis Determination - With Original Findings
aVL – axis near here
II – isoeletric

62. QRS Axis Determination
Isoelectric lead approach is tricky at first since often a true isoelectric lead is not present
Choose the best and look at the “next door” lead
If “next door” is opposite positive / negative, then you know the isoelectric direction is “inbetween” the two
The correct axis is adjusted accordingly

63. Normal or Abnormal Axis?

64. Axis?

65. Axis?

66. Questions???