1) Accidental misplacement of limb lead electrodes can cause ECG abnormalities that mimic pathology. Understanding Einthoven's triangle and how it relates the leads to electrodes is important for interpreting these abnormalities. 2) When the limb electrodes are swapped, Einthoven's triangle "flips" or "rotates" changing which leads invert, remain the same, or switch positions. Disrupting the triangle also distorts the central terminal altering all leads. 3) Specific electrode reversals cause predictable changes - LA/RA reversal inverts lead I; LA/LL reversal inverts lead III; bilateral arm-leg reversal makes lead I flat and leads II/III/aVF identical.

1. DR.PRAVEEN NAGULA
I yr pg in cardiology
Ogh,hyd

2. Accidental misplacement of the limb lead electrodes is
a common cause of ECG abnormality and may simulate
pathology such as ectopic atrial rhythm, chamber
enlargement or myocardial ischaemia and infarction.

3. When the limb electrodes (LA, RA, LL) are
exchanged without disturbing the neutral
electrode (RL/N), Einthoven’s triangle is “flipped”
180 or rotated, resulting in leads that switch
positions, become inverted or remain unchanged
(depending on their initial position and vector).

4. Exchanging one of the limb electrodes with the neutral
electrode (RL/N) disrupts Einthoven’s triangle and distorts
the zero signal received from Wilson’s central terminal,
altering the appearance of both limb and precordial leads.
Limb leads may be grossly affected, taking on the
appearance of other leads or being reduced to a flat line.

5. DEFINITIONS

6. ELECTRODES
 LA – left arm
 RA – right arm
 LL -- left leg
 RL/N- right leg (neutral electrode)

7. LEADS
 Bipolar leads: I, II, III
 Augmented unipolar leads: aVL, aVF, aVR
 Wilson’s central terminus (WCT): the ‘zero’ lead,
produced by averaging signals from the limb
electrodes.

8. EINTHOVEN’S TRIANGLE

9.  Relationship between the limb leads and electrodes is
described by Einthoven’s triangle.
 Each lead has a specific quantity and direction
(vector) produced by adding or subtracting voltages
from the recording electrodes.

10. BIPOLAR LEADS
 Lead I is the voltage difference between the LA and
RA electrodes (LA – RA), directed towards LA at 0.
 Lead II is the voltage difference between the LL and
RA electrodes (LL – RA), directed towards LL at +60.
 Lead III is the voltage difference between the LL and
LA electrodes (LL – LA), directed towards LL at +120 .

11. Augmented Unipolar leads
 Lead aVL is directed towards the LA electrode (-30 )
calculated as follows: aVL = LA – (RA+LL)/2.
 Lead aVF is directed towards the LL electrode (+90 ),
calculated as follows: aVF = LL – (LA + RA)/2.
 Lead aVR is directed towards the RA electrode (-150
), calculated as follows: aVR = RA – (LA + LL)/2.

12. WILSON’S CENTRAL TERMINUS
 This directionless “zero lead” lead is calculated as
the average input from the three limb leads:
WCT = 1/3 (RA + LA + LL).

13. An understanding of Einthoven’s triangle and
the mathematical derivations of each lead will
help us in understanding the ECG patterns
produced by each type of limb lead reversal.

14. LA/RA reversal
With reversal of the LA and RA electrodes, Einthoven’s
triangle flips 180  horizontally around an axis formed
by lead aVF.

15. This has the following effects on the ECG:
Lead I becomes inverted.
Leads II and III switch places.
Leads aVL and aVR switch places.
Lead aVF remains unchanged.
NORMAL When LA, RA changed

16. BASELINE ECG

17. LA/RA reversal

18. QUICK GUIDE to
SPOTTING LA/RA REVERSAL
 Lead I is completely inverted (P wave, QRS complex and T
wave).
 Lead aVR often becomes positive.
 There may be marked right axis deviation.

19. LA/RA reversal may simulate dextrocardia.
However, in contrast to dextrocardia there is
normal R wave progression in the precordial leads.

20. LA/RA reversal simulating dextrocardia

21. LA/RA reversal reversed

22. LA/LL reversal
With reversal of the LA and LL electrodes,
Einthoven’s triangle rotates 180 vertically around
an axis formed by aVR.

23. This has the following effects on the ECG:
 Lead III becomes inverted.
 Leads I and II switch places.
 Leads aVL and aVF switch places.
 Lead aVR remains unchanged.

24. Baseline ECG

25. LA-LL reversal

26. Quick guide to spotting LA/LL
reversal
Lead III is completely inverted (P wave, QRS complex
and T wave)
The P-wave is unexpectedly larger in lead I than lead
II (it is usually the other way around).

27. RA/LL reversal
With reversal of the RA and LL electrodes,
Einthoven’s triangle rotates 180  vertically around
an axis formed by aVL.

28. This has the following effects on the ECG:
 Lead II becomes inverted.
 Leads I and III become inverted and switch places.
 Leads aVR and aVF switch places.
 Lead aVL is unchanged.

29. Baseline ECG

30. RA/LL reversal

31. Quick guide to spotting RA/LL
reversal
Leads I, II, III and aVF are all completely inverted
(P wave, QRS complex and T wave).
Lead aVR is upright.

32. RA/RL(N) reversal
 With reversal of the RA and RL(N) electrodes, Einthoven’s triangle
collapses to very thin “slice” with the LA electrode at its apex.
 The RA and LL electrodes now record almost identical voltages,
making the difference between them negligible (i.e, lead II = zero).
 Lead aVL runs within this thin slice, facing approx. opposite to lead III.
 Displacement of the neutral electrode renders leads aVR and aVF
mathematically identical, such that they appear exactly alike (but
different to the baseline ECG).

33. RA/RL(N) lead reversal has the following ECG features:
 Lead I becomes an inverted lead III.
 Lead II records a flat line (zero potential).
 Lead III is unchanged.
 Lead aVL approximates an inverted lead III.
 Leads aVR and aVF become identical.

34. As the neutral electrode has been moved,
the precordial voltages may also be
distorted.

35. Baseline ECG

36. RA/RL(N) reversal

37. Quick guide to spotting RA/RL(N)
reversal
Lead II is a flat line.

38. LA/RL(N) reversal
 With reversal of the LA and RL(N) electrodes, Einthoven’s triangle
collapses to very thin “slice” with the RA electrode at its apex.
 The LA and LL electrodes now record almost identical voltages,
making the difference between them negligible (i.e. lead III = zero).
 Lead aVR runs within this thin slice, facing approx. opposite to lead
II.
 The displacement of the neutral electrode renders leads aVL and
aVF mathematically identical, such that they appear exactly alike
(but different to the baseline ECG).

39. LA/RL(N) lead reversal has the following ECG features:
 Lead I becomes identical to lead II.
 Lead II is unchanged.
 Lead III records a flat line (zero potential).
 Lead aVR approximates to an inverted lead II.
 Leads aVL and aVF become identical.

40. As the neutral electrode has been moved, the precordial
voltages may also be distorted.

41. Baseline ECG

42. LA/RL(N) reversal

43. Quick guide to spotting LA/RL(N) reversal
Lead III is a flat line

44. Bilateral Arm-Leg Reversal (LA-LL plus
RA-RL)
 If the electrodes on each arm are swopped with their corresponding
leg electrode (LA with LL, RA with RL), Einthoven’s triangle
collapses to a very thin slice with the LL electrode at its apex.
 The RA and LA electrodes (now sitting on adjacent feet) record
almost identical voltages, which makes the difference between them
negligible (i.e. lead I = zero).
 Leads II, III and aVF all become identical (equivalent to inverted
lead III), as they are all now measuring the voltage difference
between the left arm and the legs.
 The displacement of the neutral electrode renders leads aVL and
aVR mathematically identical, such that they appear exactly alike
(but different to the baseline ECG).

45. Bilateral arm-leg reversal has the following ECG features:
 Lead I records a flat line (zero potential).
 Lead II approximates an inverted lead III.
 Lead III is inverted.
 aVR and aVL become identical.
 aVF looks like negative lead III.

46. As the neutral electrode has been moved, the
precordial voltages may also be distorted.

47. Baseline ECG

48. Bilateral arm-leg reversal

49. Quick Guide To Spotting Bilateral
Arm-Leg Reversal
Lead I is a flat line.

50. LL/RL(N) reversal
With reversal of the lower limb electrodes,
Einthoven’s triangle is preserved as the electrical
signals from each leg are virtually identical.

51. The ECG is therefore unchanged.

52. Baseline ECG

53. LL/RL(N) reversal

54. How to spot LL/RL(N) reversal
You won’t!

55. But don’t worry, it won’t make any
difference to your ECG interpretation.