ECG LIMB LEAD REVERSAL

DR.PRAVEEN NAGULAI yr pg in cardiology

Ogh,hyd

Introduction

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.

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).

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.

DEFINITIONS

ELECTRODES

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

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.

EINTHOVEN’S TRIANGLE

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.

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 .

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.

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).

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. 

LA/RA reversal

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

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

BASELINE ECG

LA/RA reversal

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.

LA/RA reversal may simulate dextrocardia.

However, in contrast to dextrocardia there is normal R wave progression in the precordial leads.

LA/RA reversal simulating dextrocardia

LA/RA reversal reversed

LA/LL reversal

With reversal of the LA and LL

electrodes, Einthoven’s triangle rotates 180

vertically around an axis formed by aVR.

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.

Baseline ECG

LA-LL reversal 

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).

RA/LL reversal

With reversal of the RA and LL

electrodes,

Einthoven’s triangle rotates 180

vertically around an axis formed by aVL.

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.

Baseline ECG

RA/LL reversal   

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.

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).

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.

As the neutral electrode has been moved,

the precordial voltages may also be distorted.

Baseline ECG

RA/RL(N) reversal

Quick guide to spotting RA/RL(N) reversal

Lead II is a flat line.

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).

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.

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

Baseline ECG

LA/RL(N) reversal  

Quick guide to spotting LA/RL(N) reversal

Lead III is a flat line

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).

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.

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

Baseline ECG

Bilateral arm-leg reversal

Quick Guide To Spotting Bilateral Arm-Leg Reversal

Lead I is a flat line.

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.

The ECG is therefore unchanged.

Baseline ECG

LL/RL(N) reversal 

How to spot LL/RL(N) reversal

You won’t!

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