ECG LEADS

introduction

The heart produces electrical currents

The body acts as a conductor of electricity

Electrodes are able to measure the voltages of the cardiac current

introduction

An ECG is a recording of the electrical activity of the heart

Different “views” of the heart can be recorded with different electrodes

Inroduction

12 possible leads

Six limb (extremity) leads

Bipolar leads: I, II, and III

Unipolar leads: aVR, aVL, aVF

Six chest (precordial) leads

Limb leads

I + III = ?

limb leads

Electrodes

Right arm

Left arm

Left leg

Right leg (ground)

Einthoven’s triangle

Lead I

= Difference in voltage between LA and RA

Lead II

= LL minus RA

Lead III

= LL minus LA

RA LA

LL

lead i

The LA electrode detects electrical voltages of the heart that are transmitted to the left arm

The RA electrode detects electrical voltages of the heart that are transmitted to the right arm

The electrocardiograph sustracts RA from LA and the difference appears as lead I.

lead i

Lead one points horizontally.

Its left pole (LA) is postive and its right pole (RA) is negative.

Therefore, lead I = LA minus RA

Lead II

Lead II points downward diagonally

Lower pole (LL) is positive and upper pole (RA) is negative.

Lead II = LL minus RA

Lead III

Lead III points downward diagonally

Lower pole (LL) is positive and upper pole (LA) is negative.

Lead III = LL minus LA

Limb leads

Add the voltage in lead I to that in lead III and you get the volatage in lead II.

I = +2

III = +5

II = +7

Augmented limb (extremity) leads

Record the electrical voltages at one location rather than relative to the voltage at another electrode

Augmented limb leads

aVR + aVL + aVF = 0

Limb leads

Frontal plane

Limb leads

Electrode and Lead are not the same thing.

chest leads

V1 to V6

unipolar

horizontal plane

Electrode placement

V1: Fourth intercostal space to the right of the sternum.

V2: Fourth intercostal space to the Left of the sternum.

V3: Directly between leads V2 and V4.

V4: Fifth intercostal space at midclavicular line.

V5: Level with V4 at left anterior axillary line.

V6: Level with V5 at left midaxillary line.

Chest leads

Frontal Plane

Horizontal Plane

Monitor leads

12 leads are not always necessary

Sample of a monitor lead

V1 (positive)

Right shoulder (negative)

Left shoulder (ground)

Holter Monitors