Limb Leads

Chest Leads

Timing

Calculations

ECG Lead

ECG leads

ECG leads

Lead systems allow you to look at the heart from different angles. Each different angle is called a lead.

Each lead has a positive and negative pole attached to the surface of the skin, which can then be used to measure the spread of electrical activity within the heart.

ECG leads

Upward deflection on the ECG- is produced when electrical impulses travel towards a positive electrode.

Downward deflection on the ECG- is produced when electrical impulses travel towards a negative electrode.

Flat line (isoelectric line)- is produced when there is no electrical spread through the heart, or if the electrical forces are equal.

Limb leads

Limb leads

• Lead 1

• Negative right shoulder

• Positive left shoulder

• Lead 2

• Negative right shoulder

• Positive left lower chest

• Lead 3

• Negative left shoulder

• Positive left lower chest

Electrical current moving

from negative electrode to positive electrode

Einthoven’s Triangle

They are called the augmented limb leads because they are augmented (or amplified) through a modification of

Wilson’s Central Terminal (WCT). The modification was necessary because otherwise the complexes would have

been too small

aVR – positive electrode right shoulder

aVL– positive electrode left shoulder

aVF – positive electrode left

lower chest (foot)

Chest Leads

Chest Leads

Unlike limb leads that measure electrical activity in the vertical plane, the

precordial leads measure activity in the horizontal plane. Each of the 6 electrodes are set as positive

V1 = right ventricle and far left ventricle V2 = right ventricle and AV node

V3 = anterior left ventricle

V4 = anterior left ventricle

V5 = lateral left ventricle

V6 = lateral left ventricle

Gives a 2 dimensional picture of what is going on electrically in the heart

12 Lead ECG Placement

Rhythm strip

Calculations of Axis

Normal Cardiac Axis In healthy individuals you would expect the normal 11 o’clock

to 5 o’clock spread

Therefore the spread of depolarisation would be

heading towards leads I,II & III

As a result you would see a positive deflection in all of these

leads

With lead II been the most positive (it’s at 5 o’clock)

You would expect to see the most negative deflection in aVR

This is due to aVR looking at the heart in

the opposite direction to lead II

Right axis deviation

Right axis deviation (RAD) is usually caused by right ventricular hypertrophy.

In right axis deviation the direction of depolarisation is distorted to the right (1-7 o’clock)

Extra heart muscle causes a stronger signal to be generated by the right side of the heart

This causes deflection in lead I to become negative & deflection in lead II & III to be more +ve

RAD is associated with pulmonary conditions as they put strain on the right side of the heart

Left axis deviation In left axis deviation (LAD) the

general direction of depolarisation becomes distorted

to the left

This causes the deflection in lead III to

become negative

It is only considered significant if

the deflection of Lead II also becomes negative

LAD is usually caused by conduction defects & not by

increased mass of the left ventricle

Axis trick

Positive in I and II = normal

Positive in I

Negative in II = LAD

Negative in I

Positive in II = RAD

Timing

Timing

Timing

Rate

R-R interval

Is it regular?

What is the heart rate?

300, 150, 100, 75, 60, 50

300 / (# of large boxes)

1500 / (# of small boxes)

Timing

Are there P waves….?

Normally =0.08 s = 2 small sqrs

Pointy = P pulmonale (RA hypertrophy)

Bifid = P mitrale (LA hypertrophy)

PR interval

Start of P wave to start of QRS

Normal = 0.12-0.2s

Too short – can mean WPW

Too long –means AV block (heart

block) - 1st/2nd/3rd degree

QRS complex

Should be <0.12s duration

>0.12s = BBB (either LBBB or RBBB)

QRS amplitude

R in V5 or V6 < 2.6 mV

Increased amplitude indicates cardiac hypertrophy

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Timing

ST segment connects the QRS complex and the T wave and has a duration of 0.08 to 0.12 secR-R interval

ST depression

Downsloping or horizontal = abnormal

Ischaemia (coronary stenosis)

ST elevation

Infarction (coronary occlusion)

Pericarditis (widespread)

Timing

T wave 160ms

Peaked (hyperkalaemia or normal young man)

Inverted/biphasic (ischaemia, previous infarct)

Small (hypokalaemia)