Basic Electrocardiography

Paula HignettCardio-Respiratory Department

Royal Preston Hospital

Electrocardiography• Definition & Indications• The conduction system• The ECG in relation to the conduction

system• Normal ECG complex’s• Performing a 12 lead ECG

What is an ECG?• Electrocardiography is a

method of obtaining electrical activity of the heart via surface electrodes placed on the body

• The machine used to record the waveforms is called an electrocardiograph and the tracing of the electrical activity of the heart is called an electrocardiogram(ECG)

Indications For a 12 lead ECG• 1st line investigation when a more in-depth

knowledge about the hearts is required.• Can help identify or exclude suspected cardiac

pathologies including:• Enlarged chambers or thickened muscle within

the heart, • Electrolyte disturbances• Conduction abnormalities• Evidence of ischaemia or MI/MI screen• Pre and post operative assessment-guidelines

Basic Principles

To be able to understand the waveforms seen on the ECG an understanding of the

electrical system of the heart is needed:The conduction system

The Conduction System3.AV node 4.His bundle

1.SA Node

2.Internodal pathways

6.Right bundle branch

7.Purkinje fibres

5.Left Bundle

5a)posterior fascicle

5.b Anterior fascicle

1.

Back-up Mechanism Of The Heart

• The normal spread of impulses starts at the SA node

• If the SA node fails to generate an impulse the next part the conduction system will try

• Any part of the conduction system is capable of generating an impulse

• Each component has its own ‘intrinsic ‘ rate

• The lower down the conduction system the lower the heart rate

60-100bpm

45-50bpm 40-45bpm

35-40

The ECG in relation to the conduction system

• The P wave represents the spread of impulses across the atria resulting in contraction of the atria

• The P-R interval is the delay at the AV node• The QRS complex is the spread of impulses across the

ventricles which results in contraction of the ventricles• The T wave represents ventricular relaxation

P

Q

RS T

Standard ECG settings

1. Standard paper speed is 25mm/sec. Each small square is equal to 0.04 secs and 1 large square is 0.2 secs

2. Standard setting for calibration is 10mm/mV. Allows measurement of amplitude. Ensures correct sensitivity of the machine

1 2

Horizontal linesShow amplitude

Vertical lines show Time intervals

The P Wave

• The first positive deflection• Represents atrial contraction• Slow rounded wave• Duration between 0.08 and 0.11secs (2 ½ small squares)• Height less than 2.5mm (2 ½ small squares) • Upright in lead II (usual lead for a rhythm strip)• Inverted in aVR• Abnormal in atrial hypertrophy

P P P P

The PR interval• The time it takes the

impulses to travel from the atria to the AV node (atrio-ventricular conduction time

• Measured from the onset of the P wave to the onset of the QRS complex

• No more than 5 small squares in duration (0.20secs)

• Prolonged PR interval >0.20secs is 1st degree heartblock

1st Degree Heartblock

• Increased delay at the AV node• Prolonged PR interval > 1 large box or 5 small squares• PR interval is approximately the same for each complex• Normal P waves and QRS complexes• Regular rhythm • Usually benign

The QRS Complex• Represents ventricular

contraction• Measured from the onset

of the Q to the end of the S wave

• Predominantly Upward in left sided leads and negative in negative in right sided leads

• Between 0.08 and 0.12 secs in duration (3 small squares)

Q

R

S

Widened QRS complex

• A QRS > 0.12 seconds• Abnormal conduction of impulses across the ventricles• A block somewhere either in the his bundle or bundle

branches preventing normal sequence of events in the ventricles

The Q wave

• Reflects impulses travelling down the septum

• The first negative component of the QRS complex

• Should be less than 25% of the associated R wave

• Abnormal Q waves can indicate previous MI

•

Abnormal Q waves

Normal

The T wave

• Represents ventricular relaxation

• Rounded peak• Should be in the same

direction as the main QRS complex

• Abnormal if inverted, seen in LVH, Bundle branch blocks & ischaemia

Abnormal T waves

T wave inversion e.gIschaemia/old MI

Deep symmetrical T wavee.g subendocardial MI

normal

Tall peaked T wave e.g Hyperkalaemia

The ST segment

• Measured between the end of the QRS and the beginning of the T wave

• Should be no more than 1mm above or below the baseline

Baseline

Abnormal ST segments

• ST segments which are below an imaginary baseline is called ST depression.Typically seen in angina.

• ST segments above the baseline are evident during the acute stage of an MI and indicates muscle injury. Can also be seen in pericarditis and printzmental angina

ST Depression ST Elevation

Sinus Rhythms

• Beats are ‘sinus’if the P wave have originated from the SA node and the impulses have been conducted in the normal way. Meaning they have a normal PQRST complex.

• Normal sinus rhythm 60-100beats/min• Sinus Bradycardia Below 60bpm• Sinus Tachycardia Above 100bpm

Performing a 12 lead ECG

• Lie patient in a relaxed supine position with arms resting by their side

• Prepare skin in accordance to the manufacturers recommendations. ‘Biotabs’ require skin abrasion to reduce skin impedance. Shaving may be necessary.

• Apply the electrodes • Attach the limb and chest leads to the electrodes

via crocodile clips…….

(Extremity) limb leads• Colour coded electrodes

are attached to the wrists and ankles

• Right leg electrode functions as an electrical ground only. It prevents alternating current interference and can be ignored

• Can be placed anywhere on the arms or legs providing they are placed at least 10cm from the heart

aVF

N

N

aVR aVL

Limb leads-aVR, aVL & aVF (Unipolar)

• The abbreviation ‘a’ refers to augmented. A definition of augmented is to increase in size which simply means that the ECG machine amplifies the signal to make it more readable.(the machine augments the reading by 50%)

• The ‘V’ refers to voltage• R, L and F refer to right

arm, left arm and left foot

aVF

aVR aVL

Viewpoints of aVR, aVL, aVF

• The limb leads look at the heart from the side at a particular angle

• aVR looks down at the heart from the patients right shoulder

• aVL looks at the heart from the left shoulder

• aVF looks up at the heart from the left leg

Leads I, II, II• Difference in voltage

between 2 extremities• Lead I-difference in

voltage between the left arm and right arm

• Lead II-difference in voltage between the left leg and right arm

• Lead III-difference in voltage between the left leg and left arm

The 6 Chest Leads (V1-V6)• Leads are placed in a

horizontal plane• They record voltages

moving anteriorly • Each of the leads has its

own sight and a region of the heart it views best

• V1-V2 Right side heart• V3-V4 Interventricular

septum towards apex• V5-V6 Left side heart

Chest Lead placement• V1-4th intercostal space

on the right sternal border• V2-4th intercostal space

left sternal border• V3-horizontally between

V2 + V4• V4-5th intercostal space

in the mid-clavicular line• V5-same horizontal plane

as V4 at the anterior axillary line

• V6-same horizontal plane as V4 in the mid axillary line

Lead Systems Summary• The 12 leads provide a 3-D view of the electrical activity

of the heart• 6 extremity leads record voltages from the frontal plane of

the heart. These leads consist of the bipolar (I,II,III) and unipolar leads(AVR,AVL,AVF). These are recorded from the limb leads

• The chest leads record voltages from the heart in the horizontal plane by placing electrodes in specific anatomical positions

• Although we only use 10 leads we are getting 12 bits of information, which represent different areas of the heart.

The 12 lead ECG

I

V3

II

III

II

aVR

aVF

aVL

V4

V5

V6

V1

V2

Useful Tips

• Ensure paper speed and calibration signal is correct

• Chest electrodes are identified using external surface markers-don’t just guess!

• Ensure don’t transpose leads• Long rhythm strip(lead II) most useful identifying

rhythms.May need 2 pages to look for patterns.• Lead II often has waveforms with a good

amplitude for assessing complexes in more detail.

Thank-you Good luck with the rest of your study

Any Questions?