ECG

Question 1

Name the elements and positions of the conducting system of the heart

Answer 1

• Sinoatrial (SA) node Near Junction of Right atrium and SVC (top right)
• Atrioventricular (AV) node In inter- atrial septum near tricuspid valve (bottom left of atrium)
• The Bundle of His:
crosses fibrous ring of heart, from atria to ventricles
• Right bundle branches
Lies under endocardium on right side of IVS
• Left bundle branches
Lies under endocardium on right side of IVS
• Purkinje Fibres - fine branches of the bundles of His extensive throughout ventricular myocardium

Question 2

In which direction does depolaristion spread and why?

Answer 2

Depolarisation spreads downwards and to the left
slows down at AV node so atria contract before ventricles
bundle of his is continuous with av node

Question 3

What is the purpose of the fibrous rings of the heart?

Answer 3

Anchors myocardium and valves
Dense CT forming 4 fibrous rings
In plane between atria and ventricles
They are not made from myocardial cells - electrical insulator so cannnot conduct
Allows atria to contract separately from the ventricles

Question 4

Describe the speed o conduction in the heart

Answer 4

Sinoatrial (SA) node
• fastest rate (60 -100)
• sets rhythm – sinus rhythm

Atrioventricular (AV) node is continuous with
The Bundle of His: only conducting path from atria to ventricles

Right and left bundle branches

Purkinje Fibres
Rapid (4 metres/sec) spread of depolarisation throughout ventricular myocardium - fast so all parts of ventricle contract almost simultaneously

Question 5

What is the order of depolarisation in the heart?

Answer 5

SA node depolarised
Impulse spreads through atria
Held up at AV node
Spreads to ventricle via Bundle of His
Spreads rapidly down right and left bundles and purkinje system
Myocytes of IV septum are depolarised first
Apex and then left first then right ventricle walls
Base of the ventricles last
Repolarisation happens in reverse order i.e bae first, then walls then apex

Question 6

Where does ECG record electrical changes

Answer 6

EXTRACELLULAR SURFACE of cardiac myocytes

Question 7

Describe them yoy team in its resting state

Answer 7

Outside of the cell is +ve charged - flatline since no current flow

Question 8

Describe depolarisation on an ECG trace

Answer 8

• Depolarization (blue shaded area); the outside of the depolarized region becomes negatively charged relative to the inside - Na+ ions flow in
• Because the current of depolarization is directed toward the (+) electrode of the voltmeter an upward deflection is recorded.
• When the whole cell is depolarized; no current flow, so flat line.

Question 9

Describe repolarisation on an ECG trace

Answer 9

• the outside of the repolarized region becomes positively charged relative to the inside.
• Because the current of repolarization is directed AWAY from the (+) electrode of the voltmeter
• a downward deflection is recorded.
• When the whole cell is repolarized, no current flows, and a flat line recorded.

Question 10

Describe repolarisation of the ventricles

Answer 10

Repolarisation of the ventricles happens in the reverse order i.e. Last part of ventricle to depolarise is first to repolarise
• Repolarization proceeds in the direction opposite to that of depolarization in the intact heart
• Therefore, the deflections of depolarization (QRS complex) and repolarization (T wave) of the normal heart are oriented in the same direction.
• Note that the wave of repolarization is more prolonged and of lower amplitude than that of depolarization.

Question 11

Describe the wave which results from depolarisation from negative to positive

Answer 11

Positive complex, upright deflection

Question 12

Describe the wave which results from depolarisation from positive to negative

Answer 12

Negative complex, downward deflection

Question 13

Describe the wave which results from repolarisation from negative to positive

Answer 13

Negative complex, downward deflection

Question 14

Describe the wave which results from repolarisation from positive to negative

Answer 14

Positive complex, upright deflection

Question 15

How does the magnitude of deflection differ at different angles?

Answer 15

The magnitude of the deflection reflects how parallel the electrical force is to the axis of the lead being considered

• Depolarisation Wave coming directly towards (+) electrode Tall upright QRS complex
• Wave Obliquely towards electrode smaller upright QRS complex
• Wave At 900 to electrode Biphasic or No complex
• Depolarisation wave going directly away from electrode Deep -ve complex

Question 16

Describe SA node depolarisation

Answer 16

• SA node top right hand corner of Right atrium (RA)
• Near junction of SVC and RA
• First electrical event of cardiac cycle
• Insufficient signal to register on surface ECG

Question 17

Describe atrial depolarisation

Answer 17

• Spreads along atrial muscles fibres & internodal
pathways
• Throughout both right and left atria
• Direction: Downwards & to the left (Towards AV node)
• will produce a small upward deflection the p wave
• Upward because towards recording (+ve) electrode
• Lasts 80 – 100 ms

Question 18

Describe/explain what happens at the AV node

Answer 18

• Conduction is slowed down at AV node
• Allows time for atrial contraction to fill ventricle
• Signal is very small
• Isoelectric (flat line) segment
Fibrous ring between atria and ventricles No direct contact between atrial and ventricular myocytes

Question 19

What happens at the bundle of his?

Answer 19

• Fibrous ring only crossed by Bundle of His
∴ Depolarisation can only reach ventricle via this conduction through Bundle of His also gives an Isoelectric (flat) segment
• Thereafter rapidly conducted thru ventricle
• via left bundle and right bundle branches (LBB & RBB) and the Purkinje system 120 – 200 ms from start of atrial depolarisation to start of ventricular muscle depolarisation

Question 20

Describe depolarisation of the intraventricular septum

Answer 20

• First part to depolarise is Muscle in interventricular Septum
• Depolarisation spreads from left to right
• May produce a small downward deflection because moving obliquely away (or no deflection may be seen)
• Termed a Q wave = first downward deflection after p
• usually called ‘q’ (lower case) because small

Question 21

Describe the depolarisation of the apex and free ventricular wall

Answer 21

• produces a large upward deflection
• Termed the R wave
• upward because depolarisation moving directly towards electrode
• large because large muscle mass – more electrical activity
• If left ventricle was hypertrophies – Then R wave will be
correspondingly taller

Question 22

Describe the end of depolarisation

Answer 22

• depolarisation finally spreads upwards to the base of the ventricles
• produces a small downward deflection

• ‘S’ of the QRS

• downward because moving away
• small because not moving directly away

Complete venricular muscle depolarisation (QRS complex) takes 80 -120 ms

Question 23

Describe ventricular repolarisation

Answer 23

• begins on the epicardial surface
• spreads in the opposite direction to depolarisation
• produces a medium upward deflection - The T wave
• upward because it is a wave of repolarisation moving away from electrode

Question 24

Give an overview of the P Q R S and T waves

Answer 24

• P wave : atrial (muscle) depolarisation (N.B. not contraction - contraction will follow depolarisation)
• QRS : ventricular (muscle) depolarisation (N.B. not contraction)
All components of QRS may not be seen in all leads
• Q : initial downward deflection after p wave
• R : initial upward deflection after p wave
• S : downward deflection after the R

Question 25

Where are the ECG electrodes places

Answer 25

``` V1 - 4th intercostal space on right side V2 - 4th intercostal space on left side V3 - between 2 and 4 V4 - midclavicular line in 5th space V5 - between 4 and 6 V6 - midaxillary line ``` Limbs - one on each upper limb one on each lower limb - stick on bony prominences eg ankles to avoid interference from muscles

Question 26

What are leads

Answer 26

The views in each plane

Question 27

What are the limb leads

Answer 27

6 views in the vertical plane I: -ve @ right upper limb, +ve at left upper limb (looking across from left to right) II: -ve at right upper limb, +ve at left lower limb (looking diagonal up from apex) III: -ve at left upper limb, +ve at left lower limb (looking diagonal up) aVR: +ve at right upper limb (looking diagonal down) aVF: +ve at left lower limb (looking up from feet) aVL: +ve on left upper limb (looking diagonal down) See slide for images

Question 28

In which limb leads will you not see a q wave

Answer 28

Won’t see q wave in III, AVF AVR bc it would be positive so lost in the positivity of q wave only see q in leads looking from left hand side

Question 29

Draw the ecg trace for each limb lad

Answer 29

See slide

Question 30

Which leads look from which directions and what ar they good at looks for problems with?

Answer 30

• Leads I and aVL are looking at the left side of heart best limb leads for looking at problems in lateral wall of the left ventricle e.g. muscle necrosis due to occlusion of branch of left coronary artery - a lateral myocardial infarction • Leads II, III and AVF are looking at the inferior surface of the heart. best leads to detect problems in the inferior surface of the heart (diaphragmatic surface ) e.g. muscle necrosis due to occlusion of right coronary artery - an inferior myocardial infarction

Question 31

What are the check leads

Answer 31

- V1 and V2 Face the right ventricle & septum = “septal leads” V3 and V4 Face the apex and anterior wall of ventricles = “anterior leads” V5 and V6 Face the left ventricle “Lateral leads”

Question 32

Draw the ecg traces for chest leads

Answer 32

See slides

Question 33

Which leads give vies of inferior surface of ventricle?

Answer 33

II, III, aVF

Question 34

Which leads give views of the right ventricle and septum?

Answer 34

V1 and V2

Question 35

Which leads give views of the anterior surface of the ventricles?

Answer 35

V3 and V4

Question 36

Which leads give views o the IVS and anterior surface of the ventricles?

Answer 36

V1 to V4

Question 37

Which leads give views of the lateral surface of the ventricles?

Answer 37

I, aVF, V5, V6

Question 38

What does each square represent on ECG paper

Answer 38

Large square = 5mm = 0.2s Small square = 1mm = 0.04s Therefore 5 large square = 1 second 300 large squares = 1 minute

Question 39

How is the heart rate calculated?

Answer 39

• Each PQRST complex (from start of P wave – to start of next P)= 1 cardiac cycle =1 heart beat • Count number of large boxes between complexes (Easier to count R – R interval than P – P interval) R – R interval • How many complexes could be fitted into 300 large boxes ( i.e. 1 minute?) in this example = 300/ 4 (number of large boxes between two R waves) • Heart rate = 300/4 = 75 beats per minute

Question 40

How is heart rate calculated if the rhythm is irregular?

Answer 40

the rhythm is irregular | • Calculate heart rate by counting the number of QRS complexes in 6 seconds, then x by 10

Question 41

What is the typical ref range for the PR interval?

Answer 41

PR interval 0.12 – 0.20 seconds 3 – 5 small boxes Prolonged if > 1 large box Prolonged PR interval: delayed conduction through AV node and bundle of His

Question 42

What is the typical reference range for the QRS interval?

Answer 42

``` QRS interval (width of QRS complex) Time taken for ventricular depolarisation < 0.12 seconds < 3 small boxes Widened QRS: (usually) a depolarisation arising in ventricle; not spreading via the His-Purkinje system; hence takes more time ```

Question 43

What is the typical reference range for the QT interval?

Answer 43

QT interval Time taken for depolarisation & repolarisation of ventricle • Varies with heart rate • Calculation to correct for heart rate (charts) - Corrected QT interval (QTc) Upper limit of corrected QT (QTc) interval: ≤ 0.44 – 0.45 seconds (11 small boxes) Prolonged QTc: indicates prolonged ventricular repolarisation

Question 44

What is normal sinus rhythm?

Answer 44

* Depolarisation is initiated by sinus node * Is the rhythm regular? * Heart rate? (60 -100 bpm) * Are there p waves? * Are P waves upright in leads I, II ? * Is PR interval is normal (3 – 5 small boxes)? * every p wave followed by QRS? * Every QRS is preceded by a p wave? * Normal QRS width (≤3 small boxes)