The Electrocardiogram

Question 0

Explain what happens at the AVN during excitation of the heart

Answer 0

The AVN condition velocity is very slow compared to other tissues- 0.05m/s
This slow conduction delays the action potential by about 100ms

Question 1

Explain the spread of excitation through the atria ?

Answer 1

Action potential spreads rapidly from the SAN through the atrial tissue at about 1m/s
Spread of excitation occurs by 2 ways
1- from cell to cell via intercalated discs
2- through specialised conduction pathways known as the intermodal tract This pathway takes the action potential directly from the SAN to the AVN just about the tricuspid valve
The excitation of the atria causes them to contact and force blood into the ventricles

Question 2

What is the electrical connection between the atria and the ventricles ?

Answer 2

AVN

Question 3

Why is the delay at the AVN so important ?

Answer 3

It enables the atria to contract before ventricular excitation so the remaining 30% of blood can fill the ventricles

Question 4

What happens from the AVN to the ventricles in the excitation of the heart ?

Answer 4

From the AVN the action potential passes down the bundle of his and this occurs rapidly at about 5ms
The bundle of his then splits into left and right bundles to innervate each ventricle. These go down to the apex of the heart
The bundle branches then innervate the purkinje fibres which excite the ventricular muscle
Action potential spreads through the ventricles by cell to cell coupling from endocardium to epicardium

Question 5

Which surface are the purkinje fibres concentrated on ?

Answer 5

Endocardial surface

Question 6

What are the conduction velocities of the different tissues of the heart ?

Answer 6

Atrial muscle - 1ms 
AVN - 0.05 ms
Bundle of his - 5 ms
Purkinje fibres - 5 ms
Ventricular muscle - 1 ms

Question 7

What does an electrocardiogram do ?

Answer 7

It uses leads attached to the skins surface to measure the electrical currents of the heart
It is a useful diagnostic tool

Question 8

Explain the 12 lead ECG?

Answer 8

3 bipolar leads, one attached to the left arm, one attached to the right arm and one attached to the left leg
6 unipolar leads attached to the chest - 1+2 mostly influence by rv and 5+6 mostly influenced by lv
The recordings of some of the leads are combined which makes up the other 3 leads

Question 9

What is the name of the triangle around the heart produced by the leads ?

Answer 9

Einthovens triangle

Question 10

If a wave of depolarisation travels towards the positive electrode what deflection is produced ?

Answer 10

Positive deflection

Question 11

What deflection occurs when the wave of depolarisation travels away from the positive electrode ?

Answer 11

Negative deflection

Question 12

What deflection do you get when a wave of repolarisation travels towards the positive electrode ?

Answer 12

Negative deflection

Question 13

What deflection do you get when a wave of repolarisation travels away from the positive electrode ?

Answer 13

Positive deflection

Question 14

Which aspect of the heart does the lead 2 ECG look at ?

Answer 14

Looks at electrical activity of the heart in the longitudinal axis
- signal tends to be dominated by the left ventricle

Question 15

What does the P wave represent ?

Answer 15

Atrial depolarisation

Question 16

What does the QRS wave represent ?

Answer 16

Ventricular depolarisation

Question 17

What does the T wave represent ?

Answer 17

Ventricular repolarisation

Question 18

Explain the P wave ?

Answer 18

Lasts about 80 ms
Represents movement of ap from SAN throughout atria
Wave returns to baseline once the atria are fully depolarised
There is a delay after this wave due to the delay at the AVN

Question 19

Explain the Q deflection ?

Answer 19

Results from the spread of the action potential across the the septum from left ventricle to right ventricle
This spread is not towards the positive electrode it is a negative deflection

Question 20

Explain the QRS complex ?

Answer 20

Ap spreads down the bundle branches towards the apex of the heart (+ve electrode) creating a positive deflection from Q to R
This spread this goes up he walls of the heart via the purkinje fibres (away from apex) creating a negative deflection
Signal is dominated by the left ventricle
It is about 80ms

Question 21

When does atrial repolarisation occur on the ECG ?

Answer 21

Cannot see atrial repolarisation on the ECG because it is masked by ventricular excitation in the QRS complex

Question 22

Explain the S-T segment ?

Answer 22

This is a baseline segment because the ventricles are fully depolarised so there is no potential difference

Question 23

Explain the T wave ?

Answer 23

This is when ventricular repolarisation is occurring and it occurs from epicardium to endocardium- this means the wave of repolarisation is moving away from the apex (+ve electrode) so a positive deflection is created
Wave lasts about 160ms

Question 24

Why does the QRS complex have such a large spike ?

Answer 24

Because of the rapid spread of electrical activity and also because the spread of excitation is very unidirectional

Question 25

What is the effect caused by repolarisation being a lot slower than depolarisation and less unidirectional ?

Answer 25

It causes a the T wave to be broader and have a lower amplitude

Question 26

What does the P-Q interval represent ?

Answer 26

It represents the time between atrial excitation and ventricular excitation - about 160ms Includes atrial contraction and conduction of ap through AVN

Question 27

What may have occurred if an ECG indicates prolonged P-Q interval ?

Answer 27

May be damage to atrial conducting pathways Or Damage to the AVN

Question 28

Explain the S-T segment ?

Answer 28

This lasts about 120ms and it represents when the ventricles are fully depolarised

Question 29

What condition causes the S-T interval to be elevated and how is it treated ?

Answer 29

It occurs in acute myocardial infarction | Treated with thrombolysis or percutaneous coronary intervention

Question 30

What condition causes the S-T interval to be depressed ?

Answer 30

An ischaemic heart

Question 31

Explain the Q-T interval ?

Answer 31

Lasts about 400ms | Period from ventricular depolarisation to ventricular repolarisation

Question 32

What can cause a prolonged Q-T interval ?

Answer 32

Myocardial damage Coronary ischaemia Conduction abnormalities Genetic syndromes- mutations of the sodium and potassium channels

Question 33

What is the R-R interval ?

Answer 33

It is the time between successive heart beats | Used to calculate heart rate by dividing 60 with R-R

Question 34

What is a junctional rhythm ?

Answer 34

The SAN isn't functioning so it doesn't cause a P wave | This causes a slowed heart rate because the AVN takes over as the pacemaker

Question 35

What is 2nd degree heart block ?

Answer 35

Only some of the P waves are conducted through the AVN | This causes 2 p waves per cycle because the QRS complex is missing - this means there must be a problem with the AVN

Question 36

What is ventricular fibrillation ?

Answer 36

Causes a chaotic ECG Occurs during acute heart attacks The ventricles can no longer pump blood properly so it causes death quickly if it isn't fixed

Question 37

What is supra-ventricular arrhythmia ?

Answer 37

It is like having 2 pacemakers Causes inverted P waves from the secondary pacemaker The QRS and T waves are normal The abnormal wave of excitation reset the SAN CAUSING MORE TIME BETWEEN THE FIRING AT THE SAN Leads to a drop in BP Because there is a longer time between each beat the heart fills with more blood and this produces a stronger heart beat whichC causes a thump in the heart = palpitation