Flashcards in The Electrocardiogram Deck (38)
Explain the spread of excitation through the atria ?
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
Explain what happens at the AVN during excitation of the heart
The AVN condition velocity is very slow compared to other tissues- 0.05m/s
This slow conduction delays the action potential by about 100ms
What is the electrical connection between the atria and the ventricles ?
Why is the delay at the AVN so important ?
It enables the atria to contract before ventricular excitation so the remaining 30% of blood can fill the ventricles
What happens from the AVN to the ventricles in the excitation of the heart ?
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
Which surface are the purkinje fibres concentrated on ?
What are the conduction velocities of the different tissues of the heart ?
Atrial muscle - 1ms
AVN - 0.05 ms
Bundle of his - 5 ms
Purkinje fibres - 5 ms
Ventricular muscle - 1 ms
What does an electrocardiogram do ?
It uses leads attached to the skins surface to measure the electrical currents of the heart
It is a useful diagnostic tool
Explain the 12 lead ECG?
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
What is the name of the triangle around the heart produced by the leads ?
If a wave of depolarisation travels towards the positive electrode what deflection is produced ?
What deflection occurs when the wave of depolarisation travels away from the positive electrode ?
What deflection do you get when a wave of repolarisation travels towards the positive electrode ?
What deflection do you get when a wave of repolarisation travels away from the positive electrode ?
Which aspect of the heart does the lead 2 ECG look at ?
Looks at electrical activity of the heart in the longitudinal axis
- signal tends to be dominated by the left ventricle
What does the P wave represent ?
What does the QRS wave represent ?
What does the T wave represent ?
Explain the P wave ?
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
Explain the Q deflection ?
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
Explain the QRS complex ?
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
When does atrial repolarisation occur on the ECG ?
Cannot see atrial repolarisation on the ECG because it is masked by ventricular excitation in the QRS complex
Explain the S-T segment ?
This is a baseline segment because the ventricles are fully depolarised so there is no potential difference
Explain the T wave ?
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
Why does the QRS complex have such a large spike ?
Because of the rapid spread of electrical activity and also because the spread of excitation is very unidirectional
What is the effect caused by repolarisation being a lot slower than depolarisation and less unidirectional ?
It causes a the T wave to be broader and have a lower amplitude
What does the P-Q interval represent ?
It represents the time between atrial excitation and ventricular excitation - about 160ms
Includes atrial contraction and conduction of ap through AVN
What may have occurred if an ECG indicates prolonged P-Q interval ?
May be damage to atrial conducting pathways
Damage to the AVN
Explain the S-T segment ?
This lasts about 120ms and it represents when the ventricles are fully depolarised
What condition causes the S-T interval to be elevated and how is it treated ?
It occurs in acute myocardial infarction
Treated with thrombolysis or percutaneous coronary intervention
What condition causes the S-T interval to be depressed ?
An ischaemic heart
Explain the Q-T interval ?
Lasts about 400ms
Period from ventricular depolarisation to ventricular repolarisation
What can cause a prolonged Q-T interval ?
Genetic syndromes- mutations of the sodium and potassium channels
What is the R-R interval ?
It is the time between successive heart beats
Used to calculate heart rate by dividing 60 with R-R
What is a junctional rhythm ?
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
What is 2nd degree heart block ?
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
What is ventricular fibrillation ?
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