ECG Flashcards
What do myocytes have the ability to do?
Generate action potentials and rapidly conduct them throughout the heart.
This results in a wave of depolarisation through the rest of the myocardium.
This depolarisation ends with coordinated contraction of atria and ventricles.
What is the conducted system of the heart made up off?
Sinoatrial (SA) node. Near the junction of the right atrium and the SVC (top right)
Atrioventricular (AV) node. In inter-atrial septum near tricuspid valve (bottom left of atrium) Slow conduction through the AV node.
The bundle of His. Crosses fibrous rings 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 throughput ventricular myocardium.
What is the role of the fibrous ring?
- It is a layer of dense connective tissue forming four fibrous rings in a plane between the atria and the ventricles.
- It anchor the valves and the myocardium - It is the structure where all muscles insert.
- Continuous with IVS
- Not made of myocardium so can’t conduct electricity (An insulator).
- Its Insulating properties allow the atria to contract separately from the ventricles.
What is the role of the SA node?
Fastest rate (60 -100) so it I sets the rhythm.
What is the role of the AV node?
The AV node slows conduction. This gives to for the atria to contract before the ventricles.
It is continuous with the Bundle of His
What is the role of the His purkinje system?
Bundle of His is the only conducting path from atria to ventricles.
Then, depolarisation goes down the left and right bundle branches and purkinje fibres. This produces a rapid (4m/sec) spread of depolarisation through the ventricular myocardium.
Describe the pathway of the electrical impulse.
SA node depolarises
Impulse spreads through atria
Held up at the AV node.
Spreads to ventricles via bundle of His.
Spreads rapidly down the right and left bundles and purkinje system.
Myocytes of IV septum are depolarised.
Apex and left and right ventricle walls are depolarised next.
Base of the ventricles are the last to be depolarised.
Last parts to be depolarised are the first parts to be repolarised. It goes backwards.
What specifically is picked up on an ECG trace?
Muscle depolarisation and NOT electrical conduction.
It records electrical changes of the extracellular surface of cardiac myocytes.
How does an ECG trace show depolarisation?
Resting cell - outside of cell is positively charged.
Depolarisation - The outside of the depolarised region becomes negatively charged relative t the inside.
Because the current of depolarisation is directed towards the electron of the voltmeter an UPWARD DEFLECTION is recorded.
When the whole cell is depolarised, no current flow so flat line.
How does an ECG trace show repolarisation?
The outside of the depolarised region becomes positively charged relative to the inside.
Because the current of repolarisation is directed AWAY from the + electrode of the voltmeter. A DOWNWARDS DEFLECTION is recorded.
When the whole cell is repolarised, no current flows and a flaat line is recorded.
BUT, repolarisation of the ventricles happens in the reverse order
This means that the deflections of depolarisation (QRS complex) and repolarisation (T wave) of the normal heart are orientated in the same direction.
Althoguh the wave of repolarisation is more prolonged and of lower amplitude than that of depolarisation.
Describe how the direction of the wave correlates with the orientation of the complex and deflection.
.
How do you view the electrical activity from the apex?
The magnitude of the deflection reflects how parallel the electrical forces is to the axis of the lead being considered.
When causes the P wave on an ECG?
Atrial (muscle) depolarisation.
This is when depolarisation spreads along atrial muscles and internodal pathways.
This goes through both the right and left atria downwards and towards the left (Towards the AV node).
The P wave is upwards because it moves towards the +ve electrode.
Atrial depolarisation lasts 80-100ms.
What happens on an ECG during the delay at the AV node?
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 measn there is no direct contract between atrial and ventricular myocytes.
What happens on an ECG during depolarisation down the bundle of His?
Fibrous ring is only crossed by the Bundle of His.
Depolarisation can only each ventricles via this conduction through Bundle of His also gives an Isoelectric (flat) segment.
Thereafter rapidly conducted through the ventricle via the left and right bundle branches and the purkinje system.
1200-200ms from the start of atrial depolarisation to start of ventricular muscle depolarisation.
What are the 3 bipolar limb leads (views in the vertical plane)?
Limb 1 = Current flowing left to right - horizontally.
Limb 2 = Apex view (What we drew in ECG)
Limb 3 = Current flowing kinda opposite to limb 2
