Flashcards in ECG Deck (72)
What is the myocardium?
A large mass of muscle
What is the myocardium undergoing?
Electrical changes, all more or less at the same time
What does the electrical changes going on in the myocardium generate?
A large, changing electrical field
How can the electrical field produced by the myocardium be detected?
By electrodes on the body surface
What do electrodes outside the cell record?
Only changes in membrane potential
What do the skin electrodes ‘see’ with each systole?
Two signals, one depolarisation, one repolarisation
How does excitation spread over the myocardium?
Due to the interlinking of cardiac myocytes by gap junctions
What does the spread of excitation spreading over the myocardium generate?
A changing signal, which the electrodes detect
Why do we need gap junctions?
To produce coordinated depolarisation, and therefore coordinated contraction
What do gap junctions allow?
A pause before the ventricles contract after the atria contract
What is the importance of the pause before ventricles contract?
It allows for a more rapid spread through the myocytes, so more of the tissue is contracting at once
How is the ECG explained?
By a combination of the effects of depolarisation and repolarisation, and their spread over the heart
What is the starting point in conduction of the heart?
What is the passage of conduction through the heart?
SA node → internodal tracts → AV node → Bundle of His → Left bundle branch and right bundle branch → Purkinje fibres
Where are the internodal tracts?
Between the nodes
What do the internal tracts throw off?
An extra limb of depolarisation by Bachmanns bundle
What does Bachmanns bundle allow for?
Smooth contraction of the atria
What does the AV node do?
Holds everything for a moment so there is a pause in the signal to allow all of the atria to contract
What is the Bundle of His needed for?
Depolarisation to travel through to get the ventricles
How are the atria and ventricles related electrically?
They are separate, apart from through the bundle of His
What will electrical non-seperation cause of the atria and ventricles cause?
How does the left bundle branch differ from the right bundle branch?
It is fractionally faster
Why is the left bundle branch slightly faster than the right bundle?
Because the left ventricle is larger than the right
Where does the left bundle branch reach?
The apex of the left ventricle
What does the right bundle branch do?
Feeds the signal to the right ventricle
What is the result of the left and right bundle branches reaching the apex of heart?
Blood is pumped out from the bottom upwards, so blood is pushed out through large vessels
What do Purkinje fibres do?
Feed the rest of the myocardium
How long is it before cells begin to repolarise?
How does repolarisation spread?
In the opposite direction to depolarisation over the ventricles