ECG Flashcards
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?
SA node
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?
Problems
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?
About 280ms
How does repolarisation spread?
In the opposite direction to depolarisation over the ventricles
What repolarises first?
Epicardial (outer) surface
How do left ventricles contract?
In a cork screw way
How do atria contract?
Like fists
What is the advantage of the ventricular method of contraction?
It is more efficient and forceful
How many electrodes are attached in the ECG?
10
How many leads are there in the ECG?
12
In what planes is the heart inspected in the ECG?
Frontal
Horizontal
What are the types of leads in the ECG?
Unipolar
Bipolar
What leads are unipolar?
Chest leads (V1-V6) and augmented leads (AVR, AVL, AVF)
How do unipolar leads read?
From labelled positive electrode, and utilise several other electrodes as the negative
What are the bipolar leads?
I, II, III
What do bipolar leads use?
One positive and one negative electrode from the standard limb leads
What makes up Einthoven’s triangle?
Lead I, II and III
Where is lead I positioned?
From the negative electrode on the right arm to the positive electrode on the left arm
What is the purpose of lead I?
It is the ‘seeing’ electrode- the field of view
Where is lead II positioned in Einthovens triangle?
Goes from negative electrode on right arm to positive electrode on left leg
Where is lead III positioned in Einthovens triangle?
Goes from positive electrode on left leg to negative electrode on left arm
Where does lead III look?
From the left up, looking through the apex of the heart
Why is lead III the favourite lead?
Because it looks through the apex of the heart, and therefore good for looking the rhythm of heart
Where are limb leads placed?
Can be anywhere along extremities, but best over bones
Why are limb leads bess places over bones?
Because less distraction of signals
What does depolarisation moving towards an electrode lead to?
Positive deflection from the baseline
What does depolarisation moving away from an electrode lead to?
Negative deflection from the baseline
What does repolarisation moving towards an electrode lead to?
A negative deflection from the baseline
What does repolarisation moving away from an electrode lead to?
Positive deflection from baseline
Why does repolarisation cause a negative deflection when it’s moving towards you?
Because repolarisation is negative
What happens as an electrode is moved in relation to the heart?
It ‘sees’ a different deflection from the baseline
What will be seen if the current is moving directly towards the electrode?
A big spike
What will be seen if the current is passing near to the electrode?
There will be a peak/trough, but doesn’t have the same magnitude- will be a small change
What happens to deflection as you move the electrode around the heart?
It changes whether there is positive or negative deflection
What does the amplitude of a signal depend on?
How much muscle is depolarising
How directly towards the electrode the excitation is moving
How do different portions of the hear give different amplituded waves?
They have smaller or larger amounts of muscle
What causes the P wave?
Atrial depolarisation
What causes the Q wave?
Septal depolarisation spreading to ventricle
Do we always see the Q wave?
No
What causes the R wave?
Main ventricular depolarisation
What causes the S wave?
End ventricular depolarisation
What causes the T wave?
Ventricular depolarisation
What are the confounders to ECG readings?
Lead misplacement
Muscle contraction
Interference
Poor electrode contact
How can you tell if leads are placed wrong?
You get a completely wrong picture
What may cause confounding muscle contraction?
Movement
Shivering
Talking
Coughing
What can interfere with the ECG?
Alternating current from too close equipment
What can cause poor electrode contact?
Sweat
Cable pull
Hair
