Flashcards in CVS Session 7- The ECG and electrical activity of the heart Deck (27)
Which wave is absent in atrial fibrillation?
What is the Electrocardiogram?
The electrical signal produced by the myocardium of the heart when electrical changes occur.
Is is detected by electrodes placed on the body surface.
It reflects the electrical activity of the heart.
What do electrodes record?
Changes in membrane potential
What combination of effects explains the ECG?
-depolarisation and repolarisation
-their spread over the heart
How many action potentials does one heartbeat produce?
Describe conduction over the heart (5)
1. SA node fires action potential and depolarisation spreads over the atria to the AV node.
2. Delay of 120ms at the AV node.
3. Activity spreads down septum.
4. Spreads over ventricular myocardium from endo to epicardium.
5. Repolarisation occurs from epi to endocardium.
Which way does depolarisation of ventricular myocytes occur?
From endocardium to epicardium
Which way does repolarisation of ventricular myocytes occur?
From epi to endocardium
What does the view of the electrode depend on?
It's position relative to the spread of activity.
Which way does the signal go if depolarisation is moving towards the electrode?
Which way does the signal go if depolarisation is moving away from the electrode?
Which way does the signal go if repolarisation is moving towards an electrode?
Which way does the signal go if repolarisation is moving away from the electrode?
What does the amplitude of the signal depend on? (2)
-how much heart muscle is depolarising
-how directly towards the electrode the excitation is moving
Why is the area under the QRS wave and T wave the same?
Because it is the same heart muscle, producing the opposite action.
What does each wave: P, Q, R, S and T represent?
P- atrial depolarisation
Q- septal depolarisation spreading to ventricles
R- main ventricular depolarisation
S- end of depolarisation
T- ventricular repolarisation
Why does atrial repolarisation get lost in the QRS wave? (2)
-small amount of muscle therefore small signal
-happens at the same time as ventricular depolarisation
What happens to the R wave if viewed end on?
Large downward signal
How many inputs do amplifiers have?
One positive and one negative electrode.
How do amplifiers work to turn two inputs into one?
Take the signal coming in on the negative electrode and invert it.
Then combine it with the positive input signal.
Then amplify the total.
From which direction does lead I view the heart?
From the LHS
From which direction does lead II view the heart?
From which direction does lead III view the heart?
What is the QT interval?
The length of time taken for the ventricles to depolarise and repolarise.
Where are the limb leads placed, and what are the corresponding colours?
Red- right upper
Yellow- left upper
Green- left lower
Blue- right lower
What are the positions of the chest leads? What are their corresponding colours?
V1- red- RHS 4th intercostal space
V2- yellow- LHS 4th intercostal space
V3- green- LHS between V2 and V4
V4- brown- LHS 5th intercostal space, mid clavicular line
V5- black- LHS between V4 and V6
V6- purple- LHS 5th intercostal space, mid axillary line