| What threshold is needed to overcome in the cardiac myocyte to generate a action potential | -70mv |
| General overview of the movement of AP in the myocardium | Activity starts at the SA node
Depolarisation occurs and spreads around the atria
To the AV node
Where there is a delay of 120ms
After the delay, activity spreads down the septum and then over the ventricular myocardium
From the inside surface to the outside surface
After about 280ms the cells start to repolarise |
| Why when using a ECG, you can only see depolarisation and repolarisation | Since it is put on the skin, hence only drastic changes in potential can be seen e.g. depolarisation and repolarisation |
| What is the direction of spread of a action potential | Atria --> Ventricle
Endocardial cells --> Epicardial cells |
| If you put a electrode on the apex, what would atrial depolarization look like | atrial depolarisation will produce a small upward deflection
Small since it is a small muscle
Upward since it is moving towards the electrode
P wave |
| If you put a electrode on the apex, what would spread from septum look like | Excitation spreads about halfway down the septum, then out across the axis of the heart
Produces a small downward deflection because :
Downward because it is moving away
Small because not moving directly away
Q wave |
| If you put a electrode on the apex, what would spread through the ventricular myocardium | depolarisation spreads through the ventricular muscle along an axis slightly to the left of the septum
produces a large upward deflection
Upwards since it is moving towards
Large because :
Large muscle
moving directly towards electrode
R wave |
| What would you see at the end of depolarisation in a ECG | depolarisation finally spreads upwards to the base of the ventricles
produces a small downward deflection
Downward because moving away
Small since it is not moving directly away
S wave |
| What would ventricular repolarisation show on a ECG | produces a medium upward deflection
upward because moving away since it is opposite of depolarisation
medium because timing in different cells dispersed
T wave |
| What are all the parts of the ECG | P wave - atrial depolarisation
Q wave - septal depolarisation spreading to ventricle
R wave - main ventricular depolarisation
S wave - end ventricular depolarisation
T wave - ventricular repolarisation |
| Why cannot you see atrial repolarisation | It gets overshadows by QRS complex hence does not show |
| Why does the location of the electrode matter? | This is because, the location of the electrode can affect the direction and size of the actual pulse |
| What are the different locations and views | 6 chest electrodes- V1-V6 which show 6 views of the heart
4 limb electrodes --> I, II, III, aVF, aVL, aVR |
