11. Interpreting ECGs Flashcards
Define depolarisation
This is a change within a cell that causes the charge inside of the cell to become less negative. This occurs through the movement of ions e.g the movemnt of Na+ ions out of the cell.
Define repolarisation
This is a change within a cell that causes the charge inside of the cell to become more negative. This occurs through the movement of ions e.g the movement of K+ ions out of the cell.
Define resting membrane potential
This refers to the charge across the membrane of the cell when the cell is at rest and not undergoing any significant changes in ion concentration.
Define action potential
This is a brief reversal of the cell membrane polarity which is then propagated from cell to cell.
Define deflection
This refers to a deviation from the baseline.
In an ECG this is either an upward or downward wave from the baseline.
What is the function of an ECG?
An ECG is an electrocardiogram and it’s used to record the electrical activity of the heart from different angles to help to identify and locate pathology.
What causes contraction and relaxation of cardiac muscle?
Contraction and relaxation of cardiac muscle results from the depolarisation and repolarisation of myocardial cells
How does the electrical activity spread through the heart at the cell level?
- Electrical activity spreads through the cardiac myocytes of the heart.
- The action potential is generated in the primary pacemaker cells of the sinoatrial node in the right atrium.
- The action potential will be propagated from cardiac cell to cardiac cell in a depolarisation wave.
- All the cardiac cells are electrically connected through gap junctions which allows the organised transmission of electrical activity.
- The depolarisation wave causes coordinated contraction of the atria and ventricles through excitation-contraction coupling.
How does the electrical activity spread through the heart at the tissue level?
- The action potential is initiated at the SAN
- The depolarisation will pass through the right and left atrium.
- The electrical activity will then reach the AVN. The function of the AVN is to slightly slow down the action potential to allow full contraction of the atria.
- From the AVN the action potential travels down the bundle of HiS, these are fibres that travel through the Annulus Fibrosis.
- The bundle of His will enter into the interventricular septum where it’ll divide into the right and left bundle branch
- Each bundle branch travels along their respective interventricular septum side
- The right and left bundle branches will terminate at the Purkinje fibres at the apex and these will continue to conduct the depolarisation wave up to the ventricles
Where is the SAN?
at the junction of the right atrium and the superior vena cava.
Where is the AVN?
located in the inter-atrial septum near the tricuspid valve
What is the bundle of His?
Wide, fast, conducting muscle fibres that travel through the annulus fibrosis
Where does the right bundle branch travel and what does it excite?
travels along right side of interVENTRICULAR septum
– excites right ventricle
Where does the left bundle branch travel and what does it excite?
travels along left side of interVENTRICULAR septum
– excites left ventricle
Describe the Annulus fibrosis, including its function
- The annulus fibrosis is the fibrous skeleton of the heart.
- It’s used to anchor the myocardium and cardiac valves.
- It also acts as an electrical insulator between the atria and the ventricles.
- It consists of 4 fibrous rings.
List and describe the 3 parts of the heart that can initiate an action potential within myocardial cells
The SAN, AVN and LBB& RBB all possess an electrical conducting system and can all respectively initiate action potentials.
However the speed of depolarisation varies between them and the fastest conducting system(SAN) will over power the rest.
SAN> AVN> LBB & RBB
What is the function of the SA node?
- Sets heart rate and rhythm– SINUS rhythm
* Fastest rate of depolarisation in the heart
What are the 2 meanings of ECG lead
- It’s the cable that’s used to connect the electrode to the ECG recorder
- It’s the electrical view of the heart that’s obtained from any one combination of electrodes
What is an electrode?
An electrode is a conductive pad that’s attached to the skin (limbs and chest) and is used to record electrical activity.
What does an ECG measure?
Electrical activity of the heart as it is transmitted to chest wall and limbs
Measures it over a time
Respectively how many leads and electrodes are there in an ECG?
In a 12 lead ECG you have 12 leads(12 views of heart) but only 10 physical electrodes - 4 on limbs (A) and 6 on chest (B).
What is the function of the Right leg electrode?
grounding electrode – Not used for any leads/views
What is the order of waves and intervals in an ECG?
P wave
P-R interval
QRS complex
S-T segment
What does the P wave represent?
The P wave represents atrial depolarisation.
If the patient is in sinus rhythm there will ALWAYS be a p wave before the QRS complex.
The P wave is upwards because the depolarisation wave is towards the positive electrode in lead II. Direction of atrial depolarisation: Downwards & to the left (Towards AV node)