Cardiovascular System: Lecture 4 - Electrical conducting systems of the heart Flashcards
How does contraction work in cardiac myocytes?
It is spontaenous / intrinsic
Explain the pathway of cardiac conduction in the heart
- SAN sends impulses that travel through internodal pathways to AV node
- Cells of the AV node transmit AP more slowly and delay the impulse
- Impulses spread down ventricles along the bundle of His
- impulses travel down the left and right bundle branch to the apex of the heart
- Impulses travel back up the purkinje fibres
What are the two main types of cardiac action potential?
Non-pacemaker and pacemaker action potentials
Describe how a non-pacemaker (myofibril contractile cell) action potential works (5 phases)
Phase 0- Na+ channels open
Phase 1- Na+ channels close - initial repolarisation
Phase 2- Ca2+ channels open; fast K+ channels close - flattening due to decrease in K+ permeability, and increase in CA2+ permeability
Phase 3- Ca2+ channels close; fast K+ channels open - rapid repolarisation
Phase 4 - Resting potential
What does pacemaker activity refer too?
the intrinsic, spontaneous time dependant depolarisation of a cell membrane that leads to an action potential
What is the primary pacemaker defined as?
The Primary Pacemaker is defined as the tissue with the highest ‘firing’ frequency, in other words the fastest pacemaker sets heart rate and overrides all slower pacemaker tissues.
Explain the 3 types of pacemaker and their hierarcy?
SAN node is fastest
Explain the decay of a pacemaker potential and why is it important?
-important as it determines the heart rate
What are the differences between pacemaker and nonpacemaker cells?
pacemaker - left
non pacemaker -right
Which class of drugs affect phase 0 of a cardiac action potential? give an example
class 1 - Na+ channel blockers
e.g lidocaine
prevents Na+ entering
Which class of drugs affect phase 2 of a cardiac action potential?
Class 4 - Ca2+ channel blockers
e.g verapamil
prevents Ca2+ entering
Which class of drugs affect phase 3 of a cardiac action potential? give example
Class 3 - K+ channel blocker e.g sotalol
prevents K+ leaving
Which class of drugs affect phase 4 of a cardiac action potential? (the resting potential)
Class 2 - beta blockers e.g propanalol
What do depolarising currents pass through?
Gap junctions located at intercalated discs
Why do we have a delay when electrical impulses reach the AVN?
otherwise the top of the ventricles would contract and push the blood to the bottom, so the blood wouldn’t come back up
What is the order of conduction speed at the different points that impulses travel through?
How many leads and electrodes are there in an ECG?
Leads - 12
Electrodes - 10
What is the order of the first 6 lead placements on the chest when carrying out an ECG?
Order and location:
V1 - Fourth intercostal space at the right border of the sternum
» V2 - Fourth intercostal space at the left border of the sternum
» V3 - Midway between placement of V2 and V4
» V4 - Fifth intercostal space at the midclavicular line
» V5 - Anterior axillary line on the same horizontal level as V4
» V6 - Mid-axillary line on the same horizontal level as V4 and V5
What is the Einthoven triangle?
Lead placement for the limbs
works by vectors
What does this mean on an ECG?
Electrical activity towards an electrode results in a positive deflection on ECG
What does this mean on an ECG?
Electrical activity away from an electrode results in a negative deflection on ECG
how is the diagnosis of a normal ECG made?
By excluding any recognised abnormality
We ignore the bit at the top that says ‘normal/abnormal’ as ECG’s are operator dependent so they must be analysed further
What do each of these components of an ECG represent?
P wave - represents atrial depolarisation / AV node delay
PR segment - conduction through AV node and AV bundle - delay
QRS complex - depolarisation of ventricles
T wave - repolarisation of ventricles
ST segment - full ventricular depolarisation
What type of rythm is this?
Normal sinus rhythm
