Theme 3: Lecture 6 - Introduction to the ECG Flashcards
What is a syncytium
one large “cell” having many nuclei that are not separated by cell membrane
What is a functional syncytium
Many cells functioning as one
What are the three types of cardiac myocytes
- Pacemaker cells – for setting heart’s rhythm
- Conducting cells – for transmitting rhythm throughout the heart
- Contractile cells – for contracting to that rhythm (most numerous)
What is the speed of propagation for the different cardiac myocytes
- Contractile - atrial and ventricular myocytes 0.3-0.5 m/s
- Conducting system (modified cardiomyocytes), Purkinje fibres up to 5 m/s (fastest neurons ~100 m/s)
- AV node 0.05 m/s
How are cardiac myocytes linked
by low resistance pathways associated with gap junctions at the intercalated discs
What are gap junctions
They directly connect the cytoplasm of two cells, which allows various molecules, ions and electrical impulses to directly pass through a regulated gate between cells
What are intercalated discs
A lot of gap junctions located together
Describe the fibrous skeleton
- Electrical insulator located between the atria and ventricles
- Important as it means that for electrical impulses to be transported from the atria to the ventricles, they have to go through the AV node
Where do electrical impulses go once entering the AV node
- Bundle of His then
- Left and right bundle branches then
- Purkinje fibres
How are impulses conducted from the SA node to the AV node
by internodal bundles
Why are impulses conducted from SA node to AV node conducted by the internodal bundles
- Bundles ensure synchronous contraction of the atria
- Conducting via atrial muscle would be slow (0.3-0.5 m/s), conducting via bundles is much faster (1.0 m/s)
How many specialised bundles are in the atria
4
What do the specialised bundles in the atria contain
Purkinje like cells (cardiomyocytes modified to conduct) that are in direct contact with atrial muscle
Why does the AV node delay wave of excitation from atria to ventricles by 0.1 - 0.2 s
The electrical delay means that ventricles contract after atria to permit longer and more effective ventricular filling
How is the AP conducted very slowly in AV node (0.05 m/s)
- AV node composed of small modified myocytes which makes them slower in conducting
- Electrical connection between adjoining cells is weaker
What are Purkinje fibres
very large myocytes - transmit the impulse faster because bigger diameter cells conduct faster due to lower resistance
What do Purkinje fibres do
- Purkinje fibres transmit the impulse rapidly to the main mass of the ventricles from the Bundle of His
- From there, slower conduction between contractile myocytes can occur
When are different area of the ventricles depolarised
- First part of ventricular wall to be depolarised is septum, then apex
- Last part is atrioventricular groove
What is the ECG
- Electrocardiogram
- The ECG is a gross electrical measurement of the heart
How can the ECG detect a current when the individual currents of cardiac myocytes are tiny (nano amps)
because the heart is a “functional syncytium” in which large groups of cells all make electrical changes simultaneously
What can the ECG diagnose
Rate (but so can the pulse):
- Holter monitor (ECG) allows 24/7 rate determination
- Esp. useful when Atrial rate ≠ Ventricular rate
Many Subtleties:
- Not a one-stop-diagnosis
- Patient Hx essential for interpretation
What is a Holter monitor
a type of portable electrocardiogram (ECG). It records the electrical activity of the heart continuously over 24 hours or longer
What is a lead in an ECG
- a configuration of electrodes (usually consisting of a positive electrode, a negative electrode, and sometimes a ground).
- 2/3 electrodes placed on the body in standardised positions
What does what you see on an ECG depend on
Where you put the leads
