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
Describe a 12 lead ECG
looks at the heart from 12 different angles, creating measurements for 12 leads, using 10 separate electrodes.
Describe lead II
Lead II has the positive electrode on left leg, negative electrode on right arm, and the ground electrode on the right leg (although the ground could be almost anywhere).
What are the 12 standard leads of an ECG
- 3 bipolar leads
- 3 augmented leads
- 6 precordial
What are the 3 bipolar leads
I, II, III
What plane do the bipolar leads view the heart in
frontal
What are bipolar leads
they have a positive and a negative electrode at opposite ends of the heart
What does the P wave show
- Depolarisation of the atria in response to SA node triggering
- So small it’s never normally seen on an ECG
What does the PQ segment show
Delay of AV node to allow filling of the ventricles
What does the QRS complex show
Depolarisation of ventricles, triggers main pumping contractions
What does the ST segment show
- Beginning of ventricle repolarisation
- Should be flat and at the same level as before the P wave and during the PQ segment
What does the T wave show
Ventricular repolarisation
What does a wide or misshapen QRS complex show
- Abnormal ventricular conduction
- E.g from ectopic pacemakers or bundle branch block
What are large (deep) Q waves a sign of
dead tissue (old MI)
Describe sinus rhythm
- When the heart rhythm is generated from the Sino-Atrial Node
- Each P wave is followed by a QRS complex
- Each QRS complex is preceded by a P wave
- When PR interval is always normal (3-5 little boxes)
Describe sinus tachycardia on an ECG
- a tachycardia driven by the SA node beating too quickly
- It has normal PR intervals, and each P matched with a QRS therefore looks normal apart from the frequency of waves being too fast on the ECG but this is still an abnormal rhythm
What is the PR interval
from start of P wave to start of QRS complex
What is the PR segment
From end of P wave to start of QRS complex
What is the QT interval
from start of QRS complex to end of T wave
What is the ST interval
from end of QRS complex to start of T wave
What is a normal PR interval duration
120-200ms (3-5 little boxes)
What is a normal QRS complex duration
80-120ms (2-3 little boxes)
What is a normal QT interval duration
360-460ms (9-11.5 little boxes)
What is 1 second on an ECG trace
5 big boxes
How long is one little box on an ECG trace
40ms
What are the 2 types of receptor that the parasympathetic nervous system mainly uses
muscarinic and nicotinic cholinergic receptors
What is Atropine and what does it do
- Muscarinic antagonist
- Causes parasympathetic withdrawal to the heart
- So increases heart rate, contractility and conduction velocity
How does sympathetic input get to the heart
Via stellate nerves
What is the parasympathetic input to the heart
Vagus nerve
What is heart block
- A type of dysrhythmia
- any kind of impulse conduction block of the heart
- Includes AV block, Bundle Branch Block, etc
What is AV heart block
a delay or failure of atria signal stimulating ventricle
What are the causes of heart block
- Ischaemia of AV node or AV bundle
- Compression of AV bundle by scar or calcified tissue
- Inflammation of the AV node or bundle
What are the symptoms of heart block
- Can be asymptomatic
- Palpitations
- Hypotension-like: Dizziness, Malaise, Syncope
- Risk of Sudden Death
What is first degree heart block
- Delayed AV node transition
- When PR interval > 5 little boxes (200 ms) (Normal PR < 5 little boxes)
- But all P’s followed by QRS
Symptoms of first degree heart block
almost always asymptomatic
Who does first degree heart block occur in
young people
How is first degree heart block treated
rarely treated
What is second degree heart block
Some P waves are blocked and are not followed by QRS – Some QRS complexes are “”missing”
Describe Mobitz type I second degree heart block
- AKA Wenckeback
- PR interval gets longer until QRS wave fails to follow P wave
What causes Mobitz type I second degree heart block
Likely AV node damage
What is the treatment for Mobitz type I second degree heart block
Usually no treatment given
Describe Mobitz type II second degree heart block
- AKA hay block
- Some P waves are blocked and are not followed by QRS
- PR interval remains the same
- High risk and can progress to third degree heart block
What causes Mobitz type II second degree heart block
Likely problem in bundle of His
Treatment for Mobitz type II second degree heart block
Implant pacemaker
Describe third degree heart block
- Atrial signals consistently fail to arrive at ventricles
- Ventricular rate is consistent but slow as the ventricles decide to contract on their own when they don’t get a signal from the atria
- Time between atrial beats and ventricular beats is variable.
- PR interval varies radically – sometimes > 12 boxes
- Atrial beats are consistent
What are premature beats
- Individual beats that aren’t normal
- Early
- Triggered by irritable tissue
What are escape beats
- Individual beats that aren’t normal
- Late
- Triggered by natural rhythmicity of non-atrial tissue
- Occur when the atrial signal is very delayed or prevented
Where are premature and escape beats often triggered
In ventricular tissue or by AV node
What are premature and escape beats known as
Ectopic beats
What do premature ventricular contractions look like on an ECG
- Unusually wide and weird looking ventricular electrical activity
- No S wave, instead a wide negative dip where the T wave should be
What are premature ventricular contractions
- Often beat triggered in middle of myocardium
- The two ventricles will be electrically unsynchronised
- So delayed and inefficient conduction (non-Purkinje)
What is atrial fibrillation
disorganised electrical activity in atria
Who is AF common in
The elderly
What does atrial fibrillation look like on an ECG
- No P wave. Instead, flat line OR wiggly line instead of P
- Ventricular rate is fast & irregular due to many signals reaching AV node
Why can atrial fibrillation lead to thrombus formation in atrium
due to slow flow of blood because atria aren’t beating properly
What is respiratory sinus arrhythmia
- heart beat is slightly faster during inspiration, slightly slower during expiration
- Normal
Who is respiratory sinus arrhythmia seen in
children and athletes
What is respiratory sinus arrhythmia
respiratory centres in brain’s medulla
What is ST segment elevation a sign of
Acute MI
Iso electric baseline
The line on the ECG trace that runs from the end of T to the next P
