Introduction to Cardiovascular System Flashcards
What is the general role of the heart and blood?
Heart - pump blood around the body
Blood - carry metabolites around the body
What is the general role of the arteries, veins and capillaries?
Arteries - distribute oxygenated blood to cells and tissues
Capillaries - metabolite exchange
Veins - return blood to the heart
Where does the heartbeat begin and why?
The apex of the heart
This allows blood to be squeezed out of the top of the heart
Where does electrical activity begin in the heart?
Sino-atrial node
What is sinus rhythm?
The normal rhythm of the heart which is determined by the SA node
How do the valves prevent electrical impulses travelling from the atria to the ventricles?
They are insulation as they are made from non-electrically conducting material
How is the electrical activity of the heart stimulated?
How does it reach the AV node?
- action potential is initiated in SA node
2. it is propagated to the atrioventricular node via internodal tracts in the atria
How does the speed of the action potential vary in the SA node and the AV node?
Why?
Cells of the AV node transmit the action potential more slowly and delay the impulse by 100 ms
This gives time for the atria to completely empty into the ventricles before they contract
What happens to the electrical impulse as it leaves the AV node?
The impulse spreads down to the ventricles along the bundle of His
What is significant about the bundle of His?
It is the only electrical connection between the atria and the ventricles
How does the AV bundle divide?
It divides into left and right bundle branches
There are 2 bundles going to the left ventricle
There is 1 bundle going to the right ventricle
What happens to the impulse after it passes down the bundle of His and the bundle branches?
It spreads to the contractile cells of the ventricles through an extensive network of Purkinje fibres
What is diastole?
The resting period where the heart fills with blood
What is significant about the period between heartbeats?
There is a period between heartbeats where there is no electrical activity
This allows the heart to recharge ready for the next contraction
Why can electrical impulses travel so quickly through the heart?
Gap junctions in the intercalated discs allow the spread of the action potential throughout the myocardium like a functional syncytium
what is a syncytium?
A functional whole
It is one thing even though it is made up from multiple components
What is the main, secondary and backup pacemaker of the heart?
Main - SA node
Secondary - AV node
Backup - all the other electrically active myocytes
What is the SA node influenced by?
How do other heart cells respond to these stimuli?
Influenced by the blood (e.g. hormones like adrenaline) and parasympathetic and sympathetic nerves
The rest of the heart cannot respond as intelligently to the stimuli
what is an escape pacemaker?
an electronic device that will oversee the heartbeat and intervene if something goes wrong
What is the “brake” of the heartbeat?
The vagus nerve which slows down the activity of the heart
It releases acetylcholine
How does atropine work?
It is a drug that blocks the effect of ACh on the M2 receptors
ACh is released by the vagus nerve, and slows the activity of the heart
What is the “accelerator” of the heartbeat?
The “accelerator” speeds up the activity of the heart
Adrenaline is a natural way to speed up and strengthen the heartbeat
Why is adrenaline not often used as a drug?
It has more side effects than atropine
What is the resting potential of cells within the SA node?
They have no true resting potential
They generate regular, spontaneous action potentials
What is the difference in the depolarising current in non-pacemaker and pacemaker cells?
Non-pacemaker action potentials see the depolarising current carried into the cell by fast Na+ currents
In pacemaker cells, the current is carried into the cell via slow Ca2+ currents
Why are ‘slow response’ action potentials produced in SA nodal cells?
There are no fast Na+ channels in SA nodal cells
The SA nodal cells take longer to depolarise
What are the 3 phases of pacemaker action potentials, in order?
- phase 4 (If)
- phase 0 (ICa)
- Phase 3 (Ik)
what happens at the beginning of phase 4?
At the end of repolarisation, the membrane potential is very negative (-60 mV)
“funny” channels open that conduct slow inward Na+ currents
What is the “funny current”?
what does it cause?
depolarising slow inward Na+ current
It causes the membrane potential to begin to spontaneously depolarise - the pacemaker potential
What happens during phase 4 as the membrane potential reaches -50 mV?
The (transient) T-type Ca2+ channels open
The inwards directed Ca2+ current further depolarises the cell
What happens during phase 4 as the membrane potential reaches -40 mV?
The (long-lasting) L-type Ca2+ channels open
These cause more Ca2+ to enter the cell until an action potential is generated
At what magnitude is the action potential generated in the pacemaker cell?
The threshold value of -35 mV
What happens during phase 4 relating to K+?
Slow decline in the outward movement of K+
Fall in K+ conductance contributes to the depolarising pacemaker potential
What is phase 0 of the pacemaker action potential?
The depolarisation phase of the action potential
What is Phase 0 mainly caused by?
Increased Ca2+ conductance through L-type calcium channels that begin to open towards the end of phase 4
Why is the rate of depolarisation much slower in pacemaker cells than other cardiac cells?
The movement of Ca2+ through the L-type channels is not rapid
What begins to decline during phase 0?
funny currents and Ca2+ currents through T-type channels
The respective channels begin to close
What is phase 3 of the pacemaker action potential?
what is the main thing that occurs in this phase?
Repolarisation phase
K+ channels are opened which increases the outwards hyperpolarising K+ currents
What begins to decline during phase 3?
L-type Ca2+ channels close and the inwards flow of Ca2+ declines
This decreases the inwards depolarising Ca2+ currents
When is the pacemaker action potential cycle repeated?
Once the cell has completely repolarised to around -60 mV, the cycle is spontaneously repeated
How are an action potential and an ECG related?
An action potential is the electrical activity of one cell. but an ECG shows all the cardiac myocytes acting together
What is the minimum number of leads needed for an ECG and what does this produce?
A minimum of 2 leads are needed
This provides only the rhythm strip
How many wires are used in a 12 lead ECG?
Where are these placed and what views does it provide?
It uses 10 wires
4 limb leads and 6 chest leads
It provides 6 different views of the heart
Why must the ECG leads be placed some distance apart?
So the difference between them can be detected
The entire body is electrically conducting due to saline in the blood
The electrical activity of the heart can be detected from any position
On an ECG, what does the P wave represent?
Atrial depolarisation
On an ECG, what does the QRS complex indicate?
Ventricular depolarisation and contraction
On an ECG, what does the T wave represent?
Repolarisation of the ventricles
What does the PR interval represent?
Where does it stretch from?
From the start of the P wave to the start of the QRS complex
This is the time between the onset of atrial depolarisation and the onset of ventricular depolarisation
What does increasing the length of the PR interval show?
There is a problem with the AV node
What does the QT interval represent?
Where does it stretch from?
From the start of the QRS complex to the end of the T wave
It represents the time of ventricular activity, including depolarisation and repolarisation
What does the ST segment represent?
Where does it stretch from?
From the end of the QRS complex to the start of the T wave
It represents the interval between ventricular depolarisation and repolarisation
Where does the PR segment stretch from?
The end of the P wave to the start of the QRS complex
What is the mechanism behind complete heart block?
A nerve impulse generated in the SA node cannot propagate to the ventricles
How can complete heart block be identified on an ECG?
There is an increased length of the PR interval
How can complete heart block lead to fibrillation?
An accessory pacemaker in the lower heart chambers activates the ventricles
The accessory pacemaker acts independently of the SA node
There is no coordination between the frequency of contraction of the atria and ventricles
What do positive and negative deflections on an ECG show?
Negative deflection shows electrical activity moving away from the electrode
Positive deflection shows electrical activity moving towards the electrode
What is the purpose of the cardiac axis?
It describes the general direction of depolarisation
What is a normal direction of spread of electricity according to the heart axis?
The heart axis is usually down and to the left as electricity moves towards the apex
a normal direction of spread is from 11 o’clock to 5 o’clock
(aVR - II)
Which leads of the ECG does the spread of electricity head towards?
What does this show?
It heads towards leads I, II and III
There is a positive deflection in all these leads, but II has the most positive deflection as it is at 5 o’clock
What is the only ECG leads where readings are negative all the time?
aVR on the right shoulder
This is at the 11 o’clock position and all electricity is moving away from it
What are the approximate positions of the ECG leads in relation to degrees?
aVR -150 aVL -30 I 0 II 60 aVF 90 III 120
What is the left ventricular axis?
It shows the direction in which the electricity is moving
It goes from V1 to V6
V1 is mostly negative and V6 is mostly positive
What is sinus arrhythmia?
It is the physiological slowing of the SA node rate during expiration, caused by an increase in vagal activity
What condition does sinus arrhythmia lead to during inspiration and why?
Tachycardia
Negative pressure in the lungs means that there is slightly more blood returning to the heart
What condition does sinus arrhythmia lead to during expiration and why?
Bradycardia
Positive pressure in the lungs means that there is slightly less blood returning to the heart
Why does an increased heart rate during inspiration help to preserve cardiac output?
Left ventricular stroke volume falls
This is due to reduced filling as the pulmonary vascular bed expands during inspiration
What is the definition of sinus bradycardia?
When does it occur?
It is a sinus rhythm with a resting heart rate of 60 beats per minute or less
it occurs when electrical signals in the heart are slowed down or blocked
What are the 3 ways in which sinus bradycardia may be caused?
- SA node discharges electrical impulses slower than normal
- SA node pauses of fails to discharge at a regular rate
- SA node discharges an electrical impulse that is blocked before it can cause the atria to contract
What is the definition of sinus tachycardia?
What is it caused by?
A sinus rhythm with a resting heart rate of 100 beats per minute or more
Caused by SA node sending out electrical impulses more frequently than normal
What is ventricular ectopy?
An ectopic rhythm is an irregular heart rhythm due to a premature heartbeat
After an early beat of the heart, a brief pause usually follows
What condition can ventricular ectopy lead to?
Why does the early heartbeat happen?
ventricular tachycardia
due to lack of oxygen reaching the myocyte
What is atrial fibrillation?
An abnormal heart rhythm characterised by rapid and irregular beating of the atria
When does atrial fibrillation occur?
When abnormal electrical impulses start firing from myocytes in the atria
This overrides impulses from the SA node
How does atrial fibrillation reduce the heart’s efficiency and performance?
The atria contract randomly and more rapidly, meaning the heart cannot relax properly between contractions
How is atrial fibrillation shown on the ECG trace?
There is no clear P wave on the ECG trace
How does ventricular fibrillation affect the heart and why does this happen?
it causes the heart to quiver, rather than produce a pumping action
This is due to disorganised electrical activity in the ventricles
what are the consequences of ventricular fibrillation?
Cardiac arrest with loss of consciousness and no pulse
Blood pressure rapidly plummets and cuts off blood supply to the vital organs
What is the most common cause of ventricular fibrillation?
A problem with the electrical impulses travelling through the heart
This may be due to scar tissue in the heart due to a previous heart attack
What happens if ventricular tachycardia lasts for more than 30 seconds?
It leads to palpitations, dizziness and fainting
Ventricular tachycardia can lead to ventricular fibrillation
How does amiodarone work?
What is it used to treat?
Treats ventricular fibrillation
It acts on ion channels and causes a delayed repolarisation by blocking potassium ion channels
If someone has collapsed, what is the ABC method that must be followed?
A - is the airway clear?
B - are they breathing?
C - circulation
CPR is used to push blood out of the heart towards the brain to extend the 3 minutes it can survive without oxygen
What is the pathway that describes the activity of the heart, from energy to metabolism?
Energy Work Pressure Flow Perfusion Metabolism
What are the 3 problems that encompass cardiovascular problems?
Problem with the pump, pipes, electrics or all three
What is involved in perfusion?
Blood reaching small capillaries to allow metabolic exchange
In a healthy individual, how much blood is pumped by the heart per minute?
How does the amount pumped vary in the right and left ventricles?
5L per minute
The right and left ventricles pump the same amount of blood
In a heart attack, how does the amount of blood pumped per minute change?
The amount of blood pumped per minute falls
Blood is diverted to vital organs, such as the brain and heart
Blood flow to skin and digestive system is reduced
In a myocardial infarction, what is tissue infarction?
Death of tissues due to a lack of oxygenation
Why is adrenaline given to myocardial infarction patients?
It helps to increase the strength and rapidity of the heart
This helps to increase cardiac output
What type of technology is used to observe the heart?
Echocardiogram
Usually a transoeosophageal echocardiogram where a probe is swallowed as this allows scanning from the inside without obstruction from the ribs
In a myocardial infarction, what problem with the pipes may occur?
Blockage in the left coronary artery leads to the left ventricle lacking a sufficient blood supply
What is used to observe the coronary arteries?
What is used to open up a blocked artery?
An angiogram is used to observe coronary arteries
A black stent device will open up a blocked artery
How can platelets lead to a myocardial infarction?
They aggregate to form a platelet plug to stop arterial bleeding
This can occur wrongly in the coronary arteries
How does aspirin work?
It is an anti-platelet drug that inhibits thromboxane
What are the actions of Cangrelor and Clopidogrel?
They inhibit the platelets to prevent blood clots in (coronary) arteries
