Cardiac Haemodynamics Flashcards
What happens when the ventricles are 70% full with blood?
The atria contract in atrial systole
The pressure in the atria increases and forces blood into the ventricles
How is the first heart sound created?
As ventricles begin to contract, ventricular pressure exceeds atrial pressure
The atrioventricular valves close - the first heart sound
What happens after the atrioventricular valves have closed?
What type of contraction is this?
Both sets of heart valves are closed so pressure rapidly builds in the contracting ventricles
This is an isovolumetric contraction as there is no change in volume
What happens when ventricular pressure exceeds pressure in the aorta?
The aortic valve opens and blood is ejected into the aorta
What happens to ventricular pressure in diastole?
Ventricular pressure falls as blood enters the aorta
How is the second heart sound created?
When ventricular pressure falls below aortic pressure, the aortic valve closes
This creates the second heart sound
What happens when ventricular pressure drops below atrial pressure?
The atrioventricular valves open
Blood flows from the atria into the ventricles
What is different in a cardiac action potential compared with skeletal muscle and neuronal action potentials?
There is a key role for calcium in the cardiac action potential
What is the resting potential of cardiac muscle?
-90 mV
What happens during phase 0 of the cardiac action potential?
Rapid influx of sodium ions causes depolarisation
Membrane potential reaches + 10 mV
What happens during phase 2 of the cardiac action potential?
Rapid influx of calcium ions leads to initiation of the contraction
What happens during phase 3 of the cardiac action potential?
Potassium ions leave the cell and it repolarises
After repolarisation it returns to its resting state
What is significant about there being a lot of variability between cardiac myocytes?
variability helps to contribute to the geometric shape of the left ventricle
This allows different parts of the ventricle to contract in different ways
What is the state of troponin and tropomyosin before contraction of cardiac muscle?
Troponin and tropomyosin form a complex that blocks the myosin binding site on the actin filament
What happens during systole in relation to contraction?
Calcium ions arrive inside the sarcoplasm and bind to troponin
What happens when calcium ions bind to troponin?
They move the troponin-tropomyosin complex and expose the myosin binding site on the actin filament
The myosin head binds to the actin filament and forms a cross-bridge
How does contraction result from cross-bridge formation?
The myosin heads dock in and exert a pulling action on the actin
This results in a contraction
What can usually be detected in heart attack patients?
Troponin leaks out of the myocyte, and this can be detected
How much ATP is used by myocardial cells daily?
6 kg
What is stored within ATP?
How can this be converted into mechanical energy?
Chemical energy is stored within ATP
It is hydrolysed to release a phosphate group, and is converted to ADP
This process converts it into mechanical energy
What does converting ATP into mechanical energy mean for the myocardium?
It results in force generation and myofilament shortening
This process transforms basic mechanical energy into a useful hydraulic function for the whole organ
Concerning the hydraulic function of the heart, what happens when wall stress increases?
Fluid pressure in the chamber is raised
Left ventricular pressure exceeds aortic pressure and the aortic valve opens
Concerning the hydraulic function, in which ways do the myocardial fibres thicken?
Why does this happen?
There is both:
- longitudinal filament shortening
- circumferential thickening
This leads to a complex geometric reconfiguration that reduces LV chamber diameter
What is the purpose of reducing LV chamber diameter?
It causes further blood to be displaced from the ventricle into the circulation
This increases ejection fraction
Concerning hydraulic function, what happens to the ventricular muscle when the ventricle contracts?
The muscle becomes shorter and thicker
This increases ejection pressure
Why is the hydraulic function of the heart creating a high pressure important?
The system in which blood is propelled has inherent resistance as it branches out into increasingly small and dense vessel networks
What does branches of the arteries becoming smaller contribute to?
Systemic vascular resistance
What needs to be done in order to overcome increased systemic vascular resistance?
The cardiac output needs to be maximised through effective power generation in the pump
Why is the diastolic period essential?
It allows the electric system to repolarise
It allows the myocardium to relax and the heart to fill with blood
What other things occur during the diastolic period?
- aortic valve shuts
- coronary sinuses fill with blood
- myocardium receives oxygen and glucose to allow more ATP to be generated
What is cardiac functional reserve?
It is the capacity of the heart to augment performance on demand
The heart needs to pump at rest, but also needs to be able to cope with higher demands
During which conditions may it be necessary to utilise cardiac reserve?
- exercise
- intercurrent illness
- pregnancy
- fluid overload
What is the equation for cardiac output?
cardiac output =
heart rate x stroke volume
What is the equation for cardiac reserve?
cardiac reserve =
maximal cardiac output - cardiac output at rest
Which 2 mechanisms are used to increase cardiac reserve?
- increasing heart rate
2. increasing stroke volume
What is the effect of sympathetic innervation on increasing heart rate?
It increases SA node depolarisation
This leads to more frequent action potentials in the heart
This increases conduction through the AV node and bundle of His
How does adrenaline act to increase the heart rate?
It increases the heart rate through action on adrenergic receptors
This is B1 agonism
What mechanisms are used to increase stroke volume?
Stroke volume is increased through sympathetic activity
Stroke volume also depends on preload
How does the sympathetic nervous system act to increase stroke volume?
It causes prolonged opening of Ca+ channels, so more Ca2+ enters cells
This enhances calcium action in excitation-contraction coupling mechanisms
This leads to increased contraction of cardiac muscle cells
How does stroke volume change with sarcomere length?
As sarcomere length increases, there is more Ca2+ influx
The tension within the sarcomere increases
This causes an increased stroke volume
What is meant by cardiac muscle having a much smaller window of activity than skeletal muscle?
Small changes in cardiac sarcomere length result in large variations in tension
On the tension-sarcomere length graph, what is the situation at physiological stretch?
Ventricular sarcomere length is on the ascending limb
Stretching the left ventricle will aid contraction
What does left ventricle end-diastolic volume determine?
It determines how stretched the left ventricle is
What is preload?
The load on the ventricle before it contracts
Why can more contraction occur as the left ventricle becomes more stretched?
As the muscle stretches, the diameter of the myofibril is reduced
The thick and thin filaments are closer together
More myosin heads can interact with actin
What does Starling’s Law of the heart state?
The energy of contraction is a function of the length of the muscle fibre
According to Starling’s Law of the heart, what happens to stroke volume as preload increases?
As preload increases, stroke volume increases
There is more interaction between the filaments and the myofibrils
Why does an increased preload lead to an increased stroke volume?
venous return always correlates with cardiac output
exercise and other demands lead to increased venous return, which allows augmentation of stroke volume
Why does venous return always correlate with cardiac output?
It equilibrates the right and left heart output
Left ventricular cardiac output is the same as right ventricle preload
What is on the x and y axis of the Frank-Starling curve?
y axis - stroke volume
x axis - LVEDP which is the same as preload
what causes the dynamic Frank-Starling curve to shift to the left and right?
Shifts to left during exercise and pharmacological stimulation
Shifts to the right during myocardial loss and pharmacological depression
In which direction does sympathetic stimulation shift the Frank-Starling curve?
What is the mechanism behind this?
To the left
Noradrenaline and adrenaline stimulate cAMP
This means that more calcium can enter the cell and there is greater cross-bridge formation in sarcomeres
What is ejection fraction?
The amount of blood that is ejected from the ventricle with each contraction
(during each systole)
What is the equation for ejection fraction?
ejection fraction =
stroke volume + end-diastolic volume
What is physiological ejection fraction and how does this change during exercise?
physiological ejection fraction is 50 - 75%
during exercise, ejection fraction can reach 90%
What would show reduced ejection fraction?
a failing heart
In heart failure, what do the following conditions lead to?
i - diseased myocardium
ii - ischaemia
iii - viral infections/alcohol
iv - increased afterload
i - myocardium contracts less
ii - scarred myocardium
iii - wall-thinning
iv - chronic high-output
How does the body try to compensate for a failing ventricle?
Over-activation of the sympathetic nervous system to increase the heart rate
The renin-angiotensin-aldosterone system also kicks in
How do the measures to compensate for a failing ventricle work over time?
They initially increase the preload
Eventually left ventricular stretch exceeds physiological levels and it moves to the descending limb of the sarcomere tension curve
The heart cannot increase stroke volume as the muscle is not working
How is the Frank-Starling curve shifted in left ventricular failure and why?
to the right
Even though preload is increasing, the stroke volume is decreasing
How are the lungs affected by heart failure?
Left ventricular failure leads to the lungs filling with fluid
The fluid leaks into the interstitial spaces and eventually the alveoli
How can alveolar ventilation be optimised in heart failure?
High-flow oxygen
What is used to relax pulmonary vessels in heart failure?
What is the purpose of this?
Morphine
This reduces the preload and takes the strain off the left ventricle
It helps to remove some fluid from the lungs
what is morphine used for in heart failure?
To relax the pulmonary vessels
It also helps with breathing and pain
What is furosemide used for in heart failure?
It takes some fluid off the lungs to help with breathing
It also reduces the preload
Why is it important to used furosemide to help with breathing in heart failure?
Helping with breathing helps to improve oxygen saturation
