Cardiorespiratory Mechanics Flashcards
What are the mechanics of ventilation? Use the graph below to explain the changes in pressure and how they cause changes in volume:
How does the graph change with exercise?
As the external intercostal muscles move up and out, and the diaphragm moves down, it creates a negative pressure in the alveoli as the gas moecules are stretched further apart - causes air to flow in via the open airway due to the pressure gradient from the outside air into the lungs
The diaphragm then recoils up to it’s relaxed position, which increases the pressure in the thorax (positive pressure), causing exhalation of the air as the pressure gradient is now from the lungs to the outside air. The pressure in the lungs returns to the original pressure
The graph is essentially the same, but the magnitude of the curves increase
What is the flow rate and what does the graph for it look like?
How does the graph for pleural pressure look?
Flow rate is proportional to the pressure gradient when diametre of airway is the same size
Has a weird shape
Explain this graph of the pleural pressures?
What is an example for a pulmonary function test?
How is this test carried out? (Protocol)
What is the purpose of this test?
Flow-volume loop
Patient wears noseclip and places their lips around the mouth piece. Patient takes one tidal breath (regular). Then the patient is asked to breathe in slowly and deeply as much as possible, but then to exhale as hard and fast as possible until residual volume is reached. Patient must immediately inhale again
Graph gives detailed information on how the air is flowing through different compartments of the lungs - inconsistencies and interrupted flow
What does the graph for the flow-volume loop look like?
y-axis = flow rate (L/s)
x-axis = lung volume (L)
Inspiration is the graph line going downwards (negative / below 0 flow rate), amd expiration is the graph line going upwards (positive / above 0 flow rate)
Tidal breath (A and B) = low amplitude and narrow in width
Inspiration (C) = slow and steady
Expiration (D and E) = 1st litre exhaled very fast, then steady decrease in flow rate til tidal volume is reached
2nd Inspiratino (F) = fast inhalation at first
What can you measure / see from the flow-volume graph?
PEF - peak expiratory flow
Vital capacity
IRV - inspiratory reserve volume
ERV - expiratory reserve volume
TV - tidal volume
What is the difference between obstructive and restrictive diseases?
Which of the categories does obesity fall into?
Obstructive = restrict air flow into the lungs
Restrictive = restrict ability for the chest to move out
Obstructive - excessive weight makes it more difficult for air to flow in
What would you expect the flow-volume loop to look like of a person with COPD? (a mild obstructive disease)
Narrowing of the airways = lower peak, curve of the descend when breathing out (called coving)
Graph displaced to the left - because the lungs in COPD have more air in, lungers are fuller but the amount of air being accessed is less
Vital capacity decreases
What does the flow-volume graph look like of patients with severe obstructive diseases?
Similar to mild-obstructive, but more exaggerated
What does the flow-volume graph look like of patients with restrictive diseases?
Graph displaced to the right
Narrower curve
What is the difference between extrathoracic obstruction and intrathoracic obstruction?
Why does it occur?
When is the obstruction at its maximum?
Extrathoracic = no obstruction during expiration, but obstruction during inspiration
Intrathoracic = no obstruction during inspiration, but obstruction during expiration
Occurs due to the way the blockage (e.g. a tumour) moves during the ventilation process
Obstruction is at its maximum when the airflow is fast, when the airflow is slow, the obstruction is not always necessarily evident
What are the shapes of the flow-volume graphs for variable intra and extrathoracic obstructions and a fixed airway obstruction?
Intra = blunted inspiratory curve, normal expiratory curve
Extra = normal inspiratory curve, blunted expiratory curve
Fixed = blunted inspiratory curve, blunted expiratory curve (otherwise normal)
Why can you not snorkel deeper in the water?
What is Poiseuille’s law?
What does Poiseuille’s law say about resistance if the radius of the tube is halved?
What is Boyle’s law?
Because a deeper dive call for a longer snorkel tube, which adds to the dead space. Dead space causes us to breathe harder and move a greater volume of air to maintain adequate ventilation. A dead space too large in volume would be impossible to move
Resistance = [8 x viscosity of the fluid x length of the tube] / [pi x radius of the tube4]
Halving the radius increases the resistance by x16
P Gas is inversely proportional to PVol
How does resistance to airflow change as the radius of the airway passage gets smaller?
Why is this not a linear relationship?
*look at graph*
At first, decreasing the radius of the airway with each bifurcate increases resistance (as there are more frequent collisions of the particles). However, due to the increase in the cumulative cross sectional area of the airways, resistance drops - as the greater the cross sectional area, the fewer the collisions (so less resistance)
What is conductance?
How does the airway conduct air and how does conductivity change with volume?
Willingness of the airway to conduct fluid transfer (i.e. air in this situation)
The airways are able to dilate and constrict to conduct fluid transfer - conductivity of the airway increases with volume (as resistance decreases)
What are the different characteristics of the different parts of the systemic circulation?
What are the properties of the vein that allow them to be compliant and be the resevoir of blood?
The lumen size to wall thickeness ratio differs for each of these:
Arterioles = contain smooth muscles that regulate the diameter of them to alter resistance - resistant vessels control where the blood flows
Venous system = resevoir of blood, contains valves
Mechanical properties
How does pressure change as blood flows through from the arteries to the capillaries? And why?
Why should you not have a big meal then go for a run?
Which part of the circulation system presents the most resistance?
Pressure falls across the circulation system due to friction
After a meal, the arteries and arterioles in the gut vasoconstrict - that’s why people get cramps when exercising just after they have eaten
The small arteries / arterioles
What is the equation to measure BP?
What is the equation to measure MAP (mean arterial pressure)?
What are the assumptions of these equations?
BP (blood pressure) = CO (cardiac output) x R (resistance)
MAP (mean arterial pressure) = CO (cardiac output) x PVR (peripheral vascular resistance)
Steady flow (which does not accur due to the intermittent pumping of the heart), rigid vessels, and right atrial pressure is negligible
What are the 3 different variables that affect / cause resistance to blood flow in a tube?
How is arterial diameter a determinant of resistance? What is Poiseuille’s equation?
- Viscosity
- Length (of the tube)
- Inner radius
*look at image* Relatively small changes in vascular tone can produce large changes in flow
What is the typical cardiac volume?
What is the typical stroke volume?
What is the typical HR?
So what is the typical cardiac output?
During exercise, how much can the cardiac output increase to?
5L/min
70ml / beat
70 BPM
4.9 L / min
20 L / min - due to increased blood flow to the muscles and skin (for heat radiation)
What are the 2 different types of flow?
How are these 2 types of flow formed? What are they useful for?
Laminar and turbulent flow
Laminar = Velocity of the fluid is constant at any one point and flows in layers, blood flows fastest at the centre of the lumen
Turbulent = Blood flows erratically forming eddy’s and is prone to pooling, associated with the pathophysiological changes to the endothelial lining of the blood vessels
Useful to measure BP
Which type of flow is caused by the slow release of the cuff in clinic when measuring BP?
Where is the cuff positioned when measuring BP?
Slow deflation of the cuff causes turbulent flow, which an be heard using the stethescope to measure systolic and diastolic pressure (systolic = pressure when you first hear a sound, diastolic = pressure when the sound is gone)
Upper arm, as it is easily accessible and in line with the heart
What is pulse pressure? What is the equation to work out pulse pressure?
What is the MAP equation?
What is MAP approximately?
Pulse pressure is the difference between systolic and diastolic BP. Pulse pressure = systolic BP - diastolic BP
MAP (mean arterial volume) = Diastolic BP + a third of the PP (pulse pressure)
All units = mmHg
93 mmHg
What is the transmural pressure?
How does transmural pressure change during inspiration?
How does transmeural pressure change during forced, fast inspiration?
So why do the extrapulmonary airways have ‘C’ shaped cartilage?
Refers to the pressure insidethe airways relative to outside of the airways i.e. the pleural cavity
Pressure across membrane changes during inspiration but remains a positive change - *look at image*
If inspiration is forced = end with negative pressure *look at image*
For structural support, the airways (trachea and primary bronchi) would collapse from a large transmural pressure when breathing really hard
What are the two structural properties of lung tissue?
What are the equations associated with these two?
Compliance = willingness to distort when pressure is being applied to it; more distortion = more compliant
Elastance = tendency to retain it’s shape i.e. recoil into its original shape and volume
The elasticity of a vessel is related to which structural property?
How is this property shown in the aorta?
Compliance - more elastic fibres = more compliant
When the aortic valve closes, the ventricular pressure falls rapidly, but the aoritc pressure falls slowly due to the elasticity of the vessel - the aorta absorbs energy during systole
What is the windkessel effect?
Why can the blood flow not be intermittent / pulsatile?
How does the blood flow become continuous?
How does the aorta support continuous flow?
Intermittent blood flow (starting and stopping)
Ventricles = forms intermittent flow naturally, if there were vessels with no compliance (the blood flow would start and stop). The stopping period would cause blood to clot
By having compliant vessels - elastic arteries are able to redistribute the energy - e.g. the aorta and its compliance enable for continuous flow
Aortic compliance: Aorta = absorbs energy, stretched during systole, recoils and regenerates more pressure during diastole (diastolic pressure)
What is the purpose of valves in veins?
How are the 2 ways venous return is facilitated?
Maintains unidiretional flow - stops blood flowing backwards
- Skeletal muscle pump - contraction of skeletal muscles, which compresses the vessels causing the blood to flow
- Respiratory pump - as diaphragm moves down = low intrathoracic pressure = stretches out the inferior vena cava, so it increases pressure gradient between vena cava and right atrium
What clinical signs may show a failure in valve activity?
Varicose veins - dilated superficial veins in the leg
Oedema of the feet from prolonged elevation of venous pressure
What is the law of LaPlace?
The aorta and brachial artery have the same BP, but the aorta has more than 2x the radius of the brachial artery? How do the vessels differ to keep their BP the same?
Relationship between tension, pressure, radius and thickness of a vessel
Wall thickness differs (to reduce splitting of the wall from tension) - therefore must be at least double the wall thickness
What is an aneuralsmal disease? (i.e. aneurysms)
How can the law of LaPlace be applied to aneurysms and their rupturing?
What is the most common aneurysm in the UK?
When the wall tension exceeds the tension capacity of the vessel, the vessel walls weaken over time causing a localised balloon like distension
Vascular aneurysms increase the radius of the vessel, so to form the same internal pressure, more force must be exerted by the vessel walls. However, due to the weakening of the elastic fibres in the vessel walls, they cannot exert more force so instead continue to expand til they rupture
AAA - abdominal aorta aneurysm (screening for this in males over 60)
Are veins or arteries more compliant?
If there is great blood loss, how and why does the venous system give the blood back to the arteriole system?
How does the graph of compliance change in this fight or flight situation?
Veins - at low pressures
Veins = blood resevoir, venocontriction as a result of sympathetic NS activation from nerves and hromones e.g. adrenaline (fight or flight situation)
Venous curve changes more like the arterial curve as there are high pressure
What is ventilation and what is perfusion?
How does ventilation and perfusion change across the lung?
How does gravity affect the ventilation and perfusion of the lung?
Ventilation = breathing and gas exchange, perfusion = passage of fluid
At rest, the top of the lung is more stretched than the bottom due to gravity - lung rests more towards the bottom of the pleural cavity due to gravity so squishes lung tissue towards the bottom. The transmural pressure at the top of the lung is greater than at the bottom
Therefore, the bottom of the lung is easier to ventilate due to lower transmural pressure - the alveoli are smaller and more compliant. Also, air generally flows towards the base of the lung (due to gravity)
There is lower intravascular pressure towards the top of the lung than the bottom. So there is more blood flow at the bottom of the lung due to less resistance and greater recruitment = greater perfusion
What are the 3 different zones of the lung and what is their ventilation and perfusion?
Blood = preferentially flow to the base due to gravity
Zone 1 = least amount of ventilation and least amount of perfusion
As the zone number increases, so does ventilation and perfusion
The relationship of ventilation and perfusion from the apex to the base of the lung differs as blood is thicker than air, steeper relationship with blood than with air
What is the ventilation-perfusion ratio?
What is wasted perfusion and wasted ventilation?
When are the ventilation and perfusion curves optimal (perfect ventilation-perfusion ratio)?
What happens if the ventilation and perfusion are not matched?
How much ventilation is there
Bottom of lung has more perfusion than needed = some of the perfusion is wasted
Ventilation at the top of the lung is greater than the perfusion at the top = some of the ventilation is wasted
When the lines cross on the graph, as there would be equal amounts of ventilation and perfusion
More issues / disorders, abnormal pathology etc.
How would compliance and resistance be affected by chronic obstructive pulmonary disease?
A. Increase in compliance and increase in resistance - emphysemic breakdown of structural lung tissue increases compliance and bronchitic swelling of the airways and mucus hypersecretion increase resistance to airflow
What is the most likely reason for under-reading FVC in a patient with COPD?
A = poor technique (hurts them to do the steps properly), so the reading may be lower than the true value
