Cardiorespiratory Mechanics Flashcards
Graph of volume and pressure changes during inspiration and expiration
Diaphragm pulls down, decreases thoracic pressure below 0 and ambient pressure is 0- creating pressure gradient
Air flows down pressure gradient to increase volume- this increases pressure in lungs until it’s 0 again (same as outside)
Inspiratory effort is removed and lungs are squeezed which increases pressure in lungs until pressure gradient between inside lungs and outside air is created which air moves down, decreasing volume in lungs
Limitations with snorkeling at great depths
- The amount of dead space increases (the distance between alveoli and outside air) which we need to move oxygen through
- At 0.5m with 2.2cm diameter snorkel the dead space is 1.21 x pi x 30 cm = 114mL which is 1/4 of resting tidal volume
- ## At 100m this increases to 38L of dead space which is 7x total lung capacity
How does resistance change wit size and number of airways
Resistance is inversely proportional to the fourth power of radius of airways,resistance increases as airways get smaller
Resistance decreases with increasing airway number
Why does this change after the 4th gen?
The amount and velocity of air going through the smaller pipes is less than higher pipes
Higher airywas have different structural support as they have cartilaginous discs which limit dilation of airways
How does resistance and conductivity of lungs change with lung volume
When we breathe in our airways dilate which decreases resistance and conductivity increases
How and why do veins and venules have large proportions of blood volume
Blood at high pressure gets pushed from arteries + arterioles through capillaries into venules + veins where it slows down
Veins + venules are highly compliant and act as a reservoir for blood volume
This is because it can distort allowing more blood to be held
Why does pressure fall across circulation
Due to viscous pressure losses
As blood moves through the heart’s chambers and vessels, there is friction between the layers of blood, resulting in energy loss in the form of hea
Why is pulmonary circulation at lower pressure than systemic
Lungs are close to heart does heart doesn’t need to push hard to pump blood
Equation for blood pressure
Blood pressure=CARDIAC OUTPUT TIMES RESISTANCE
change in pressure gradient=Q times TPR
Assume there is steady flow,rigid vessels,right atrial pressure is negligible
What three variables does resistance of a tube to flow depend on
Fluid viscosity
Length of tube
Inner radius of tube
Laminar blood flow
Velocity of the fluid is constant at any 1 point and flows in layers
Blood flows fastest closest to centre of lumen
Turbulent blood flow
Blood flows erratically, forming eddys, and is prone to pooling
Associated with pathophysiological changes to endothelial lining of blood vessels
More likely to activate clotting factors and produce a thrombus
How is turbulent flow used to measure bp
- BP usually measured on upper arm as its easily accessed and at heart level
- Slow deflation of cuff causes turbulent flow which can be heard with a stethoscope
Pulse pressure equation
Pulse pressure=systolic bp-diastolic bp
- How could airway transmural pressure changing through inspiration and expiration cause our airways to collapse?
Patent means open airways- through inspiration our airways are open since transmural pressure is positive
However if we do a hard expire (contract stomach muscles, breathe out really hard, hunch over etc) then we increase the pressure in the airways a lot but also increase the intrapleural pressure
If intrapleural pressure > pressure of airway at any point, the airway at that point will collapse
What does body have to prevent collapse
Cartilaginous support is large extra pulmonary airways
Compliance
Tendency to distort under pressure
Elastance
Tendency to recoil to original volume
How would compliance and resistance be affected by COPD
increased compliance due to emphysemic breakdown
Increased resistance to airflow due to bronchitic swelling of airways
Why are ventricular and aortic pressures different
Once aortic valve closes ventricular pressure falls rapidly but aortic pressure falls slowly
Due to elasticity of aorta and arteries which buffer change in pulse
How does arterial compliance lead to continuous blood flow rather than pulsatile flow of heart beats
- During ejection, blood enters aorta and other downstream elastic arteries faster than it leaves them (40% of SV is stored by elastic arteries)- the left ventricle contracts and aorta + arteries stretch
- When aortic valve closes, ejection ceases but due to recoil of the elastic arteries, pressure falls slowly and there is diastolic flow in downstream circulation- the left ventricle relaxes and aorta + arteries recoil
- This aortic recoil is basically what squeezes the blood on after the ventricles squeeze it to provide continuous blood flow rather than pulsatile
If arterial compliance decreases how does bp change
- Systolic pressure will increase as ability of vessels to stretch in response to ventricular systolic pressure has decreased
- Diastolic pressure will decrease as ability of vessels to recoil once stretched to push blood to create 80 mmHg of diastolic pressure has decreased
- Pulse pressure will go up (as its systolic - diastolic)
2 pumps of facilitated venous return
1) Skeletal muscle pump- in lower limbs, effect of muscle compressing veins in presence of functional vein valves means the blood can’t push back down to foot but goes up to heart
2) Respiratory pump- in thorax, we have IVC. When diaphragm pulls down to create -ve intrathoracic pressure, this vacuums the vein out which pulls vein apart- this reduces pressure in thorax relative to abdomen which causes blood to flow up IVC back to heart
What two failure of venous return causes which problem
Incompetent valves cause dilated superficial veins
Prolonged elevation of venous pressure causes oedema
How are aortic aneurysms caused
- Over time, vessel walls can weaken causing a balloon-like distension
- This is a pathological example of Law of Laplace- vascular aneurysms increase vessel radius so for the same internal pressure the inward force exerted by the muscular wall must also increase
- However, if muscle fibres have weakened, the force needed can’t be produced so aneurysm will continue to expand (often until it ruptures)
- The way this happens is the same for formation of diverticuli in gut
- Abdominal aortic aneurysms are most common and more common in men than women
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Arterial vs venous compliance
Compliance is relationship between transmural pressure and vessel volume depending on vessel elasticity
Venous compliance is greater than arterial compliance at low pressure
How can venous compliance be changed
Increasing smooth muscle contraction decreases venous volume and increases venous pressure
Most blood volume is stored in veins
Relatively small changes in venous pressure distend veins and increase volume of blood
How does ventilation change as you go down the length of the lung
- Higher up in lung:
- the pleural pressure is more negative (-8 cmH2O)
- greater transmural pressure gradient of (0 vs -8)
- alveoli are larger and less compliant
- you get less ventilation
- Lower down in lung:
- Pleural pressure is less negative (-2 cmH2O)
- smaller transmural pressure gradient (0 vs -2)
- Alveoli are smaller and more compliant
You get more ventilation
How does perfusion change as you go down the length of the lung
- Higher up in lung:
- Lower intravascular pressure (due to gravity)
- Less recruitment
- Greater resistance
- Lower flow rate
- Lower down in lung:
- Higher intravascular pressure (due to gravity)
- More recruitment
- Less resistance
- Higher flow rate
- Flow tends to follow path of least resistance- it’s easier for heart to pump downhill to bottom of lung (
Transmural pressure
Transmural pressure refers to the pressure difference across the wall of a hollow structure or organ
Eg pressure inside airway vs pressure outside lung
Cardiopulmonary exercise test
Cycle ergometer or treadmill used under clinical supervision. ECG ventilation oxygen and carbon dioxide measured
Can find peak VO2
Adv-precise and reproducible,quantifies performance in relation to metabolism
Dis-expensive,needs skilled technicians
Six min walk test
Objective is to cover greatest distance in six mins
Sub maximal test
Primary outcome is to find total distance,secondary variables include perceived exertion,heart rate,pulse oximetry
Adv-patient driven pace,cheap,valid
Dis- needs unobstructed course,pace not regulated
Incremental shuttle walk test
Uses 10m circuit,paced by an audio,each minute has one extra length that previous
Primary outcome is distance walked,secondary is exertion,heart rate and pulse oximetry
Adv-cheap,paced,valid
Dos-patient can be penalized for poor pace management,increments may be difficult
