Cardiorespiratory Mechanics

Question 1

Graph of volume and pressure changes during inspiration and expiration

Answer 1

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

Question 2

Limitations with snorkeling at great depths

Answer 2

• 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

Question 3

How does resistance change wit size and number of airways

Answer 3

Resistance is inversely proportional to the fourth power of radius of airways,resistance increases as airways get smaller
Resistance decreases with increasing airway number

Question 4

Why does this change after the 4th gen?

Answer 4

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

Question 5

How does resistance and conductivity of lungs change with lung volume

Answer 5

When we breathe in our airways dilate which decreases resistance and conductivity increases

Question 6

How and why do veins and venules have large proportions of blood volume

Answer 6

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

Question 7

Why does pressure fall across circulation

Answer 7

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

Question 8

Why is pulmonary circulation at lower pressure than systemic

Answer 8

Lungs are close to heart does heart doesn’t need to push hard to pump blood

Question 9

Equation for blood pressure

Answer 9

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

Question 10

What three variables does resistance of a tube to flow depend on

Answer 10

Fluid viscosity
Length of tube
Inner radius of tube

Question 11

Laminar blood flow

Answer 11

Velocity of the fluid is constant at any 1 point and flows in layers

Blood flows fastest closest to centre of lumen

Question 12

Turbulent blood flow

Answer 12

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

Question 13

How is turbulent flow used to measure bp

Answer 13

• 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

Question 14

Pulse pressure equation

Answer 14

Pulse pressure=systolic bp-diastolic bp

Question 15

• How could airway transmural pressure changing through inspiration and expiration cause our airways to collapse?

Answer 15

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

Question 16

What does body have to prevent collapse

Answer 16

Cartilaginous support is large extra pulmonary airways

Question 17

Compliance

Answer 17

Tendency to distort under pressure

Question 18

Elastance

Answer 18

Tendency to recoil to original volume

Question 19

How would compliance and resistance be affected by COPD

Answer 19

increased compliance due to emphysemic breakdown
Increased resistance to airflow due to bronchitic swelling of airways

Question 20

Why are ventricular and aortic pressures different

Answer 20

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

Question 21

How does arterial compliance lead to continuous blood flow rather than pulsatile flow of heart beats

Answer 21

• 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

Question 22

If arterial compliance decreases how does bp change

Answer 22

• 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)

Question 23

2 pumps of facilitated venous return

Answer 23

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

Question 24

What two failure of venous return causes which problem

Answer 24

Incompetent valves cause dilated superficial veins
Prolonged elevation of venous pressure causes oedema

Question 25

How are aortic aneurysms caused

Answer 25

• 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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Question 26

Arterial vs venous compliance

Answer 26

Compliance is relationship between transmural pressure and vessel volume depending on vessel elasticity
Venous compliance is greater than arterial compliance at low pressure

Question 27

How can venous compliance be changed

Answer 27

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

Question 28

How does ventilation change as you go down the length of the lung

Answer 28

• 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

Question 29

How does perfusion change as you go down the length of the lung

Answer 29

• 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 (

Question 30

Transmural pressure

Answer 30

Transmural pressure refers to the pressure difference across the wall of a hollow structure or organ
Eg pressure inside airway vs pressure outside lung

Question 31

Cardiopulmonary exercise test

Answer 31

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

Question 32

Six min walk test

Answer 32

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

Question 33

Incremental shuttle walk test

Answer 33

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