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Respiratory > Week 3 - gas exchange and lung function tests > Flashcards

Week 3 - gas exchange and lung function tests Flashcards

1
Q

What is Fick’s first law of diffusion?

A

Flux of molecules across a barrier is proportional to the permeability of the molecules times the transfer surface area over which diffusion can occur times the concentration gradient

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2
Q

What factors affect the rate of diffusion?

A
  • Pressure difference
  • Solubility of the gas in solution
  • The cross-sectional area of the fluid
  • The distance the molecules must diffuse
  • The molecular weight of the gas
  • Temperature of the fluid
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3
Q

What does the rate of diffusion depend on when gases pass through other gases?

A
  • The rate of diffusion is inversely proportional to the square root of its molar mass
  • So lighter gases diffuse more rapidly
  • O2 is a smaller molecule than CO2 so would tend to diffuse more quickly
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4
Q

What does the rate of diffusion depend on when gases pass liquids?

A
  • Rate of diffusion is dependent on the solubility of the gas
  • CO2 is much more soluble than O2, so diffuses in a liquid 20 times faster than oxygen
  • – So the gradient is smaller
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5
Q

How do you compensate for the difference in diffusion coefficient between CO2 and O2?

A

By the differences in partial pressures:

  • Larger pO2 compensates for the slower diffusion of O2
  • In a diseased lung, oxygen gas exchange is thus more impaired than CO2 because of oxygen’s slower diffusion rate
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6
Q

What factors affect the rate of gas diffusion through the respiratory membrane?

A
  • Thickness of membrane
  • – Increase as a result of oedema fluid in the interstitial space and in alveoli (slows rate)
  • Surface area of membrane
  • – Decreased by removal of an entire lung
  • – Emphysema results in alveoli combining, decreasing surface area
  • Diffusion coefficient of gas in the substance of the membrane
  • Pressure difference of the gas between the 2 sides of the membrane
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7
Q

What are the layers making up the diffusion barrier at the air-blood interphase?

A
  • Epithelial cell of alveolus
  • Tissue fluid
  • Endothelial cell of capillary
  • Plasma
  • Red cell membrane
    The barrier is 0.6μm thick
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8
Q

What is serial (anatomical) dead space?

A
  • The volume of the airways
  • Can be measured by nitrogen washout
  • Typically about 0.15L
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9
Q

What is distributive dead space?

A

Some parts of the lung (that are not airways) do not support gas exchange

  • Dead/damaged alveoli
  • Alveoli with poor perfusion
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10
Q

What is physiological dead space?

A

Serial dead space + distributive dead space (i.e. the total!)
- Typically 0.17L

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11
Q

How quickly is alveolar air replaced and why is this important?

A

Multiple breaths are required to totally exchange alveolar air

  • Important, as it guards against sudden changes in blood gas levels
  • If respiration is temporarily interrupted, blood gas levels and pH are unaffected
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12
Q

What is the alveolar ventilation rate?

A
  • The amount of air that actually reaches the alveoli
  • To calculate AVR, you need to allow for ‘wasted’ ventilation of dead spaces
  • AVR = PVR – DSVR = (TV x RR) – (DSV x RR)
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13
Q

What is simple spirometry?

A
  • How it is done:
  • – Patient fills their lungs from the atmosphere
  • – They breath out as far and fast as possible through a spirometer
  • Allows the measurement of many lung volumes and capacities
  • – Vital capacity is particularly significant
  • – Tables can be used to predict the vital capacity of an individual of known, age, sex and height
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14
Q

What factors affect vital capacity?

A
  • Inspiration = compliance of the lungs and force of inspiratory muscles
  • Expiration = airway resistance
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15
Q

What is residual volume?

A

Volume remaining after maximal expiration

  • Cannot be measured by spirometry
  • Contributes to the total lung capacity, so important
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16
Q

What are capacities?

A

2 or more lung volumes added together

- Volumes over the cycle can change depending on the pattern of breathing, but capacities are fixed

17
Q

What is vital capacity?

A
  • Measured from max inspiration to max expiration
  • About 5L in a typical adult
  • VC = IRV + TV +ERV
  • Often changes in disease
18
Q

What is inspiratory capacity?

A
  • Biggest breath that can be taken from resting expiratory level
  • Typically about 3L
  • IC = IRV + TV
19
Q

What is functional residual capacity?

A
  • Volume of air in the lungs at resting expiratory level
  • Typically 2L
  • FRC = ERV + RV
20
Q

What is the total lung capacity?

A
  • Volume of gas in the lungs at the end of maximal inspiration
  • Typically 5.8L
  • TLC = IRV + TV + ERV + RV
21
Q

How do you measured forced vital capacity?

A
  • FVC = the maximum volume that can be expired from full lungs
  • Plot a graph of volume expired over time, following spirometry
  • FVC is the highest point on the graph
22
Q

How do you measure forced expiratory volume in 1 second?

A
  • The volume expired in the 1st second of expiration from full lungs
  • Affected by how quickly air flow slows down
  • – .: It is low if the airways are narrowed
  • Again, plot a graph of volume expired over time
  • – Find the volume expired after 1 second
  • If FEV1 is > 70% of FVC then it is normal
23
Q

What is a restrictive deficit?

A
  • Maximal filling of the lungs is determined by the balance between the maximum inspiratory effort and the force of recoil of the lungs
  • If the lungs are unusually stiff, or inspiratory effort is compromised by muscle weakness, injury or deformity, then a restrictive deficit is produced
  • FVC is reduced
  • FEV1 > 70% FVC so normal
24
Q

What is an obstructive deficit?

A
  • During expiration, particularly when forced, the small airways are compressed
  • – This increases flow resistance, eventually to the point where no more air can be driven out of the alveoli
  • If the airways are narrowed, then expiratory flow is compromised much earlier in expiration, producing an obstructive deficit
  • FEV1 is reduced
  • FVC is relatively normal
25
Q

What are flow volume curves?

A
  • A graph of volume expired against flow rate, derived from a Vitalograph trace
  • When the lungs are full, the airways are stretched so resistance is minimum
  • – Flow is hence at maximum (peak expiratory flow rate)
  • As the lungs are compressed, more air is expired and the airways begin to narrow, so resistance increases and flow rate decreases
  • In normal individuals, peak flow is affected mostly by the resistance of large airways, but will also be affected by severe obstruction of the smaller airways
26
Q

How can you measure residual volume?

A
  • Patient breathes a known volume of gas (V1) containing a known concentration of helium (C1), starting at FRC
  • As the patient breathes, the helium concentration changes as it gets diluted because it is in a larger volume
  • The patient continues rebreathing this gas until it is in equilibrium
  • The new concentration of helium is C2
  • – C1 x V1 = C2 x V2 and V2 = V1 + FRC, so hence can work out FRC
  • – Residual volume = FRC – ERV
27
Q

How can you measure serial dead space?

A

Nitrogen washout

  • Patient takes a maximum inspiration of 100% oxygen
  • The oxygen that reaches the alveoli will mix with alveolar air, and the resulting mix will contain nitrogen
  • However the air in the conducting airways (dead space) will still be filled with pure oxygen
  • The person exhales through a 1 way valve
  • – This measures the % of nitrogen in the air and the volume of air expired
  • – [N2] is initially 0 as the patient exhales the dead space oxygen
  • – As alveolar air begins to mix with dead space air, nitrogen concentration gradually climbs until it reaches a plateau where only alveolar gas is being expired
  • A graph can be drawn to determine the dead space, plotting nitrogen % against expired volume
  • Anatomical dead space is the volume at which area A = area B

Respiratory (8 decks)

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