Week 3 - gas exchange and lung function tests Flashcards
What is Fick’s first law of diffusion?
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
What factors affect the rate of diffusion?
- 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
What does the rate of diffusion depend on when gases pass through other gases?
- 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
What does the rate of diffusion depend on when gases pass liquids?
- 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
How do you compensate for the difference in diffusion coefficient between CO2 and O2?
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
What factors affect the rate of gas diffusion through the respiratory membrane?
- 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
What are the layers making up the diffusion barrier at the air-blood interphase?
- Epithelial cell of alveolus
- Tissue fluid
- Endothelial cell of capillary
- Plasma
- Red cell membrane
The barrier is 0.6μm thick
What is serial (anatomical) dead space?
- The volume of the airways
- Can be measured by nitrogen washout
- Typically about 0.15L
What is distributive dead space?
Some parts of the lung (that are not airways) do not support gas exchange
- Dead/damaged alveoli
- Alveoli with poor perfusion
What is physiological dead space?
Serial dead space + distributive dead space (i.e. the total!)
- Typically 0.17L
How quickly is alveolar air replaced and why is this important?
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
What is the alveolar ventilation rate?
- 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)
What is simple spirometry?
- 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
What factors affect vital capacity?
- Inspiration = compliance of the lungs and force of inspiratory muscles
- Expiration = airway resistance
What is residual volume?
Volume remaining after maximal expiration
- Cannot be measured by spirometry
- Contributes to the total lung capacity, so important
What are capacities?
2 or more lung volumes added together
- Volumes over the cycle can change depending on the pattern of breathing, but capacities are fixed
What is vital capacity?
- Measured from max inspiration to max expiration
- About 5L in a typical adult
- VC = IRV + TV +ERV
- Often changes in disease
What is inspiratory capacity?
- Biggest breath that can be taken from resting expiratory level
- Typically about 3L
- IC = IRV + TV
What is functional residual capacity?
- Volume of air in the lungs at resting expiratory level
- Typically 2L
- FRC = ERV + RV
What is the total lung capacity?
- Volume of gas in the lungs at the end of maximal inspiration
- Typically 5.8L
- TLC = IRV + TV + ERV + RV
How do you measured forced vital capacity?
- 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
How do you measure forced expiratory volume in 1 second?
- 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
What is a restrictive deficit?
- 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
What is an obstructive deficit?
- 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
What are flow volume curves?
- 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
How can you measure residual volume?
- 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
How can you measure serial dead space?
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
