Respiratory Session 3- Gas Exchange And Lung Function Tests Flashcards
What is alveolar pO2 determined by?
Balance between rate of removal of O2 by the blood and rate of replenishment of O2 by alveolar ventilation
What is alveolar pCO2 determined by?
Balance between the rate at which CO2 enters the alveoli from blood and the rate at which it is removed from alveolar gas by ventilation
Why is the balance between perfusion and ventilation important?
Keeps the partial pressure of oxygen and carbon dioxide in the alveolar gas stable at its normal values of 13.3kPa and 5.3kPa respectively
What factors determine the rate at which gases exchange?
Area available for exchange
Resistance to diffusion
Gradient of partial pressure
What is the exchange area in a normal lung?
70m^2
From the alveoli, what must gas diffuse through to reach haemoglobin?
Alveolar epithelial cell Interstitial fluid Capillary endothelial cell Plasma Red cell membrane
What property of a gas affects its rate of diffusion the most?
Solubility of the gas in water
More soluble= faster diffusion
Does CO2 or O2 diffuse faster and by how much?
CO2 diffuses 21 times as fast as oxygen
What are the normal partial pressures of O2 and CO2?
O2 = 13.3kPa CO2 = 5.3kPa
Are that partial pressures of O2 and CO2 in arterial blood the same as the partial pressures in alveolar gas?
YES! In a normal subject
How are the partial pressures of O2 and CO2 kept very close to their normal values?
Exchange of gas between alveolar gas and atmospheric air brought close to it through the airways of the lung by the process of ventilation
How long does full gas exchange take?
500ms
What are the 3 main diseases that cause diffusion defects and why?
Interstitial lung disease- excessive deposition of collagen in interstitial space and thickening of alveolar walls = lengthening of diffusion pathway
Pulmonary oedema- fluid in interstitium and alveolus increases length of diffusion pathway
Emphysema- destruction of alveolar walls = large air spaces which reduces total surface area available for gas exchange
How is diffusion resistance measured?
Carbon monoxide transfer factor- measuring CO uptake following single, maximal breath of a gas mixture containing air, 14% helium and 0.1% CO
Why is CO used to calculate diffusion resistance?
It has a very high affinity for Hb
All the CO entering blood binds to Hb so concentration gradient for pCO across alveolar capillary membrane is maintained for the entire time blood is in contact with alveolar gas
What is the anatomical or serial dead space?
The volume of the conduction airways (containing air that is not available for gas exchange)
How much dead space is there normally and how is it measured?
150ml- measured by the nitrogen washout test
What is the alveolar/distributive dead space?
The volume of air in alveoli not taking part in gas exchange
What is the physiological dead space and how is it determined?
Anatomical dead space and alveolar dead space
Can be determined by measuring pCO2/pO2 of expired and alveolar air
Define pulmonary ventilation rate
Total rate of movement of air into and out of the lungs
Dead space ventilation and alveolar ventilation
What is dead space ventilation?
Movement of air into and out of the dead space
Is fewer deep breaths or many shallow breaths more effective and why?
Deeper breathing is more effective as at each breath the dead space is fully ventilated so the deeper the breath the higher the proportion of air available for gas exchange
How is the movement of air during breathing measured?
Spirometry
Define tidal volume
Volume of air that enters and leaves the lungs with each breath
Define inspiratory reserve volume
Maximal amount of additional air that can be drawn into the lungs by determined effort after normal inspiration
Define expiratory reserve volume
Additional amount of air that can be expired from the lungs by determined effort after normal expiration
Define residual volume
Volume of air left in the lungs after forced expiration
Can lung capacities change?
Nope
How are lung capacities defined?
Fixed points in the breathing cycle- maximum inspiration, maximum expiration and end of quiet expiration
Define inspiratory capacity
From the end of quiet expiration to maximum inspiration
Define functional residual capacity
Volume of air in the lungs at the end of quiet expiration
Define vital capacity
Inspiratory capacity and expiratory reserve OR
Inspiratory reserve volume and tidal volume and expiratory reserve volume
How do you work out total lung volume?
Vital capacity and reserve volume
Why can vital capacity be reduced?
Lungs not filled normally in inspiration, emptied normally in expiration or both
What is maximal filling of the lungs determined by?
Balance between maximum inspiratory effort and the force of recoil of the lungs
What can cause a ‘restrictive’ deficit?
Unusually stiff lungs
Inspiratory effort compromised by muscle weakness, injury or deformity
How is an ‘obstructive’ deficit produced?
Expiratory flow compromised much earlier in expiration than normal is airways are narrowed
How can restrictive and obstructive deficits be separated?
Asking patients to breathe out rapidly from maximal inspiration through a single breath spirometer which plots volume exhaled against time
What is a vitalograph?
A time-volume plot generated during spirometry
Describe a normal vitalograph
Rapid increase that slows down over time to a maximum volume
What do you measure to distinguish between obstructive and restrictive defects?
Forced expiratory volume in one second
Forced vital capacity
Describe the vitalograph findings in an obstructive defect
FVC is nearly normal
FEV1 is reduced markedly
FEV1/FVC ratio is <70%
Describe the vitalograph findings in a restrictive defect
FVC is reduced
FEV1 is reduced proportionally
FEV1/FVC ratio is normal
What is plotted on each axis in an expiratory flow volume loop?
X axis- lung volume
Y axis- expiratory flow rate
When does the peak expiratory flow rate occur?
At the start of expiration, when the lungs are expanded and airways stretched open
In normal individuals what affects peak flow the most?
Resistance of the large airways
What can reduce PEFR?
Severe obstruction of small airways
What dos the flow-volume curve look like with mild obstruction of the small airways?
It is scooped out- reduces very rapidly then slows as it reaches the minimum
How can you measure residual volume?
Helium dilution test
How can you measure dead space?
Nitrogen washout method
