Week 3 - Lung Function Tests Flashcards
What do lung function tests give an assessment of?
Mechanical conditions of the lungs, resistance of airways, alveolar membrane
What is spirometry?
Breath from a closed chamber over water and the volume changes with ventillation
What is tidal volume?
Volume in and out with each breath
What is inspiratory reserve volume?
Extra volume that can be breathed in past the usual tidal volume
What is expiratory reserve volume?
Extra volume that can be breathed out past the usual tidal volume
What is residual volume?
Volume remaining in lungs after maximal expiration
Can residual volume be measure by spirometry?
No
Why is it important to calculate residual volume?
Because it contributes to total lung capacity
What is the difference between capacity and volume?
Capacity is 2 or more volumes added together, volumes can change depending on breathing pattern, capacities are fixed
What is vital capacity?
Measured from maximum inspiration to maximum expiration, often changes with disease, usually about 5l in an adult
How is vital capacity calculated?
Inspiratory reserve volume + tidal volume + expiratory reserve volume
What is inspiratory capacity?
Biggest breath that can be taken from resting expiratory level which is lung volume at end of quiet expiration
What is the typical value for inspiratory capacity?
3l
What is functional residual capacity?
Volume of air in lungs at resting expiratory level
How do you calculate functional residual capacity?
Expiratory reserve volume + residual volume
What is the typical value for functional residual volume?
2l
What is total lung capacity?
Volume of gas in lungs at end of maximal inspiration
What is the typical value for total lung capacity?
5.8l
What factors influence vital capacity?
Inspiration - compliance of lungs, force of inspiratory muscles
Expiration - airway resistance, increases as expiration proceeds
What factors do you have to take into account when assesing capacity?
Height, sex, age
What is single breath spirometry?
Fill lungs, breathe out as far and fully as possible, volume is measured over time, measuring how much and how fast the air is breathed out
What is a vitalograph?
Plot of volume expired vs time
What is FEV1.0?
Volume expired in first second
What si the usual difference between FEV1.0 and forced vital capacity?
Normally >70%
How will FEV1.0 and forced vital capacity be affected by affected by narrowed airway?
Will still fill easily by resistance increases on inspiration so air will come out more slowly
This means FEV1.0 will be reduced but FEC will be relatively normal
How is spirometry used to measure reversibility?
Performed before and after treatment with bronchodilator drugs and if its reversible suggests asthma
If lungs are difficult to fill what does this suggest?
Stiff, weak muscles, problem with chest wall
How would lungs that are difficult to fill affect FVC and FEV1.0?
They will start less full so FVC will be reduced but the air will come out normally so FEV1.0 will still be >70% of FVC
What are flow volume curves?
Plots volume against flow
What is peak expiratory flow rate used for?
Measures with a simple cheap device so often used as a screening test for airway narrowing but it is very insensitive
Are flow volume curves good for interpreting disease?
Yes, can detect problems earlier in the course of disease than simple spirometry, can discriminate where in the tract the problem lies
How would obstruction be shown on a flow volume curve?
Lungs are compressed, more air expired, airways begin to narrow, resistance increases and flow rate falls, the narrower the airways to start with the more rapidly it falls, curve will have normal peak but decreases faster
How would a restrictive pattern show on a flow volume curve?
Curve narrowed, shape generally the same as normal but flow is always less at comparable volumes
How is residual volume measured?
Helium is not metabolised by the body, patient breathes in a know volume of gas containing a known concentration of helium starting at functional residual capacity, as patient breathes the helium concentration changes as it gets diluted because its in a larger volume - adding air already in the lungs
How is serial dead space measured?
The last gas in the airways is always the first gas out, take one normal breath of pure oxygen, breath out via a meter measuring % nitrogen, initially only oxygen expired and then a mixture of oxygen and air (inc nitrogen) from alveoli, volume that has been expired at this transition is the serial dead space
What is fowlers method for calculating serial dead space?
Plot the nitrogen washout results on a graph, draw a line through the curve so the 2 areas are equal and the volume up until this line is the dead space
How is diffusion conductance measured (ease at which gas can cross the membrane)?
Inhale gas mixture containing CO to total lung capacity, hold breath for 10 seconds, rate of CO diffusion is estimated, gives measure of conductance of CO from alveolar gas to Hb in pulmonary capillary blood cells
