Lung Function Testing Flashcards
What are the benefits of Lung function testing?
Non-invasive
Cheap
Technically Simple: measurement of Volume and Flows
Technically Complex: Measurement of the composition of gases
What is the Tidal volume?
The volume that enters and leaves the lungs with each breath (0.5L)
What is the Inspiratory Reserve Volume?
The extra volume that can be breathed IN over that at rest (2.5L)
What is the Expiratory Reserve Volume?
The extra volume that can be breathed OUT over that at rest (1.5L)
What is Residual Volume?
The volume remaining after maximal expiration, cannot be measured by spirometry but contributes to lung capacity (0.8L)
What is the difference between lung volume and lung capacity?
Lung volumes change with tidal volume.
Lung capacities do not change with tidal volume, as they are defined relative to fixed points in the breathing cycle. Capacities are fixed.
How is the Vital capacity calculated?
Biggest breath in
(IRV+ TV+ ERV= VC
Inspiratory reserve volume + Tidal Volume + Expiratory Reserve Volume
What is the normal Vital capacity in a typical adult?
~5L (3.7L F)
What is Inspiratory Capacity?
The biggest breath that can be taken in from resting expiratory level (the lung volume at the end of quiet expiration)
Typically 3L
What is the typical inspiratory capacity?
~3L (3.8L M, 2.7L F)
What is the function residual capacity of the lung?
The volume of air in the lungs at the end of quiet respiration.
ERV + RV
typically ~ 2L
What is the typical FRC?
~2.3L
What is the total lung volume?
The total volume of gas in the lungs at the end of maximal inspiration
Vital Capacity + Reserve Volume
Typical 5.8 L
(4.4L F)
What is the typical Total Lung Volume
~5.8L (4.4L F)
What does Vital capacity depend on?
capacity not volume
Depends on maximal inspiration and maximal expiration, i.e maximal inspiratory effort and force of recoil.
- Inspiration: Compliance of the lungs Force of inspiratory muscles - Expiration: Airway resistance: increases as expiration proceeds
Or both.
What produces a restrictive deficit?
If lungs are unusually stiff, or inspriratory effort is compromised by muscle weakness, injury or deformity
What produces a obstructive deficit?
During expiration (particularly forced) the small airways are compressed, increasing flow resistance, eventually to a point where no more air can be driven out of the alveoli.
if airways are narrowed, then expiratory flow is compromised much earlier in expiration.
e.g. asthma
Discuss the affect on the FEV1/FVC ratio in an obstructive defect
In an obstructive defect:
- FVC is nearly normal as lungs are easy fill
However the resistance will increase on inspiration, the air will come out more slowly
- FEV1 is reduced markedly
- FEV1/FVC ratio is less than 70%
What is the normal FEV1/FVC ratio?
70%
what is FEV1?
The Forced Expiratory Volme in one second
What is FEV1 used for?
To distinguish between restrictive and obstructive defects
Discuss the affect on the FEV1/FVC ratio in an restrictive defect
In a restrictive defect:
The lungs are more difficult to fill; stiff, weak muscles, problem with chest wall
- FVC is reduced
The air will come out normally
- FEV1 is reduced proportionally
- FEV1/FVC ratio is normal (or even higher than normal)
What is a vitalograph?
volume plotted against time
What is the PEFR?
The peak expiratory flow rate- at the start of expiration when the lungs are expanded the airways are stretched open, the expiratory flow rate is at is maximum.
Resistance at its minimum
Discuss the effects a obstructive deficit would have on a flow volume curve?
e.g. asthma
The volume expired remains the same.
However as the lungs are compressed, more air is expired and airways begin to narrow. Resistance increases and the flow rate falls. The narrower the airways are to start (i.e in a obstructive defect) the more rapidly the flow rate falls (scalloping of flow vol curve)
Discuss the effects a restrictive deficit would have on a flow volume curve?
e.g. emphysema
the lungs fail to ill normally, however the PEFR is reached as the flow rat is unaffected.
the shape of the curve is generally the same, but narrower (as the volume is decreased)
and flow rate is greater at than normal at comparable volumes
How do you measure the residual volume in the lungs/
Using the helium dilution test
Describe the helium dilution test
Helium is not metabolised
The patient breathes in a known volume of air containing a known concentrate of helium.
This concentration changes and helium becomes diluted as the lungs fill, because the helium comes in contact with air already in the lungs.
What is serial/physiological dead space?
The volume of air inhaled that does not enter the lungs either because it does not enter the conducting airways or because of poor perfusion across the alveoli
What test would you use to measure the dead space in the lungs?
Nitrogen wash out test
Describe the nitrogen wash out test
The last gas in is the first gas out
pt breathes in pure O2
then breathes out, % N is measured, inially only pure o” then a air which inc. N2
vol expired at transition = serial dead space.
How would you measure diffusion capacity of the lungs?
The Carbon monoxide transfer factor measurement
Describe the CO Transfer Factor measurement
Calculated by measuring the CO uptake following a single maximal breath of a gas mixture containing air, 14% helium and 0.1% CO. Inhaled CO is used because of its very high affinity for Hb.
Since almost all the CO entering the blood binds to Hb, the concentration gradient for pCO (between alveolar gas and plasma) across the alveolar capillary membrane is maintained (and remains the same) for the entire time blood remains in contact with alveolar gas.
The amount of CO transferred is an estimate of the diffusion resistance barrier
What is diffusion conductance?
the resistance to diffusion across the alveolar membrane, and is estimated by the CO transfer factor
When would a CO transfer factor measurement be indicated?
When spirometry suggests a reduction in Vital Capacity, Residual Volume or Total lung capacity
Looks at diffusion conductance (resistance)
True or False
A 70Kg man with normal lungs
Would achieve an alveolar conc of 5% when breathing in 5% helium for 2 normal breaths
True
He is not metabolised
What are the benefits of interpreting a Flow vol curve in disease?
Sensitive
Can detect problems early compared to spirometry
Can discriminate where in the respiratory tract the problem lies
