Pulmonary function tests Flashcards
What are lung function tests used for?
To help establish a diagnosis in lung disease
Helpful in obstructive and restrictive lung disease which are recognisable findings on the tests
What is the average tidal volume of the lung?
500ml
What is the average dead space volume of the lung?
150ml
How much volume of air goes to the alveoli?
350ml
What is the tidal volume (TV)?
The volume of air taken in during normal quiet breathing
Divided into:
TVi - volume of air inspired
TVe - volume of air expired
What is the inspiratory reserve volume (IRV)?
Volume of air inspired past the TVi
What is the expiratory reserve volume (ERV)?
Volume of air expired past the TVe
What is the residual volume (RV)?
The volume of air that has to remain in the lung to prevent it from collapsing
What is the functional residual capacity (FRC)?
The total amount of air remaining in the lungs during normal quiet breathing
ERV + RV
Remember you don’t usually breathe out ERV unless you force air out of your lungs, so in normal breathing ERV remains in your lungs
What is the inspiratory capacity (IC)?
The total amount that your lung is capable of inspiring
TVi + IRV
What is the total lung capacity (TLC)?
The total amount of air your lungs are capable of holding
VC + RV
What is the vital capacity (VC)?
The “functional” capacity of your lung
ERV + IRV + TV(i+e)
What are the 3 main types of pulmonary function tests?
Spirometry
Lung volume testing
Diffusion capacity
What is spirometry?
A simple test that helps diagnose and monitor pulmonary diseases (e.g., COPD, asthma)
It measures how much air an individual can breathe out in one forced breath
What are the components of spirometry?
FEV1 = forced expiratory volume in 1 second
FVC = forced vital capacity - total/highest amount of air an individual can force out irrespective of time
Spirogram
Flow loop volume
What FEV1/FVC ratio suggests obstruction?
< 0.7
What happens to FEV1 and FVC in obstruction?
FEV1 reduced
FVC slightly lower or normal
This is due to reduced elasticity and increased compliance
So the lungs can take air in easily but lack the elasticity to recoil sufficiently to push enough air forcefully hence FEV1 decreases
What happens to FEV1 and FVC in restriction?
Reduced FVC
Normal/slightly low FEV1
Increased recoil and reduced compliance
The lungs become too stretchy/recoils easily and is not able to expand to take enough air in
So the amount of air available to forcefully expire reduces (i.e., FVC reduces)
What are the characteristics of a spirometry?
Function of GENDER + AGE + HEIGHT + RACE
No defined absolute normal or absolute abnormal values
Compare actual values to predicted values
Abnormal result often defined as % predicted < 80
Exception is for FEV1/FVC ratio, which expressed and evaluated as an absolute ratio
What is a positive response in a bronchodilator responsiveness?
Increase FEV1 (and/or FVC) by 12% AND 200ml
This indicates reversible obstruction
NOTE: the lack of a bronchodilator response in the lab does not preclude a clinical response to bronchodilator therapy in real life
What are the different flow loop volume charts and what do they indicate?
What is the methacoline challenge test (MCT)?
A test of airway responsiveness
Looks at how “twitchy” the airways are, by seeing whether they narrow after inhaling (breathing in) methacholine
Used in asthma testing
What is methacoline?
Methacoline is a non-selective muscarinic receptor agonist that acts on airway smooth muscle to induce bronchoconstriction
How is the MCT carried out?
- Patient does a spirometry test to check what the baseline lung function is. If this is not too low, they move to the next step
- Patient may be asked to breathe in a saline solution via nebuliser before repeating the spirometry test
- If there is no change in lung function, the patient will be asked to breathe in a low dose of methacholine via a nebuliser
- Spirometry test is repeated immediately after inhalation of methacholine, and again after a few minutes
- If lung function does not change after the lowest dose of methacholine, the patient inhales another stronger dose of methacholine. They then do the spirometry test again
- As long as they remain unreactive to methacholine, the patient continues the test; inhaling stronger dose of methacholine (usually 5-10 doses in total), followed by spirometry testing
- As soon as the lungs show a significant drop in FEV1 of 20% or more, the test is considered complete. The [methacholine] is the test result
- Patient will then be given a bronchodilator treatment (usually salbutamol or ipratropium) to inhale
- Spirometry test is repeated to ensure lung function has returned to normal
How should you advise patients to prepare for their MCT?
On day of test, don’t eat or drink products containing caffeine e.g., coffee, tea, cola drinks, energy drinks, chocolate. Caffeine can act as a weak bronchodilator and cause inaccurate test results
Wear loose fitting clothing to your appointment, to allow your chest to expand freely as you will be asked to take deep breaths in and blow out hard during the tests
Don’t do any vigorous exercise for 30 minutes before the appointment. This may tire you out before the tests
Do not eat a large meal within 2 hours before the test. This may make you feel uncomfortable whilst taking the tests
What is the gas transfer test (TLco)?
Measures how well the lungs take up O2 from the inspired air
The test result is called the transfer factor (or the diffusing capacity - DLco)
TLco refers to the transfer capacity of the lung, for the uptake of CO
What is the TLco used for?
To help diagnose and monitor lung conditions including COPD and pulmonary fibrosis
Can also be used to assess the lungs before surgery or to see how a person’s lungs react when they receive chemotherapy
Which pulmonary conditions will reduce TLco?
Emphysema
Lung infiltration
Lobectomy
Which cardiovascular/haematological diseases will reduce TLco?
Pulmonary HTN
Low cardiac output
Pulmonary oedema
Anaemia
Which conditions will increase the TLco?
High cardiac output
Pulmonary haemorrhage
Polycythaemia
What should patients expect during a TLco test?
Breathe in air containing tiny amounts of helium and CO
Asked to take a big breath through a mouthpiece while wearing a nose clip
Hold breath for a minimum of 8 seconds, then breathe our steadily into the machine
Need to do this a few times, with a pause of a few minutes in between
It could take several attempts to get a reliable reading
What should patients not do before the TLco test?
Not smoke
Results can be affected by smoking
Lung diffusion capacity relies on capillary blood volume and membrane diffusivity. In smokers there is a significant decrease in capillary blood volume
Which results does a gas transfer test give?
TLco - shows how efficiently the lungs work to take O2 from the inspired air. Calculated from Kco and VA
Kco - measures how well the airways are performing
VA (alveolar volume) - estimates the lung surface area available for gas exchange
Low lung efficiency = < 80% of normal predicted values
What is the PEFR?
The peak expiratory flow rate
Measured during spirometry but also with hand-held devices at home
Do same time each day (there are morning dips)
Useful for patients with asthma
- picks up exacerbations
- assesses response to Tx
How would you tell a patient to do a PEFR?
- Move the dial all the way to the bottom
- Take a deep breath and inhale as much air as you can and then hold your breath
- Place you mouth over the mouthpiece and form a tight seal
- Blow out as hard and as fast as you can
- Record the PEFR reading
- Repeat steps 1-5 three more times
- Take the highest reading as your PEFR value
How does restriction affect the RV, FRC, & TLC?
Causes a reduction
What conditions can cause decreased lung volumes?
Lung parenchymal process : expansion of the interstitial space (water, scar or inflammation) or “airspace filling process”
Pleural disease
Chest wall disease
Weakness
- focal or global
- nerve or muscle based
Sources
https://www.ouh.nhs.uk/patient-guide/leaflets/files/59679Pmethacholine.pdf
https://www.blf.org.uk/support-for-you/breathing-tests/gas-transfer-tlco
Measuring Lung function Module 103 05.01.2018 lecture
