5.6 Principles of Pulmonary Function Testing (PFT) Flashcards
List some things that PFTs can measure
- Lung mechanics
- Diffusion properties
- Actions of different stimuli
List some situations where PFTs are used
- Assessing response to treatment
- Diagnosis and/or monitoring of respiratory disease
- Pre-operative and functional assessments
How are predicted values for PFTs found?
Testing a large volume of patients; normal distribution
What is meant by the lower limit of the normal (LLN) in terms of predicted values?
Lower limit of the normal is the bottom 5%
What is the upper limit of the normal?
Top 5%
Why is the LLN better than a fixed value?
Takes into account anthropometric characteristics
What is the most common lung function test?
Spirometry
Describe spirometry
- Clip nose
- Fill lungs completely
- BLAST air out
- repeat 3-8 times (or until repeatable and reliable results)
- Bronchodilator given
- Tested again
How long does spirometry take?
20 minutes
How does spirometry measure the amount of air left in the lungs after expiration?
It doesn’t
Obstructive vs restrictive lung disease
Obstructive: Hard to exhale all air in lungs
Restrictive: hard to fully expand your lungs with air
Does FEV1 increase or decrease during obstructive lung defect? Why?
- It decreases
- It becomes harder for air to move out of the lungs as quickly, so the amount of air that leaves in 1 second decreases
Effect of obstructive ventilatory defect on FEV1/FVC ratio (relative to LLN)
- Decreased
- Takes longer to empty lungs and reach FVC
What does obstructive lung disease look like on a flow volume loop?
- “Scooped out” downward slope
Findings of restrictive ventilatory defect on flow volume loop/spirometry testing
- Near normal FEV1
- Normal or slightly elevated FEV1/FVC
- Usually, lower forced expiratory time
List some examples of obstructive ventilatory defects
- Asthma
- Chronic bronchitis
- Emphysema
List some examples of restrictive ventilatory defects
- Pleural effusion
- Congestion
- Fibrosis
(anything that pushes in on the lungs)
Why is bronchodilator response significant in obstructive disorders? Give some examples to support your answer. What is the threshold for a significant response?
- Because they indicate whether or not the pathology can be reversed
- Conditions such as asthma are reversible
- Conditions such as COPD and emphysema aren’t
Threshold for significant response is 10% improvement.
Does decreased FEV1/FVC indicate obstructive or restrictive lung disease?
It indicates obstructive; there is an airflow limitation, meaning air is not getting out fast enough (therefore obstructive)
Does decreased FVC indicate obstructive or restrictive lung disease? Is this always the case, and why?
- Can indicate restrictive disease (lungs can’t expand as much, so total amount of exhaled air decreases)
- Might not be the case: maybe there is air left in the lungs, which would indicate something else. Confirm by measuring total lung capacity.
Functional residual capacity
From end of quiet tidal breath to no more air in lungs at all (ERV + RV)
Inspiratory capacity
Point from end of tidal volume (expiration) to maximum inspiratory capacity (VT + IRC)
Explain how lung volume by helium dilution works
Uses boyles law: C1V1 = C2V2
- Create closed system (C1 and V1)
- Get patient to breathe in an out helium gas, and measure conc (gives C2)
- Use values to calculate V2., which is total lung capacity
What happens to residual volume as we age? Why?
- It increases
- We lose elastic tissue and it becomes harder to empty our lungs
Why does TLC increase in emphysema?
- Lungs are overstretched, like the elastic in a pair of old underpants
- As a result, TLC increases
Describe changes in lung capacities in someone with pulmonary fibrosis
- Everything decreases
- It is harder to expand the lungs, but it is also harder to empty them, since elastic recoil is reduced
Describe changes in lung capacities in someone with neuromuscular disease
- Decreased volume
- Increased reserve volume -> muscles are not working properly
What is the probability that a healthy patient will have a lung function value below the lower limit of the normal?
5%; therefore, there is some lenience
Does a normal lung function value always imply no disease? What are the implications of this?
- Not always
- Clinical examination remains really important
Relationship between tissue area and rate of diffusion
Proportional
Relationship between diffusion constant and rate of diffusion
Proportional
Relationship between partial pressure across tissue barrier and rate of diffusion
Proportional
Relationship between tissue thickness and rate of diffusion
Inversely proportional
Explain how DLCO tests work
- Inhale tracer gas
- Inhaled CO
- Inhale regular air in the mixture
- The less CO left in the exhalation, the better the lungs are working, since the haemoglobin has taken up CO quickly
Factors that will increase diffusing capacity of CO
- Increased Hb binding sites
- Increase amount of blood in pulmonary capillaries
Factors that will decrease diffusing capacity of CO
- Decreased ability of CO to bind to Hb
- Reduces ability of CO to cross the air-blood barrier
- Reduces the amount of blood in the pulmonary capillaries
