Why is testing lung function useful?
Useful in symptomatic or patients at risk of pulmonary disease.
Uses:
- Evaluation of lung function patterns provides aid to diagnosis
- Can follow changes in lung function e.g. normal compared to treatment
- Assess changes in response to specific stimuli
What is measured in Lung function tests?
Air flow rate = measures diameter of airways
Vital capacity = vol. from max. inhalation to max. exhalation = measure total lung volume
V/Q matching = measure how quickly gas diffuses into blood
How is peak flow used to asses lung function?
- Easy to perform
- Cheap equipment (peak flow meter + diary to record values/symptoms)
- Max. achievable flow determined by diameter of bronchial tree + muscle power available (L/min)
How is peak flow performed?
- Sit in comfortable position (same every time)
- How peak flow meter horizontally
- Deep breath out then deep breath in
- Wrap lips tightly round mouthpiece
- Blow from max. inhalation so muscles + bronchioles are at max. diameter
- Air flow not limited as airways open
- Quick, sharp, hard single breath out into peak flow meter
- Check reading
What different inhalers can be given?
- Reliever - beta (sympathetic) antagonist, opens airways directly + gives immediate relief
- Preventer - reducing inflammation, long-term treatment
Explain the principles of Spirometry
Includes measurement of forced expiratory volume in 1 second (FEV1) and forced vital capacity (FVC)
What are the advantages/disadvantages of spirometry?
Advantages:
- readily available
- most useful test
- takes 10-15 mins
- internationally accepted criteria (ensures results are optimal + repeatable)
- high utility
Disadvantages:
- uses £800 equipment
- requires trained technician
- results heavily relied on
What does forced vital capacity (FVC) measure?
Measure from max. inspiration to max. expiration
Good indicator of total lung volume + flow
What is residual capacity/volume?
Volume of air left in lungs after FVC (difficult to measure)
What is the flow volume loop?
Not the same flow volume as measured with peak flow meter as it is not a forced volume
Flow volume increases drastically with inhalation and then drops dramatically as lungs/airways get narrower/smaller (diameter becomes only limiting factor)
What is Restrictive Spirometry?
Small airways = restricted so spirometric pattern = restrictive
Reduction in vital capacity = best index of lung restriction (measured from full inspiration with forced exhalation until cannot exhale further)
Used to support clinical working diagnosis but not used in isolation
What is the most common cause of restrictive spirometry?
Failure to breathe out fully during test
What is the flow like in lung restriction?
Flow maintained as lungs airways aren’t restricted BUT FVC is affected (decreases)
If airways are narrowed, time taken to empty lungs (exhalation) will…
Increase
How do you get a wheeze?
Amount of air breathed out 1 sec in hard exhalation will thus be reduced + airway narrowing results in turbulent flow (due to obstruction)
What signs would show airway obstruction?
Residual volume increase (more air trapped behind obstructed bronchi at end of expiration) Barrel shaped (hyper inflated chest)
What can affect FEV1 and FVC?
Height Weight Age Sex Ethnicity
What is normal FEV1 and FVC?
> 80% of predicted
What happens to the ratio of FEV1:FVC in a) normal b) obstructive lung disease c) restrictive lung disease?
a) ~0.7 (not strict)
b) <0.7 as FEV1 reduced more than FVC
c) >0.7 as FVC reduced more than FEV1
What is Diffusion Lung Capacity for Carbon Monoxide (DLCO)?
- Can directly measure rate at which CO diffuses across lung membranes into blood to assess permeability of entire respiratory system.
- Can then be compared to normal values for population and expressed as percentage of normal
- Both restrictive + obstructive lung diseases can reduce diffusion capacity.
