Basic Lung Function Flashcards
Why is testing lung function useful?
- Evaluate lung function patterns in conjunction with patient’s history + exam aids diagnosis
- Can follow changes under normal circumstances + in response to treatment
- Assess changes in response to specific stimuli in lab or environment e.g. occupational asthma
What 3 different things can you measure in terms of lung function?
- Air flow rate: diameter of airways indication
- Vital capacity: measure of total lung volume
- How quickly gas diffuses into blood: measure of permeability of lung membranes + V/Q matching
What is vital capacity?
Measured from max inspiration to max inhalation so patient must take biggest breath in = good indicator of total lung volume
Air left in lungs is residual volume
What is a expiratory peak flow diary?
Uses a peak flow meter + diary to record values + symptoms
Max achievable expiratory flow determined by diameter of bronchial tree + muscle power available in L/minute
What are the advantages + disadvantages of the expiratory peak flow diary?
Adv: easy + cheap equipment
Diadv: technically easy to mess up so patient must know how to do it
What is the correct way of using a peak flow meter?
- Check pointer is at 0
- Stand/sit comfortably (keep same position if repeated)
- Hold it horizontally keeping fingers away from pointer
- Take deep breath out, deep breath in + wrap lips tightly round mouthpiece
- Blow as hard as you can using max force
- Check reading on pointer
- Rest back to 0
- Do 3 x + record highest value obtained
How will you know using a peak flow chart if someone has asthma? What should happen when treatment with a preventer inhaler starts?
> 20% variability between morning + evening readings (mornings usually lower) because airways are opening + closing (wheeze when closed)
Readings improve although slight variability still (which is normal) - response to treatment can aid diagnosis
What is used as a asthma preventer inhaler + relief inhaler?
Preventer: steroids
Reliever: B-agonist
What is spirometry?
Measures forced expiratory volume in one second (FEV1) + forced vital capacity (FVC)
Patient must do the FVC manouvere + the machine will print out a reading + drawing
What are the advantages + disadvantages of spirometry?
Adv: most readily available + useful lung test, takes 10-15 mins, little risk, internationally accepted criteria exist ensure results are optimal/repeatable + normal values been defined for + differ between populations
Disadv: expensive, requires lung function technician experts with significant training
Is the exhalation performed in expiratory peak flow + spirometry the same?
No
Expiratory peak flow is a big, very sudden + forceful breath out
But, spirometry exhalation should be a big breath out slowly + continued for as long as possible
What is the advantage of taking FEV1 in spirometry rather than peak expiratory flow?
Gives muscle independent flow so essentially gives an indicator of diameter of airways
What will a flow volume loop look like from a spirometry test in a healthy patient?
Flow will rapidly increase at first + then decrease because the lung is getting smaller + diameter of air pipes is getting smaller becoming the limiting factor despite muscle power (should get around ~ 550)
Declining line will hit volume axis at tight angle whereas emphysema lungs for e.g. will plateau to 0 as they cannot get air out of lungs as quickly
What will be lower in diseased lungs on a time volume loop than shown for a healthy lung?
FVC i.e. max volume that patient gets too - plateau will be a lot lower than in a healthy lung
What does restrictive lung disease (e.g. fibrotic/scarred lungs) look like in spirometry readings?
Small lungs show a restrictive spirometry pattern - reduced FVC (tall narrow curve)
Compatible spirometry supportive of clinical working diagnosis if used alongside supportive history + cracking noise on chest examination (alveoli collapsing early + popping open again)
What is the commonest cause of restrictive spirometry?
Failure to breathe out fully during the test so need technician to tell patient to carry on pushing out till they cannot anymore
What happens to the spirometry reading in airways obstruction disease e.g. emphysema?
Airways are narrowed so time taken to empty lungs in exhalation is increased -> FEV1 reduced (concave up curve)
Increased residual volume (more air trapped behind in obstructed bronchi) - not measured by spirometry
In conjunction with narrowing causing turbulent flow which can be heard as wheeze + barrel shaped/hyperinflated chest on examination
What is total lung capacity?
Vital capacity + residual volume
What will a spirometry curve shown if the patient has a fixed obstruction i.e. tumour around their trachea?
Fixed obstruction which will not change with breathing as trachea will not be able to open or close so you will see straight horizontal lines for inspiratory + expiratory flow i.e. fixed maximums either way
How can FEV1 and FVC be used together?
Normal FEV1 +FVC = > 80% of predicted
If you look at ratio, FEV1 reduced more than FVC = ratio < 0.7 (obstructive)
FVC is reduced more than FEV1 = ratio > 0.7 (restrictive)
What are FEV1 and FVC predictions based on?
- Normal distribution of population
- Height
- Weight
- Sex
How can we directly measure gas diffusion?
Direct measurement of rate at which carbon monoxide diffuses across lung membranes into blood
Compared to normal values for population + expressed as % of normal
= Diffusion Capacity for Carbon Monoxide (DLCO) - reduced in restrictive + obstructive lung disease due to less SA
What is direct measurement of gas diffusion used to assess?
Permeability of entire respiratory system including:
- Shunts
- Less permeable membranes
- V/Q mismatch
What are contraindications to lung function?
Risk vs benefit:
- Haemoptysis
- Closed or recent (2 weeks) pneumothorax
- Unstable CV status, pre-eclampsia + hypertension (MAP > 130 mmHg)
- Thoracic, abdominal or cerebral aneurysms
- Acute illness
- Recent thoracic, ear, ocular or abdominal surgery
- Active communicable diseases
Why is there contraindications to lung function tests?
As maximal pressure is generated in the thorax so there will be impact on thoracic + abdominal organs
Also, large swings in BP + expansion of chest wall + lungs occurs
What other conditions may affect the lung function results?
Acute respiratory conditions
Pain
Pregnancy
