Why test lung function?
Useful in conjunction with patient history and clinical examination.
Symptoms appear once patients are down to around 60% of lung function- lung function tests can be used to assess declining lung function in asymptomatic patients and provide education.
What does histamine cause?
How can it be used in assessment of lung function?
Patient complains of respiratory symptoms but lung function appears normal when assessed (e.g. asthma- airways return to normal when well).
Histamine can be given to check responsiveness of airways.
What is air flow rate a good measure of?
What is the vital capacity?
What does it indicate?
Volume from maximal inhalation to maximal exhalation.
Measures total lung volume
What can be used to measure how quickly gases diffuse across the alveolar membrane? (V/Q matching)
What is the residual volume?
The volume remaining in the lungs at maximum exhalation
What determines maximal achievable flow (on peak flow measurement)?
Diameter of the bronchial tree
How can a peak flow chart be used to diagnose asthma?
>20% variability in peak flow
What is FEV1?
Forced expiratory volume in 1 second
What measurements does spirometry use?
Why is it more accurate than peak flow?
FEV1 and FVC
As patients take a long, slow breath out, lungs decrease in volume making the airways smaller. Allows measurement of flow when the only limiting factor is airway diameter and is independent of muscle power.
In peak flow, patients with high muscle mass can have deceptively high readings.
What is the best indicator of lung restriction?
Reduction in vital capacity
Explain how the following features of obstructive airways can be explained:
Barrel shaped chest
FEV1 reduced: reduced airway diameter means the flow will be reduced therefore less air volume expired in 1 second.
Wheeze: Smaller airway diameter creates turbulent flow.
Barrel shaped chest:
- Air trapped behind obstructed (narrowed) airways increases residual volume
- Reduced elastic recoil of lungs means resting position of lungs is now at higher functional residual capacity- balance between elastic recoil of lungs and chest wall favours chest wall.