Theme 4: Lecture 2 - Measuring lung function Flashcards
Spirometry
a common office test used to assess how well your lungs work by measuring how much air you inhale, how much you exhale and how quickly you exhale.
FVC
Forced vital capacity
Describe FVC or Tiffeneau manoeuvre
- Take a deep breath in
- Don’t hold your breath
- Put your lips round the outside of the tube and blow out as hard as you can for as long as you can
FEV1
Forced expiratory volume in 1 second
What is a normal FEV1
3.25 litres
What is a normal FVC
4.2 litres
What is a normal FEV1/FVC
77%
What are abnormal spirometry results
- any result < 80% of the predicted value
- any result < lower limit of normal
What is the lower limit of normal
taken to be equal to the 5th percentile of a healthy, non-smoking population
When is obstruction present
When FEV1/FVC is less than 70%
Why might FEV1/FVC be unreliable in more severe obstruction
In severe obstruction the patient sometimes has trouble reaching a full FVC
What can tell us a lot more about the characteristics of air flow than spirometry
A flow volume loop
Describe an expiratory flow volume loop in early airflow obstruction
- PEFR may be normal
- Mid-expiratory flow rates usually more affected
PEFR
peak expiratory flow rate
Describe an expiratory flow volume loop in severe airflow obstruction
- Lower FVC
- Lower PEFR
- Lower mid expiratory flow rates
Describe an expiratory flow volume loop in extra thoracic obstruction
- Unchanged FVC
- Flow rate plateaued
What can flow volume loops indicate
Where the obstruction is located in the tracheobronchial tree
Describe peak flow rate
-Easy to perform
-Easy to maintain device
-Useful for:
Diagnosis – asthma, not COPD
Monitoring day to day variation
Picking up exacerbations
Assessing response to treatment
-Mandatory for patients on nebulised Rx
What causes a decrease in the radius of the airway
- Mucus or other obstruction
- Bronchoconstriction
- Compression (from a mass)
Can lung volumes be obtained from spirometry
No
Methods of measurement for lung volumes
- Helium dilution
- Plethysmography ‘body box’
Characteristics of restrictive lung disease
- Reduced TLC, FRC, IC and RV
- Preserved tidal volume
- Reduced IRV (inspiratory reserve volume) / inspiratory capacity
- Reduced vital capacity
Causes of restriction and decreased lung volumes
- Alveolar filling process (e.g. pneumonia)
- Lung tissue disease: Fibrotic lung disease
- Pleural disease: pneumothorax, large pleural effusion, fibrosis of pleural tissue (“trapped lung”)
- Chest wall disease (e.g. kyphoscoliosis)
- Weakness (due to nerve and/or muscle disease)
Describe what happens in emphysema
- Loss of elastic recoil leads to compliance curve plateau occurring at a larger volume which leads to an increased TLC
- Occurs in COPD
