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
Lung volumes in COPD
- Preserved TV
- Decreased IC, ERV and VC
- Increased FRC and TLC
- Significantly increased RV
Describe gas exchange
- Takes place at alveoli
- Depends on adequate ventilation of alveoli
- Influenced by alveolar surface area and thickness of alveolar membrane
- Delivers oxygen to blood where it combines with Hb
- Gets rid of CO2 into exhaled air
When does abnormal gas exchange occur
- Airway disorders (asthma & COPD)
- Alveolar destruction (emphysema)
- Fibrotic lung disease (idiopathic lung fibrosis, asbestosis etc)
- Abnormal ventilatory control
- Abnormal environment (altitude)
What is gas exchange measured as
CO transfer factor
Describe how gas transfer is measured
- CO diffuses like Oxygen
- CO binds to Hb and is carried away
- Inhale known volume of gas with low concentrations of CO and Helium
- Hold breath for known time
- Measure CO and He in expired air
- He dilution gives alveolar volume
What does a low diffusion capacity (TLCO) indicate
Abnormal gas exchange
What are the pulmonary diseases that decrease TLCO
- Emphysema
- Alveolar filling process
- Lobectomy
- Scarring or inflammation around the alveolar wall
What are the cardiovascular/haematological diseases that decrease TLCO
- Pulmonary hypertension
- Low cardiac output
- Pulmonary oedema
- Anaemia
What causes increased TLCO
- High cardiac output
- Pulmonary haemorrhage
- Polycythaemia
Name 4 other pulmonary function tests used in routine clinical practice
- Assessment of airway reversibility
- Assessment of ventilation
- Fitness to fly
- Respiratory muscle assessment
