4. Compliance and Lung Function Tests Flashcards
What is surfactant?
Detergent like fluid produced by Type II alveolar cells
What does surfactant do?
- Reduces surface tension on alveolar surface membrane thus reducing tendency for alveoli to collapse
- Increases lung compliance
- Reduces lung’s tendency to recoil
- Makes work of breathing easier
Where does surface tension occur?
Where ever there is an air-water interface and refers to the attraction between water molecules
Why is surfactant more effective in small alveoli than large alveoli?
Surfactant molecules come closer together and are therefore more concentrated
When is surfactant produced?
Production starts at 25 weeks gestation and is complete by 36 weeks
What stimulates surfactant production?
Thyroid hormones and cortisol which increase towards the end of pregnancy
What can premature babies suffer from due to incomplete production of surfactant?
Infant Respiratory Distress Syndrome (IRDS)
Why is there less change in pressure in a saline filled lung compared to lung inflation in utero?
There is no need to overcome surface tension as there is not= air-water interface
Compliance
Change in volume relative to change in pressure
What does compliance represent?
The stretchability of the lungs
What does high compliance mean?
A large increase in lung volume for small decrease in ip pressure
What does low compliance mean?
Small increase in lung volume for large decrease in ip pressure
What can affect compliance?
- Disease states
- Age
Why does expiration require effort in emphysema?
Loss of elastic tissue
Why does inspiration require effort in fibrosis?
Inert fibrous tissue
Why is expiration usually passive?
Normally effort (work) of inspiration is recovered as elastic recoil during expiration
Why does the pressure volume curve vary between apex and base of the lung?
At the base, the volume change is greater for a given change in pressure
Why does compliance decline with height from base to apex?
- The alveoli at the apex are more inflated due to FRC.
- At the base the alveoli are compressed between the weight of the lung above and the diaphragm below, hence more compliant on inspiration
How does alveolar ventilation vary between apex and base?
Declines with height from base to apex
What does a small change in ip pressure result in?
A larger change in volume at the base compared to the apex
Obstructive Lung Disease
- Obstruction of air flow, especially on expiration
- Increased airway resistance
Restrictive Lung Disease
Restriction of lung expansion
What are 2 examples of obstructive lung disorders?
- Asthma
- COPD
Chronic bronchitis
Inflammation of the bronchi
Emphysema
Destruction of the alveoli, loss of elasticity
What is the prevalence of COPD?
- 80 million people worldwide moderate to severe COPD
- Increases with age
What are examples of restrictive lung disorders?
- Fibrosis
- Infant Respiratory Distress Syndrome
- Oedema
- Pneumothorax
What is fibrosis?
Formation or development of excess fibrous connective tissue
What are 2 causes of fibrosis?
- Idiopathic
- Asbestosis
What are restrictive lung diseases cause by?
- Loss of lung compliance
- Lung stiffness
- Incomplete lung expansion
Spirometry
Technique commonly used to measure lung function
How can spirometry measurements be classed?
Static or dynamic
Spirometry: Static
Where the only consideration made is the volume exhaled
Spirometry: Dynamic
Where the time taken to exhale a certain volume is what is being measured
What volumes can be measured directly by spirometry?
-Tidal volume
-Inspiratory reserve volume
-Expiratory reserve volume
Inspiratory capacity
Vital capacity
What is the normal value for forced vital capacity?
5L
What is the normal value for forced expiratory volume in 1 second?
4L
What is the normal value for FEV1:FVC?
80%
What is the value of FEV1:FVC for obstructive lung diseases?
42%
What is the value of FEV1:FVC for restrictive lung disease?
90%
Describe COPD
- Rate at which air is exhaled is much slower
- Total volume is also reduced
- Major effect is on airways and so FEV is reduced to a greater extent than FVC
- Ratio also reduced
Describe pulmonary fibrosis.
- Absolute rate of airflow is reduced
- Total volume due to limitations to lung expansion
- Ratio remains constant or can increase as a large proportion of volume can be exhaled in the first second
What limitations exist of the FEV1/FVC ratio?
- Obstructive, both FEV and FVC fall but FEV more so, so ratio is reduced
- Restrictive, both FEV and FVC fall so ratio remains much the same despite severe compromise of function
- Therefore normal FEV1/FVC ratio not always indicative of health
What is the forced expiratory flow?
Average expired flow over the middle of an FVC
How does FEV1 and FEF differ?
- FEF correlates with FEV1 but changes are generally more striking
- However, normal range is greater
