Ventilation and Compliance 2 Flashcards
What is surfactant?
Detergent-like fluid produced by Type II Pneumocytes
What is the function of surfactant?
Reduces surface tension on alveolar surface membrane and reduces its tendency to collapse
What is surface tension?
Surface tension occurs at air-water interfaces and refers to the attraction between water molecules
What the is the overall effect of surfactant on the compliance of the lung?
Increases its distensibility
What is the Law of LaPlace?
Shows the relationship between pressure and a equation of surface tension and radius of the alveoli
What is the equation of the Law of LaPlace?
P=2T/r
How does the Law of LaPlace marry the different pressures in differently sized alveoli and what role does surfactant play?
The Law of LaPlace shows that smaller alveoli have higher pressures, surfactant reduces surface tension and the pressure is equalised
What are the gestational milestones for surfactant production?
approx. 25 weeks - Surfactant production begins
Complete by 36 weeks
What hormones stimulate surfactant production in foetuses?
Thyroid hormones and cortisol
When babies are born prematurely what pathology can arise to do with incomplete surfactant development?
Infant Respiratory Distress Syndrome (IRDS)
What is compliance?
Change in volume relative to change in pressure; represents stretchability of the lungs
What is meant by a high compliance?
Large increase in volume for a small decrease in pressure
What is meant by a low compliance?
Small increase in lung volume for large decrease pressure
Describe the pressure-volume relationship
It requires a greater change in pressure (from FRC) to reach a particular volume because the work done during inspiration is recovered in elastic recoil for expiration (why its described as passive at rest)
Describe the pressure-volume relationship associated with Emphysema
Loss of elastic tissue means expiration requires effort
Describe the pressure-volume relationship associated with fibrosis
Inert fibrous tissue means effort of inspiration increases
How does the pressure volume curve vary in different regions of the lung?
At the base, the volume change is greater than the apex for a given change in pressure.
Alveolar ventilation and compliance also decrease from base to apex
Why is the volume change at the base of the lung greater than the apex for a given change in pressure?
At the base, alveoli are more compressed by the rest of the lung and the diaphragm therefore are more compliant on inspiration
What is the difference between obstructive and restrictive lung diseases?
Obstructive - obstruction of airways, particularly on expiration
Restrictive - restriction of lung expansion (inspiration)
What are two examples of obstructive lung disease?
Asthma and COPD (a combination of chronic bronchitis and emphysema)
What are three examples of restrictive lung diseases/disorders?
IRDS
Oedema
Pneumothorax
What is spirometry?
A technique used to measure lung function. Measurements can either be classes as static or dynamic
What is the difference between static and dynamic measurement s in spirometry?
States - where the only consideration made is the volume exhaled
Dynamic - where the time taken to exhale a certain volume is being measured
What lung volumes and capacities can’t be directly measured by spirometry?
RV, TLC and FRC
What is a typical FEV1 value for healthy males?
4L
What is a typical FVC in healthy males?
5L
What is the typical FEV1/FVC for a healthy male as a percentage?
80%
What FEV1/FVC percentage is typically seen in obstructive pulmonary diseases?
<80%
What FEV1/FVC percentage is typically seen in restrictive diseases?
> 80%
Why are obstructive diseases characterised by an FEV1/FVC percentage <80%?
Rate at which air is exhaled is slower; reduced FEV1 and little effect on FVC therefore ratio is reduced
Why are restrictive diseases characterised by an FEV1/FVC percentage >80%?
Total lung volume is reduced therefore ratio can increase as large proportion of volume can still be exhaled quickly
What are the limitations for the FEV1/FVC for both obstructive and restrictive diseases?
Both FEV and FVC fall - ratio stays the same despite seriously compromised function
What is FEF25-75?
Forced expiratory flow - average expired flow one the middle of the FVC
What is the advantage of using FEF25-75?
Correlates with FEV1 but changes are generally more striking
What is the disadvantage of using FEF25-75
“Normal” range is greater
