Compliance Flashcards
What is elasticity?
The resistance of an object to deformation by external force.
What is compliance?
The ability to stretch.
The inverse of elasticity, or 1/E.
What are the features of high elasticity?
High elasticity
Low compliance
Hard to stretch
Easy to recoil
What are the features of low elasticity?
Low elasticity
High compliance
Easy to stretch
Hard to recoil
What are the forces on the lung at rest?
Opposing forces act on the lung and chest wall.
Chest wall has high outward elastic recoil.
Lung has high inward elastic recoil.
Lung distending pressure is always positive, which prevents the lung from collapsing.
What happens to lung distending pressure during inspiration?
When inspiring, the thorax expands, and the chest wall pulls away from the lung.
The pressure in the fluid-filled pleural cavity decreases further, because fluid cannot expand or compress.
The lung then expands further, and the lung distending pressure becomes more positive.
What happens to intrapleural pressure?
The intrapleural pressure (in between lung and chest wall) is about -0.5kPa at FRC.
As we breathe in, the pressure becomes more and more sub-atmospheric, and the lung expands more and more.
See picture.
What happens to different pressures during breathing?
Alveolar pressure (PA) is equal to barometric pressure (PB) at the end of expiration.
Alveolar pressure (PA) is equal to barometric pressure (PB) at the end of inspiration.
Lung distending pressure (PA - Ppl) is greatest at the end of inspiration.
What happens to air flow during breathing?
Air flow is zero at the end of expiration.
Air flow is zero at the end of inspiration.
What is the relationship of lung expansion?
The degree of lung expansion is proportional to distending pressure.
Pdist = Palv - Ppl
So, generating a greater outward, distending pressure leads to a greater change in lung volume.
What is the lung compliance curve?
The slope = lung compliance
Compliance = δV/δP distending
Compliance = change in lung volume - change in distending pressure.
Lung distending pressure becomes more positive during inspiration.
What is distending pressure at FRC?
The distending pressure is about 0.5kPa.
The lung volume is about 2L.
The lung volume then increases lots for a small change in distending pressure.
How does compliance change with lung volume?
At very low lung volumes, lung compliance is low - it is hard to expand the lung, lots of pressure is required.
At normal volumes compliance is high - the lung is easily inflated.
At high lung volumes, compliance if low - the lung does not expand much with increasing pressure.
What is the effect of low compliance on breathing?
Interstitial fibrosis, there is hardening of the alveolar walls, due to collagen and scarring usually from smoking.
This causes decreased compliance - it is hard to expand the lung.
The distending pressure has to change more for the lungs to expand.
What is the effect of high compliance on breathing?
Emphysema - the lung walls collapse, the alveoli break down.
Compliance increases.
A small change in distending pressure leads to a large change in volume.
It is easy to inflate the lungs, but hard to expire, because the elastic recoil is gone, especially when coughing.
What does the effect of compliance look like on a graph?
see picture
What determines the compliance of the lung?
The elastic properties of the tissue - elastic connective tissue forces. Therefore thickening lung tissue decreases compliance.
Surface tension at air-water interfaces within alveoli, this accounts for more than half lung elastic recoil.
What is surface tension in the lungs?
Surface tension is a collapsing force, but it is very low in the lungs due to the release of surfactant from type 2 pneumocytes, so the lungs do not collapse.
What does surfactant do?
High surface tension would normally collapse alveoli.
Surfactant molecules have strong attraction for each other, but low attraction for other molecules.
Surfactant molecules accumulate at the surface and reduce surface tension.
What would happen if there is no surfactant?
e.g. in premature babies.
Compliance would decrease.
Very high elastic recoil - lots of effort to inflate lungs.
Can be given artificial surfactant.
The figure below shows a compliance curve for a normal lung.
What would the curve look like if we inflated the lung with fluid instead of air?
If filled with fluid there would be no surface tension.
So the lung would be more compliant
Line 1
see picture.
What is Laplace’s law?
P = 2T / r
T = surface tension force.
The pressure in a round structure is inversely proportional to the radius.
Pressure is directly proportional to the surface wall tension. If r goes down, T goes down by the same amount, so P stays the same.
Why do smaller alveoli not collapse into larger alveoli?
Surfactant reduces surface tension so pressure is the same in the alveoli with a different radius.
So the pressures in the different sized alveoli are the same.
How is the effect of surfactant area dependent?
Smaller radius alveoli means a lower surface area.
This increases the density of the surfactant.
So it is more effective at reducing surface tension in smaller alveoli and during deflation of the lung.
What is chest wall distending pressure?
The chest distending pressure is always negative - pushes the chest in and prevents it springing outward.
How does chest compliance change?
In scoliosis, the spinal deformity means the ribcage cannot move as efficiently,
In spondylitis - the vertebrae is fused, so is rigid and prevents the moving of the ribcage.
Pectus excavatum - the sternum is deformed inwards.
Obesity - reduces chest recoil.
What does the distending pressures look like on the graph?
At FRC, the distending pressures of the lung and chest wall are equal and opposite - the slopes have the same gradient.
This means the lung and chest wall have the same compliance and elastic properties.
See picture.
What happens to distending pressures in pneumothorax?
The distending pressures are lost, and the linkages between the lung and chest wall.
So the lung collapses to the minimum volume, and the chest wall springs out.
The overall compliance curve is more shallow.
See picture.
What happens to distending pressures in emphysema?
The chest wall compliance stays the same, but the lung compliance increases.
So the lung distending pressure becomes steeper, and the total compliance becomes steeper - the curve rests at a higher volume.
See picture.
What happens to distending pressure in interstitial fibrosis?
The chest wall compliance stays the same, but lung compliance increases.
The lung distending pressure becomes more shallow, so the overall compliance curve becomes more shallow - the curve rests at a lower volume.
See picture.
