Mechanical Ventilation- Lung Compliance
Lung Compliance = how distensible is the lung, or how easily will it change shape?
Elastance = the property of resistance to changing shape- i.e. the opposite of compliance.
Opposition to inflation is caused by:
- Tissue elastic forces- the tissue within the lung itself has an elastic force which is also opposing inflation of the lung. Examples of such fibrous tissue would be collagen and elastin.
- Surface tension- the forces within a spherical shape are all pulling inwards, trying to collapse the sphere, as in the alveoli. This force will oppose the inflation of the lung.
The relationship makes sense when you work through it. Don't be frightened of all formulas!
If you have a large change in volume with a smaller rise in pressure then lung compliance is increased.
If you have a small change in volume with a large change in pressure then lung compliance is reduced.
Enter some figures and see!
So compliance is a change in volume for a given change in pressure.
Lets look at the pressure/volume curve above.
Note the lung volume at (1) does not start at zero. There is a reserve volume in the lungs- they are never completely empty.
The ventilator then starts to increase the pressure in the lung as it initiates the breath. However the lung volume does not change initially as the lung compliance is low at this stage (2).
Remember the balloon analogy here. It is initially hard to get the balloon to inflate....the effects of surface tension play a part in this.
When the pressure reaches a certain point the compliance will change markedly and becomes much greater. This is known as the lower inflection point (LIP) (3).
The compliance is much greater so the volume will increase rapidly (4).
As the airways become fuller the compliance will then fall again at the upper inflection point (UIP) (5). Again remember the balloon analogy...when the balloon becomes very full it becomes harder to blow into it.
In the normally compliant lung a change of 1cm H2O will result in a change in volume of 200 mls.
When the patient is ventilated this changes to 1cm H2O will result in a change of 60-80mls.
This reduced compliance is due to the changed lung mechanics when breathing via positive pressure as a opposed to negative pressure.
When the lungs deflate, for a given volume they are at a higher pressure compared to inspiration. That is why the line returns via a different path. this is known as hysteresis.
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