Mechanical Ventilation- How lung compliance affects ventilation in volume controlled ventilation.

Put simply the lung compliance is about its ability to inflate and deflate in relation to the pressures needed to make it do so.

Normal Lungs- Normal inflation/deflation

So, if it only takes a very small rise in pressure to instill a known volume into the lungs, then the lung is said to be very compliant, or floppy in our example. If it takes a larger amount of pressure to achieve the same movement of volume then the lung is said to have low compliance, or stiff in our example.

Stiff lung- Hard to inflate- deflate quickly.

So the rigid or stiff lung with the low compliance could be as a result of fibrosis or interstitial lung disease. This results in thickening in the pleura. This means that their lungs will not inflate easily but will deflate more readily.

'Floppy' lung- Easy to inflate, deflates slowly

When we have the overly compliant lungs, for example in the patient with COPD or emphysema where some of the structural tissue is broken down, simply put the lungs don’t hold themselves together so well. As a consequence they become much easier to inflate, but will deflate only slowly as they lose some of their recoil.

If we assume that we have set a tidal volume of 500mls in the normal lung then we will achieve a set pressure in order to do so. We measure this via the Peak Inspiratory Pressure (PiP) (see Mechanical Ventilation- Peak Pressure and Plateau Pressure) on the ventilator and for arguments sake lets say that we get a PIP of 20cmH20 to get all 500mls in.

We now assume that something has happened to the patient to make the lungs stiffer, or less compliant. We are still aiming to get 500mls into the lung but now the ventilator has to generate a higher PiP to do so. The PiP may go up to 30cmH2O for example. So now it takes 10cmH2O more pressure to achieve the same tidal volume. The patients compliance has gone down.

If the same patient has lungs which become more floppy then it will take a lower pressure to inflate the lung. It will take only 15cmH2O for example. The compliance has reduced.

Whilst this might sound like a good thing, the problem it causes is that the lung does not deflate so well as it has lost some of its recoil. This means that not all the gas will come out with each breath, as a consequence they can start to trap gas within the lungs. As the next breath comes in there is still some air left in from the previous breath.

To ensure that the pressures don’t get too high then we set a high pressure limit on the ventilator, for example 40cmH2O. If the ventilator meets this target then it will automatically cycle to the next stage of the breath, i.e. expiration, rather than continuing to deliver the same breath.

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