Mechanical Ventilation- Pressure/Volume loop.
This is the Pressure/Volume loop. Pressure is on the bottom axis and volume is on the side axis. this loop is generated every time there is a breath generated, either by the ventilator or by the patient.
This is the normal shape (rugby ball shaped), and should be positioned in the centre.
Notice that as the pressure increases initially there is not a great rise in volume. Remember the balloon analogy again. Its hard to start the inflation. But once it gets going at the lower inflection point the lungs start to inflate quite quickly.
At the top of the curve we have the peak inspiratory pressure (PiP) and achievement of the tidal volume (Vt).
The curve then moves down as the exhalation phase takes place. Pressure falls and the volume does also.
What if the curve looks like this? See the 'beak' at the top?
This may mean that you are giving too much volume thereby creating too much pressure.
This can cause barotrauma to the lungs.
In this instance you should decrease the volume if in volume controlled ventilation or decrease the pressure if in pressure controlled ventilation to achieve the upper inflection point only.
This ensures that you do not create too much stretch on the lung, a stretch which is not insufflating much more air anyway.
The patient may, through their illness, develop less compliant lungs.
In other words their lungs become stiffer and harder to inflate.
In volume controlled ventilation, where the machine will deliver a set volume their airway pressure will go up.
So the peak inspiratory pressure will rise.
With reduced compliance in pressure controlled ventilation the curve will still shift to the right but will look slightly different.
Here you can see that the pressure has not changed, the PiP has not increased as in the previous example, but the tidal volume is now reduced.
Mechanical Ventilation
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