Mechanical Ventilation- Peak and Plateau Pressure
Peak pressure applies when there is airflow in the circuit, i.e. during inspiration. What determines the peak pressure is the airway resistance in the lungs. So it follows that if there is a problem with the airways the peak pressure will rise.
Plateau pressure applies when there is not airflow in the circuit. That is when inspiration is complete. This pressure is determined by the lung compliance. So it follows that if there is a problem with the compliance the plateau pressure will rise.
So lets work through this diagram to help our understanding of the difference, and how it can help us interpret the information.
If we ventilate the lung then the airway pressure will go up and the flow will increase (1).
When we release the breath, the pressure will go down, and the flow will move in the opposite direction as the air moves out (2). The flow then returns to base line (3).
Let us then say that we inflate the lungs but then hold them at inflation for a period of time. The pressure will reach a peak (4), and then as the breath hold comes in there will be an equilibrium pressure (5), (remember the balloon analogy here: it takes high pressure to inflate the balloon, but once inflated, a lot less effort to keep it inflated) until release of the pressure as exhalation takes place (6).
You can see on the flow line the period between (5) and (6) where there is no flow as the breath hold takes place.
So, remember, peak pressure (circled) is a reflection of the airways and plateau pressure is a reflection of lung compliance.
Remembering that important fact, lets look at a couple of examples that we might encounter.
So firstly we can look at a wave form demonstrating a high peak pressure.
If the difference between the peak pressure and the plateau pressure is greater than 5 mm Hg then that is defined as an elevated peak pressure.
The other extreme would be where there is not a problem with the peak pressure but instead with a high plateau pressure.
So if the pressure alarm is going off you need to know whether it is a high peak pressure causing the problem or a high plateau pressure.
Why is that important?
Because a high peak pressure is an airway issue, for example:
- Retained secretions
- Mucous plug
- ETT tip occlusion
...and a high plateau pressure is a compliance issue, for example:
- Pulmonary oedema
So understanding the difference between peak pressure and plateau pressure will help you diagnose what may be wrong with your patients ventilation
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