Performing the inspiratory hold manoeuvre
This is very similar to measuring intrinsic PEEP with an expiratory breath hold.
Once again, airway pressure has 2 components: (the resistance of the airways and the pressure in the alveoli)
What did we just measure?...
That was Pplat, the plateau arway pressure, which is directly related to alveolar pressure.
It is the alveolar pressure you are interested in. This is what determines your oxygenation.
However, you are never measuring that directly, because the pressure gauge is deep inside the ventilator.
You are measuring the pressure in the circuit, that is to say, the airway.
Airway pressure = (resistance of airways) + (alveolar pressure)
Resistance of airways = flow x resistance
Alveolar pressure = (volume over compliance) + PEEP
If airway pressure = flow x resistance + (volume over compliance) + PEEP,
…. and you take away flow (by stopping the inspiration), and you ignore (or subtract) PEEP, then...
Airway pressure = (0 x resistance) + (volume over compliance)
Thus, in absence of flow,
Airway pressure = alveolar pressure
The alveolar pressure should not get above 30 cmH2O.
In asthmatics, this manoeuvre also reveals the degree of dynamic hyperinflation. In those people, the alveolar pressure