Alveolar pressure and the inspiratory hold manoeuvre

The inspiratory hold manoeuvre abolishes the pressure contribution from the airway resistance and reveals the pressure in the alveoli. This is available on virtually every specialist-grade ventilator, and consists of a manual override of the expiratory valve, forcing it to close and essentially producing a super-syringe-style test of lung compliance, where the entire respiratory system (including the ventilator circuit) is challenged with a static volume. 

From an exam point of view, there is no point in knowing this at the Part I stage.  It does not appear in the CICM primary syllabus (2017), nor is it a part of the CICM WCA ("Ventilation"), nor has it ever appeared in the Part I exam. It is, therefore, possible to go through one's early training program without ever having this knowledge undergo any level of standardised testing. Then, suddenly it might come up as it did in Question 28 from the first paper of 2014, where a minor sub-question asked "include in your answer how Ppl is measured" in relation to a patient with dynamic hyperinflation. To be fair, the WCA ("Ventilation") does have the expectation that a trainee "demonstrates methods to measure iPEEP" and the inspiratory hold manoeuvre is probably the most reliable method of doing this, so this chapter narrowly escapes complete irrelevance. 

In summary:

The best peer-reviewed resource for this is the 2014 article by Dean Hess, which happens to also have a massive amount of information on respiratory system mechanics. The time-poor exam candidate, to save precious seconds, may wish to skip straight to page 1775 where plateau pressure is discussed.

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). 

Here is a real-life example of this being done:

You can see that this is a patient with a combination of substantial airway resistance (contributing 21 cm H₂O to the total peak inspiratory pressure) and poor static lung compliance (16 ml/cm H₂O). During the inspiratory hold the pressure drifts down somewhat, demonstrating that lung unit with long time constants are filling while units with faster time constants are emptying; this pendelluft redistribution of pressure accounts for some of the pressure drop during the hold. Another reason for this gradual pressure drop is the relaxation of lung tissue and the chest wall, which is the main reason in people with normal healthy lungs (in a normal person there should be minimal pendelluft because their lung units should all have a very similar time constant). Anyway, a hold of about 2 seconds is recommended. From the diagram above it can be plainly seen that after two seconds of breath-holding the pressure has stabilised and holding the button down any longer than that is clearly going to be pointless).

What did we just measure?...

That was Pplat, the plateau airway pressure, which is directly related to alveolar pressure. It is the alveolar pressure you are interested in, which is a major determinant of 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.