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.

Performing the inspiratory hold manoeuvre

Alveolar pressure and 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

References

Most of this information comes from only two textbooks. With "Basic Assessment and Support in Intensive Care" by Gomersall et al (as well as whatever I picked up during the BASIC course) as a foundation, I built using the humongous and canonical "Principles and Practice of Mechanical Ventilation" by Tobins et al – the 1442 page 2nd edition.