I:E and Peak Pressure (PIP)

[DartmouthDave]

Hello,

A quick question for those in the know.

With volume-cycled ventilation a shorter I-time means an increased flow rate and in turn an increased peak pressures.

Is this increased peak pressure just due to the resistance of the ET and the circuit?

Or, is this increased pressure transmitted to the lung?

Cheers

[Paramagic]

PIPs isn't that important with regards to what is being transmitted to the lung, it is not an accurate representation of what is happening at an alveoli level, just the circuit and larger airways

Plateau pressure is what the alveoli is seeing and you want that under about 30mmH20 (off the top of my head, I could be wrong) Most ventilators have an inspiratory hold button that will get you your plateau pressure when you hold it down.

[chbare]

The Pplat may be a good approximation of static compliance, but does not represent airway resistance (Raw). In fact, the common formulae we use to calculate Raw uses the PIP. Since we are talking about flow, (change in flow = change in velocity = acceleration = dynamic state) and thus movement, an examination of the PIP is valid.

To gain a better quantitative understanding of the flow relationship, make a simple model of the lung. Assume a straw is placed into a balloon. The measurement at the distal end of the straw can be called airway pressure or Paw. The measurement in the balloon would be the alveolar pressure or Palv.

The straw it's self represents the airways or the conduit for gas movement, thus pressure here will be a function of the product of flow and resistance. However, the alveoli measurement would be a function of volume divided by compliance. Of course, you would have to add whatever PEEP you have in the system as well.

Since we are interested in the Paw as it is a function of the PIP as it relates to flow, we can develop a formula to describe all of the concepts as they relate to Paw:

Paw = Flow * Resistance + Volume/Compliance + PEEP

As you can see, an increase in flow will lead to an increase in Paw.

Edited October 2, 2011 by chbare