Vent waveform for dummies

[fleshwound]

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Marino ICU book Fig 24.1 - for constant flow volume cycled ventilation - the proximal airway pressure rises immediately as shown by the initial vertical upsloping segment. Why? Why not a constant increase like the alveoli?

X-axis is unmarked by I'm assuming it's Time.

 

[Mad Jack]

Do you mean this picture: https://www.inkling.com/read/the-icu-book-paul-marino-4th/chapter-25/figure-25-1

 

[Mad Jack]

If that's the diagram you're talking about, the initial spike in pressure is a function of the resistance of the ETT. Since the circuit is pressurized, the instant a breath begins, gas flows through the ETT. The ETT is smaller than the vent circuit, so there's a bit of back pressure that occurs at the moment of inspiration.

 

[fleshwound]

Figure 24.1 from 3rd edition, which doesn't seem to be in the 4th. http://imgur.com/4CbiRua

The initial pressure rise looks more pronounced in the older edition. So hypothetically if ETT was larger than vent circuit, then we wouldn't see that back pressure? And it'd be a smoother line?

 

[Hernandez]

Over simplified answer, Compliance, you have to overcome chest wall compliance and intrinsic airway resistance before and the volume actually starts moving. Just pick up a balloon and try and inflate it and you can feel what that picture shows