question about PEEP and venous admixture

[fleshwound]

I'm currently reading from "Critical Care Medicine" (Parrillo) on vents and came across something I'm having difficulty reasoning through.

"Mechanical ventilation may INCREASE the venous admixture to approximately 10% [from 2-5%] in the NORMAL individual. ...if positive pressure ventilation produces overdistension, redistribution of pulmonary blood flow to unventilated regions may occur, resulting in hypoxemia."

- I get that PEEP opens up alveoli in lung dz, but why would it be detrimental in the normal individual?
- if peep is opening up alveoli to the point of causing overdistension, what "unventilated regions" remain for blood flow to redistribute to? (are we talking about dead space of the conductive airways??)

thanks

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[Hernandez]

The lung isn't a homogenous sponge with the same compliance but my personal bias is that peep induced Hypoxemia is hard to induce normo or hypervolunemic, it's easier to induce in volume depleted PTs so over distinction on peep decreases cardiac output and often clinically is also accompanied by hypotension.