Let's start with a little multiple choice. Which mechanism(s) could contribute to hypoxemia in this patient?

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The answer is ALL of the above!

But why? To answer, we need to understand what PEEP is and what effects it has on the lung.
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Positive end expiratory pressure (PEEP) is the pressure above atmospheric that is applied in between breaths while on a mechanical ventilator.

PEEP is beneficial for two reasons:
1️⃣ PEEP recruits collapsed lung (see video)
2️⃣ greater alveolar pressure drives more O2 into the blood (Henry's law)

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As an aside, it's worth noting that the benefits of PEEP in ARDS were discovered *accidentally*.

In 1967, two doctors caring for a patient on a ventilatory spotted a knob labeled "expiratory retard" and - not knowing what it did - decided to give it a try. It worked!

Nowadays we call that knob PEEP and it's an indispensable part of mechanical ventilation.

https://pubmed.ncbi.nlm.nih.gov/28731363/

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Ok so we understand why PEEP can help, but why can PEEP be *harmful*?

We need to understand the relationship between lung volumes & pulmonary blood flow.

Let's take a closer look at the alveoli. With the help of an electron microscope we can see that alveoli are surrounded by a dense interconnected network of blood vessels.

(btw, if you were wondering, they obtain these amazing images by injecting the vessels with a polymer, dissolving the remaining tissue, then taking SEM micrographs)
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