M19PEEP, oxygenation and permissive hypercapnia in pregnancy

Clinical overview

Oxygenation and ventilation are separate problems with separate controls, and confusing them is why a patient ends up on 100% oxygen with a normal carbon dioxide and a collapsing lung. Oxygenation is a function of how much lung is open and perfused — PEEP and FiO₂ are its dials. Ventilation is a function of how much gas moves — rate and tidal volume are its dials.

Pregnancy shifts both. Functional residual capacity falls as the uterus lifts the diaphragm, so there is less reserve holding alveoli open and atelectasis forms faster, particularly supine and post-operatively. Oxygen consumption rises, so any interruption desaturates quickly. And the maternal-fetal oxygen relationship means the mother's saturation is the fetus's supply line — but chasing a supranormal maternal PaO₂ with high pressures and high FiO₂ helps neither.

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Figure M19.1 — Oxygenation depends on an open, perfused lung and is adjusted chiefly with PEEP and FiO₂; ventilation depends on gas movement and is adjusted with respiratory rate and tidal volume. Pregnancy reduces FRC, increases oxygen consumption and lowers the PaCO₂ baseline, accelerating desaturation and magnifying a given carbon-dioxide rise.

Core knowledge

What PEEP actually does

PEEP holds pressure at end expiration so that recruitable alveoli do not collapse between breaths. Its benefits and its harms come from the same mechanism:

• Benefit: more lung open at end expiration means better oxygenation, less injurious opening-and-closing, and a more compliant lung to ventilate.
• Harm: pressure applied to lung that is already open overdistends it, raises pulmonary vascular resistance, impedes venous return and can drop cardiac output — delivering less oxygen to the tissues even as the saturation improves.

That last point is the one to hold onto: the target is oxygen delivery — cardiac output multiplied by arterial oxygen content — not the number on the pulse oximeter. See the worked calculation in M2. A PEEP increase that improves the saturation while dropping the blood pressure and the urine output has made the patient worse.

Finding a workable PEEP

There is no single correct method and no pregnancy-specific table. What matters is that you change it deliberately and judge the result on more than one variable:

1. Choose a starting point (commonly 5 cmH₂O, higher in moderate–severe ARDS).
2. Change it in small steps, one variable at a time.
3. After each change look at: oxygenation, compliance or driving pressure, blood pressure and perfusion.
4. Keep the setting that improves oxygenation and mechanics without a haemodynamic penalty. If oxygenation improves while driving pressure rises, you are overdistending rather than recruiting.
5. Reassess after every position change, every fluid shift and after delivery — the abdomen that was pressing on the diaphragm at 34 weeks is not pressing on it two hours postpartum, and the same PEEP now behaves differently.