PEEP

Blow up a balloon and let it go fully flat, and the next inflation is hard work because the walls have stuck together. Keep just a little air in it, and the next breath is far easier. PEEP — positive end-expiratory pressure — applies this idea to the lungs: instead of letting the airway pressure fall to zero at the end of each breath out, the ventilator holds a small positive pressure, so the air sacs never fully collapse.

Keeping air sacs open between breaths has two big benefits. It stops the repeated open-and-slam cycling that itself injures fragile lung tissue, and it keeps more of the lung available to take up oxygen, which improves oxygenation in conditions like ARDS where many air sacs would otherwise sit flooded and shut. Because it recruits and stabilises lung, raising PEEP is one of the main levers used to lift a stubbornly low blood oxygen.

PEEP is set in centimetres of water, often starting around five and raised as needed, and it can also be delivered without a ventilator by CPAP masks. An honest caveat: more is not always better — too much PEEP can overstretch healthier air sacs and, by raising the pressure inside the chest, can squeeze the large veins and reduce the blood returning to the heart, lowering blood pressure. The right level balances opening the lung against these costs.