JOVANA
Library Glossary Getting Started Three Levels Fields How it works Mission
Join the mission
All guides

ARDS: When the Lungs Flood From Within

Acute respiratory distress syndrome is inflammation flooding the air sacs, turning healthy lung into a wet, stiff, shrunken organ. Learn what defines it, why the oxygen is so stubborn, and the gentle-ventilation philosophy that saves lives.

What ARDS actually is

ARDS is a sudden, severe inflammation of the lung — set off by pneumonia, sepsis, aspiration, trauma, or pancreatitis — that makes the alveolar–capillary membrane leaky. Protein-rich fluid pours out of the capillaries and floods the alveoli. Air sacs that should be open and dry become wet and collapsed. The result is a lung that is heavy, stiff (low compliance), and full of shunt — blood flowing past flooded sacs that get no air at all.

The hallmark is brutal hypoxemia — pure type 1 respiratory failure — that barely responds to extra oxygen, because shunted blood never touches air no matter how rich the gas you offer. On the chest film both lungs look white with widespread fluffy shadows, and the heart is not the cause. It is defined formally by sudden onset (within a week), bilateral opacities not explained by heart failure, and a low oxygen ratio.

ARDS oxygenation grade — the P/F ratio (Berlin definition)

P/F ratio = PaO2 / FiO2   (measured with PEEP >= 5 cmH2O)

Worked example:
  PaO2 = 80 mmHg on FiO2 = 0.60
  P/F = 80 / 0.60 = 133

Severity bands:
  Mild      P/F  200-300
  Moderate  P/F  100-200   <- this patient (133) = MODERATE ARDS
  Severe    P/F  <= 100

The lower the ratio, the worse the shunt. Note how a 'good-looking'
PaO2 of 80 is alarming once you see it took 60% oxygen to get there.
The P/F ratio grades how severe the gas-exchange failure is.

The small, stiff lung — and why we ventilate it gently

A useful picture is the baby lung: because so much lung is flooded, only a small fraction is left to receive air. If you push normal-sized breaths into that tiny remnant, you over-stretch it and cause more injury — a vicious circle called ventilator-induced lung injury. So the central rule of ARDS care is lung-protective ventilation: small breaths, modest pressures, and just enough end-expiratory pressure to keep sacs from collapsing.

  1. Set a low [[tidal-volume-setting|tidal volume]] — about 6 mL per kilogram of *predicted* (ideal) body weight, based on height, not actual weight.
  2. Keep the plateau pressure low (aim under ~30 cmH2O) so you don't over-stretch the baby lung.
  3. Apply enough PEEP to splint the flooded sacs open between breaths and recruit lung for gas exchange.
  4. Accept a higher CO2 and a mildly low pH (“permissive hypercapnia”) as the price of protecting the lung.

Turning the patient over

When oxygen stays dangerously low despite good lung-protective settings, one of the most powerful manoeuvres is simply to flip the patient face-down: prone positioning. The flooded, heavy parts of the ARDS lung sit at the back when lying on the back, crushed by the heart and the weight above. Turning prone redistributes air and blood more evenly, opens up more lung for gas exchange, and in moderate-to-severe ARDS has been shown to save lives.