🏠 Study Hub 🖼️ Infographics
Nursing Field Notes / Respiratory · Med-Surg Course

ARDS 🫁

Refractory hypoxemia · stiff wet lungs · the oxygen that does not work

NG-395 Respiratory ADHD-friendly visual edition

Something injures the alveolar-capillary membrane, it leaks, and the alveoli fill with protein-rich fluid. The defining feature is hypoxemia that does not improve with oxygen — that is what the word refractory means.

The definitionHypoxemia that oxygen will not fix
The x-rayWhite-out — bilateral infiltrates, both lungs
Not the heartNormal wedge pressure — this is not fluid overload
The ventLow tidal volume 4 to 6 mL/kg, plus PEEP

🧨 What starts it

One alveolus healthy and the same alveolus in ARDS - membrane leaking, sac full of fluid, surfactant gone - with blood running past and picking up no oxygen. Beside it the white-out film and the ventilator settings.
Refractory means the oxygen does not work, and this is why: blood cannot collect oxygen from a sac that is full of fluid, however rich the gas above it. Swipe it sideways if it is cut off, or tap to open it full size.
How a normal lung becomes ARDS
  • An insult — sepsis is the commonest — triggers a systemic inflammatory response.
  • The alveolar-capillary membrane becomes permeable. Protein-rich fluid pours into the alveoli.
  • Surfactant is destroyed, so alveoli collapse and will not reopen.
  • Blood flows past collapsed, fluid-filled alveoli and picks up no oxygen — a true shunt. Adding oxygen cannot fix a shunt.
  • Days later the lung starts to fibrose and becomes stiff.
What sets it off
  • Sepsis — number one
  • Aspiration of gastric contents
  • Severe trauma, multiple fractures, burns
  • Near-drowning, smoke or toxic gas inhalation
  • Pancreatitis, massive transfusion, pneumonia
  • COVID and other severe viral pneumonias
ARDS or heart failure?

Both give bilateral infiltrates and terrible oxygenation. The separator is the pulmonary capillary wedge pressure: normal in ARDS, high in cardiogenic pulmonary edema. ARDS is a leaky membrane, not a failing pump.

🔎 What you will see

How it unfolds
  • 12 to 48 hours after the insult — she gets dyspnoeic and tachypnoeic.
  • Restlessness and anxiety first — that is hypoxia, and it comes before the saturation drops.
  • Retractions, accessory muscle use, then refractory hypoxemia: you raise the FiO₂ and the saturation barely moves.
  • Crackles throughout. Later the lungs get so stiff the chest barely rises.
  • Untreated it ends in multi-organ dysfunction.
The numbers
  • P/F ratio (PaO₂ ÷ FiO₂): under 300 = mild, under 200 = moderate, under 100 = severe.
  • ABG early: respiratory alkalosis from hyperventilating.
  • ABG late: respiratory acidosis as she tires and CO₂ climbs. That is the ominous one.
  • Chest x-ray: bilateral white-out, no cardiomegaly.
The sign that means she is failing

A rising PaCO₂ in someone who was blowing it off. She is running out of effort. That is an intubation call, not a wait-and-see.

🩺 What you do

Ventilator management — the part that is examined
  • Low tidal volume, 4 to 6 mL/kg of predicted body weight. Big breaths tear stiff lungs — this is called lung-protective ventilation and it saves lives.
  • PEEP holds alveoli open at the end of expiration. It is the treatment for the shunt.
  • Watch PEEP for barotrauma (sudden pneumothorax — absent breath sounds, tracheal deviation) and for dropping blood pressure from reduced venous return.
  • Permissive hypercapnia: a higher CO₂ is accepted to keep the volumes small.
  • Prone positioning for 12 to 16 hours improves oxygenation in severe cases.
Everything else
  • Treat the cause — antibiotics for sepsis, and quickly.
  • Sedation and sometimes paralysis to stop her fighting the ventilator.
  • Conservative fluids — a leaky membrane leaks more when overloaded.
  • Nutrition early; she is profoundly catabolic.
  • VAP prevention: head of bed 30°, oral care, daily sedation holiday.
  • DVT and stress-ulcer prophylaxis.
What the nurse watches for
  • Sudden loss of breath sounds on one side plus dropping pressure = pneumothorax from barotrauma. Emergency.
  • Falling blood pressure after a PEEP increase — report it.
  • Restlessness is hypoxia until proven otherwise. Do not sedate restlessness before checking the oxygen.
  • Suction only when indicated — coughing, rising airway pressures, visible secretions. Not on a schedule.

⚡ Quick recall

The definitionHypoxemia that oxygen will not fix
The x-rayWhite-out — bilateral infiltrates, both lungs
Not the heartNormal wedge pressure — this is not fluid overload
The ventLow tidal volume 4 to 6 mL/kg, plus PEEP
What single word defines ARDS?
Refractory — the hypoxemia does not improve with supplemental oxygen.
What tidal volume, and why?
4 to 6 mL/kg predicted body weight. Large volumes injure already-stiff lungs.
What separates ARDS from cardiogenic pulmonary edema?
The wedge pressure. Normal in ARDS, high in heart failure.
The PaCO2 is climbing. What does that mean?
She is tiring. Prepare for intubation — that is a late and dangerous sign.
Two things PEEP can cause?
Barotrauma (pneumothorax) and hypotension from reduced venous return.