Acute Respiratory Distress Syndrome — ARDS

A patient's PaO2 stays dangerously low despite 100% oxygen — that refractory hypoxemia is the hallmark that separates ARDS from other respiratory failures and changes everything about management.

Acute respiratory distress syndrome is a life-threatening inflammatory response in which damage to the alveolar-capillary membrane causes protein-rich fluid to flood the alveoli, collapsing them and creating massive intrapulmonary shunting. The defining feature is refractory hypoxemia — PaO2 does not improve adequately with supplemental oxygen because blood passes through fluid-filled alveoli without gas exchange. Diagnosis uses the Berlin criteria: acute onset within 1 week of a known insult, bilateral opacities on chest X-ray not fully explained by effusions or atelectasis, respiratory failure not fully explained by cardiac failure, and PaO2/FiO2 ratio ≤ 300 mmHg (mild), ≤ 200 (moderate), or ≤ 100 (severe). Common triggers include sepsis, aspiration, pneumonia, massive transfusion, and trauma. Nursing priorities center on mechanical ventilation with lung-protective strategy: low tidal volumes (6 mL/kg ideal body weight), plateau pressure ≤ 30 cmH2O, and adequate PEEP to recruit collapsed alveoli. Prone positioning for 12–16 hours per day improves oxygenation in moderate-to-severe cases. Monitor for complications: barotrauma, VAP, and multiorgan failure. Strict I&O and conservative fluid management reduce pulmonary edema progression.

Key Distinctions

Don't confuse ARDS with cardiogenic pulmonary edema — ARDS shows bilateral infiltrates with a normal or low PCWP (≤ 18 mmHg), while cardiogenic edema shows elevated PCWP. Note: the Berlin criteria replaced the strict PCWP cutoff with clinical judgment that respiratory failure is not fully explained by cardiac failure, but the PCWP distinction remains a widely tested differentiator. Students mistake refractory hypoxemia for ordinary hypoxemia; the key difference is that cranking up FiO2 alone does not fix ARDS because the problem is shunting, not ventilation. The chest X-ray 'white-out' in ARDS is bilateral and diffuse — unilateral findings suggest pneumothorax or effusion instead.

Clinical Pearl

Think of ARDS alveoli as tiny water balloons — oxygen can't cross flooded walls no matter how much you deliver. PEEP is the squeeze that reopens them.