Acute Respiratory Distress Syndrome (ARDS): Pathophysiology, Diagnosis, and Nursing Management

Acute Respiratory Distress Syndrome (ARDS) is a life-threatening complication of an underlying illness in which widespread alveolar inflammation causes severe hypoxemia that does not respond to supplemental oxygen alone. This note reviews the pathophysiology, causes, Berlin diagnostic criteria, and the lung-protective and nursing strategies most commonly tested on the NCLEX.

What Is ARDS?

• A life-threatening condition in which widespread inflammation causes fluid to leak into the alveoli.
• Fluid accumulation prevents oxygen from reaching the bloodstream.
• Develops rapidly — usually within 6 to 72 hours of the triggering event.
• Not a disease itself, but a complication of an underlying condition.

Pathophysiology

• Injury to the alveolar-capillary membrane increases permeability.
• Fluid, protein, and inflammatory cells leak into the alveoli.
• Surfactant production decreases, leading to alveolar collapse.
• Loss of surfactant produces stiff, non-compliant lungs.
• Ventilation–perfusion (V/Q) mismatching causes severe hypoxemia.

Causes of ARDS

Direct lung injury (pulmonary):

• Pneumonia (most common direct cause)
• Aspiration of gastric contents
• Inhalation injury (smoke, chemicals)
• Pulmonary contusion
• Fat embolism (long bone fractures)
• Near-drowning

Indirect lung injury (extrapulmonary):

• Sepsis (most common indirect cause)
• Severe trauma, burns
• Massive blood transfusion (TRALI)
• Pancreatitis
• Shock states (hypovolemic, cardiogenic)
• Drug overdose (opioids, salicylates)

Clinical Presentation

• Rapid-onset dyspnea within 6–72 hours of the trigger.
• Tachypnea with respiratory rate > 20 breaths/min.
• Refractory hypoxemia — does not improve with supplemental oxygen alone.
• Bilateral crackles/rales on auscultation.
• Frothy pink sputum indicating pulmonary edema.
• Cyanosis and accessory muscle use in severe cases.
• Restlessness, anxiety, and confusion from hypoxia.

Diagnostic Tests

• Chest X-ray: bilateral diffuse infiltrates, often described as a "white-out" appearance.
• Arterial blood gas (ABG): hypoxemia, early respiratory alkalosis, late metabolic acidosis.
• Pulse oximetry: SpO₂ < 90% despite high-flow oxygen.
• Echocardiogram: rules out cardiogenic pulmonary edema by showing normal cardiac function.
• CBC: elevated WBCs may identify an underlying infection.

Berlin Definition (Diagnostic Criteria)

• Timing: onset within 1 week of a known clinical insult.
• Chest imaging: bilateral opacities not fully explained by effusion or collapse.
• Origin of edema: respiratory failure not fully explained by heart failure or fluid overload.
• Severity by PaO₂/FiO₂ ratio:
  • Mild ARDS: PaO₂/FiO₂ 201–300 mmHg
  • Moderate ARDS: PaO₂/FiO₂ 101–200 mmHg
  • Severe ARDS: PaO₂/FiO₂ ≤ 100 mmHg

P/F Ratio

• P/F ratio = PaO₂ ÷ FiO₂.
• < 300: impaired oxygenation (mild ARDS).
• < 200: moderate ARDS.
• < 100: severe ARDS.

Management Strategies

Ventilator Management (Lung-Protective Strategy)

• Low tidal volume: 4–6 mL/kg of ideal body weight to prevent volutrauma.
• Plateau pressure < 30 cm H₂O to prevent barotrauma.
• High PEEP: 10–15 cm H₂O to recruit alveoli and improve oxygenation.
• Permissive hypercapnia: accept higher PaCO₂ to limit lung injury.

Prone Positioning

• Placing the patient face down improves oxygenation in severe ARDS.
• Redistributes blood flow to better-ventilated lung regions.
• Patients are typically turned every 12 to 16 hours.

Pharmacologic Management

• Sedatives and paralytics reduce oxygen demand and prevent patient–ventilator dyssynchrony.
• Neuromuscular blockers (e.g., cisatracurium) may be used in early severe ARDS.
• Antibiotics treat the underlying infection.
• Diuretics manage fluid balance and reduce pulmonary edema.

Fluid Management

• Use conservative fluid management — it improves oxygenation.
• Goal: neutral or negative fluid balance.

Complications

• Ventilator-associated pneumonia (VAP) from prolonged mechanical ventilation.
• Barotrauma — pneumothorax or subcutaneous emphysema from high airway pressures.
• Pulmonary fibrosis as scar tissue forms.
• Multiple organ dysfunction syndrome (MODS) from prolonged hypoxia.
• Acute kidney injury from hypoperfusion and sepsis.
• Mortality of 30–50% despite optimal treatment.

Nursing Care

Assessment

• Monitor respiratory rate, depth, and effort continuously.
• Assess oxygen saturation and titrate oxygen as ordered.
• Track ABG results and P/F ratio trends.
• Auscultate for crackles or diminished breath sounds.

Interventions

• Maintain a patent airway and confirm proper endotracheal tube placement.
• Position prone if ordered to improve oxygenation.
• Provide frequent oral care with chlorhexidine to prevent VAP.
• Suction the airway as needed using sterile technique.
• Monitor for barotrauma: sudden respiratory distress or decreased breath sounds.
• Monitor for VAP: fever, purulent sputum, worsening oxygenation.
• Keep head of bed elevated 30–45° to reduce aspiration and VAP risk.

Common NCLEX Traps

• ARDS is not cardiogenic pulmonary edema — PAOP (wedge pressure) is normal in ARDS.
• P/F ratio < 300 is diagnostic — not oxygen saturation alone.
• Low tidal volume prevents lung injury — high tidal volume worsens it.
• Prone positioning improves oxygenation but does not cure ARDS.
• Patients on neuromuscular blockers must also receive continuous sedation — they cannot move but remain conscious.
• VAP prevention requires oral care and head-of-bed elevation (30–45°).

Key takeaways

• ARDS is sudden, widespread alveolar inflammation causing refractory hypoxemia within 6–72 hours of a trigger such as pneumonia or sepsis.
• Diagnosis requires bilateral opacities, a P/F ratio ≤ 300, and no primary cardiac cause (Berlin criteria).
• Use lung-protective ventilation: tidal volume 4–6 mL/kg IBW, plateau pressure < 30 cm H₂O, and high PEEP.
• Prone positioning and conservative fluid management improve oxygenation in moderate-to-severe ARDS.
• Nursing priorities: monitor the P/F ratio, prevent VAP (oral care, HOB 30–45°), and watch for barotrauma.
• Mortality remains high (30–50%), and complications include VAP, barotrauma, pulmonary fibrosis, MODS, and AKI.