ARDS Quiz

ARDS (Acute Respiratory Distress Syndrome): A Core Review for Clinical Exams

Acute Respiratory Distress Syndrome (ARDS) is a form of acute, diffuse, inflammatory lung injury leading to increased pulmonary vascular permeability, increased lung weight, and a loss of aerated lung tissue. This guide breaks down the essential concepts needed for exam success, focusing on diagnosis, pathophysiology, and management cornerstones.

Understanding the Berlin Definition

For exam questions, memorizing the four core components of the Berlin Definition is critical for diagnosing and classifying ARDS severity. This definition replaced the previous American-European Consensus Conference (AECC) definition of ALI/ARDS.

• Timing: Onset within 1 week of a known clinical insult or new/worsening respiratory symptoms.
• Chest Imaging: Bilateral opacities on chest radiograph or CT scan, not fully explained by effusions, atelectasis, or nodules.
• Origin of Edema: Respiratory failure not fully explained by cardiac failure or fluid overload. An objective assessment (like echocardiography) is needed to exclude hydrostatic edema if no risk factor is present.
• Oxygenation Impairment (on PEEP ≥ 5 cmH₂O):
  • Mild: 200 mmHg < PaO₂/FiO₂ ≤ 300 mmHg
  • Moderate: 100 mmHg < PaO₂/FiO₂ ≤ 200 mmHg
  • Severe: PaO₂/FiO₂ ≤ 100 mmHg

The Three Phases of ARDS Pathophysiology

ARDS typically progresses through three overlapping phases. Questions may test your ability to associate clinical findings with the correct phase.

1. Exudative Phase (Days 1-7): Characterized by diffuse alveolar damage (DAD). Injury to the alveolar-capillary barrier causes a leak of protein-rich fluid into the alveoli, forming hyaline membranes and causing severe hypoxemia.

2. Proliferative Phase (Days 7-21): The lung begins to repair itself. Type II pneumocytes proliferate, and fibroblasts deposit collagen. Patients may improve and be weaned from the ventilator, or the lung injury may progress.

3. Fibrotic Phase (After Day 21): Not all patients enter this phase. It is characterized by the obliteration of normal lung architecture by fibrosis, leading to high dead space, increased risk of pneumothorax, and long-term pulmonary dysfunction.

Core Principles of Lung-Protective Ventilation (LPV)

LPV is the cornerstone of ARDS management and a proven mortality-reducing strategy. Its goal is to minimize ventilator-induced lung injury (VILI).

• Low Tidal Volume (Vₜ): Target 4-6 mL/kg of predicted body weight (PBW), not actual weight.
• Plateau Pressure (Pplat): Maintain Pplat < 30 cmH₂O. This pressure reflects alveolar distension and is more critical than peak inspiratory pressure (PIP).
• Driving Pressure (ΔP): Maintain driving pressure (Pplat – PEEP) as low as possible, ideally < 15 cmH₂O. High driving pressure is strongly associated with mortality.
• Permissive Hypercapnia: Tolerate an elevated PaCO₂ and resulting respiratory acidosis (pH typically >7.20-7.25) to achieve low Vₜ and Pplat goals.
• PEEP Titration: Use sufficient PEEP to prevent alveolar collapse (atelectrauma) and improve oxygenation, while avoiding overdistension and hemodynamic compromise.

The Role of PEEP in Alveolar Recruitment

Positive End-Expiratory Pressure (PEEP) is applied to keep alveoli open at the end of expiration. This action “recruits” collapsed alveoli, increasing functional residual capacity (FRC), improving V/Q matching, and enhancing oxygenation.

Prone Positioning: When and Why?

Prone positioning has a proven mortality benefit in patients with severe ARDS (PaO₂/FiO₂ ≤ 150 mmHg). It should be initiated early and maintained for at least 16 hours per day. It works by recruiting dorsal lung regions, improving V/Q matching, and promoting more homogenous lung inflation.

Differentiating Direct vs. Indirect Lung Injury

Understanding the initial insult helps anticipate the pattern of lung injury. Exam questions often test this distinction.

Direct Causes (Injury starts in the lung): Pneumonia, aspiration of gastric contents, pulmonary contusion, near-drowning.

Indirect Causes (Systemic inflammation hits the lung): Sepsis (most common overall cause), severe pancreatitis, major trauma, multiple transfusions (TRALI).

Fluid Management: The Conservative Approach

After initial hemodynamic stabilization, a conservative fluid strategy is recommended. The FACTT trial showed this approach leads to more ventilator-free days and a shorter ICU stay compared to a liberal strategy by reducing pulmonary edema.

Key Takeaways for Exam Day

• ARDS is a non-cardiogenic pulmonary edema defined by the Berlin criteria (Timing, Imaging, Origin, Oxygenation).
• The primary pathology is diffuse alveolar damage leading to leaky capillaries and impaired gas exchange.
• Management is centered on treating the underlying cause and providing lung-protective ventilation (low tidal volume, plateau pressure <30 cmH₂O).
• Prone positioning is a life-saving intervention for severe ARDS (PaO₂/FiO₂ ≤ 150).
• A conservative fluid strategy after initial resuscitation improves outcomes by limiting lung edema.

This content is intended for educational and informational purposes only to supplement exam preparation. It is not a substitute for professional medical advice, diagnosis, or treatment from a qualified healthcare provider.