Acute Respiratory Distress Syndrome (ARDS)

Introduction

• ARDS: Acute Respiratory Distress Syndrome
• Part of clinical medicine section
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Definition and Criteria (Berlin Criteria)

• Acute Hypoxemia: SpO2 < 90% or PaO2 < 60 mmHg
• PF Ratio: Must be < 300
• Timing: Onset within less than one week
• Imaging: Diffuse bilateral infiltrates on chest x-ray or CT scan
• Non-Cardiogenic: Exclude cardiogenic pulmonary edema using Swan-Ganz catheter (PCWP < 18) or echocardiogram

Pathophysiology

• Diffuse Alveolar Damage (DAD)
  • Damage to type 1 and type 2 alveolar cells and pulmonary capillaries
  • Type 1 Cell Damage: Fluid, proteins, and immune cells leak into alveolus leading to exudative phase and poor ventilation (shunt)
  • Type 2 Cell Damage: Decreased surfactant production, increased surface tension, and alveolar collapse (shunt)
• Causes of DAD
  • Direct Lung Injury: Pneumonia, aspiration, inhaled toxins
  • Indirect Lung Injury: Sepsis, pancreatitis, transfusion-associated lung injury (TRALI)

Clinical Features

• Symptoms: Rapid breathing, increased work of breathing, shortness of breath
• Signs of Hypoxemia: Profound hypoxemia, respiratory distress

Complications

• Pulmonary Hypertension
  • Hypoxic vasoconstriction, increased pulmonary vascular resistance, right heart failure
  • Signs: JVD, hepatomegaly, ascites, pedal edema
• Ventilator-Associated Pneumonia (VAP)
  • Intubation > 2 days, bacterial colonization (Pseudomonas, MRSA)
• Ventilator-Induced Lung Injury
  • High PEEP and tidal volumes can cause pneumothorax or pneumomediastinum
  • High tidal volumes can cause increased inflammation and worsened lung injury
  • Low PEEP can cause alveolar de-recruitment and increased work of breathing
• Hyperoxia
  • Excessive FiO2 can cause tissue destruction and worsened V/Q mismatch due to free radicals

Diagnosis

• Criteria
  • Acute onset < 1 week
  • PF ratio < 300
  • Diffuse bilateral infiltrates on imaging
  • Non-cardiogenic: PCWP < 18 (Swan-Ganz) or normal echo
• Severity
  • Mild: PF ratio 200-300
  • Moderate: PF ratio 100-200
  • Severe: PF ratio < 100

Ventilator Management

• Modes
  • CMV: Controlled mechanical ventilation for non-spontaneous breaths
  • PSV: Pressure support ventilation for spontaneous breaths
• Adjusting Parameters
  • High PCO2: Increase respiratory rate or tidal volume
  • Low PCO2: Decrease respiratory rate or tidal volume
  • High PO2: Decrease PEEP or FiO2
  • Low PO2: Increase PEEP or FiO2
• Key Concepts
  • PEEP: Keeps alveoli open to improve ventilation and gas exchange
  • Pip: High PIP indicates resistance in ventilator tube, low PIP indicates air leak
  • Plateau Pressures: High pressures indicate poor lung compliance (reduce tidal volume)
• Liberation from Ventilator
  • Criteria: Resolved intubation reason, minimal FiO2 and PEEP, acceptable respiratory rate and tidal volumes

Treatment

• Initial Management
  • Sedation (Propofol, Midazolam)
  • Maintain euvolemia
  • Low tidal volume ventilation (< 6 cc/kg ideal body weight)
  • High PEEP to prevent alveolar collapse
• Advanced Management
  • Neuromuscular blockade: For PF ratio < 150 and failed initial measures
  • Proning: For PF ratio < 150 to reduce dependent atelectasis
  • Pulmonary vasodilators: For pulmonary hypertension and RV dysfunction
  • VV ECMO: Last resort for refractory hypoxemia