Acute Respiratory Distress Syndrome (ARDS)

Jessica Reed

Background

ARDS is a form of non-cardiogenic pulmonary edema characterized by acute onset (<7 days), bilateral pulmonary infiltrates, and severe hypoxemia as a result of diffuse systemic inflammation which damages the alveoli/capillary endothelium interface causing fluid and protein accumulation within the interstitium and alveoli leading to impaired gas exchange, decreased lung compliance, and pulmonary hypertension

Diagnostic Criteria

  1. Timing: Onset within one week of a known clinical insult or new/worsening respiratory symptoms
  2. Chest Imaging: Bilateral opacities on CXR/CT and/or consolidations on US. Not fully explained by effusions, atelectasis, or nodules/masses
  3. Origin of Edema: Respiratory failure not fully explained by cardiac failure or fluid overload
  4. Oxygenation: Diagnosis defined with PaO2:FiO2 ≤ 300 or SpO2:FiO2 ≤ 315 with respiratory support of PEEP ≥5 or HFNC ≥ 30L. Severity based on PaO2/FiO2 ratio with PEEP or CPAP ≥ 5cm H2O (see below)

Triggers of ARDS

Direct

  • Pneumonia (viral and bacterial)
  • Aspiration
  • Lung contusion d/t trauma
  • Direct inhalation injury
  • Primary Graft dysfunction of transplant

Indirect

  • Non-pulmonary Sepsis (most common cause)
  • Massive Blood Transfusion/TRALI
  • Hematopoietic stem cell transplantation
  • Drugs (Opioids, TCAs, ASA, cocaine, amiodarone, chemotherapy, salicylates)
  • Pancreatitis
  • Burns
  • Radiation

Evaluation

Severity:

  • Based on PaO2/FiO2 ratio (“the P to F ratio”) with PEEP ≥ 5 cm H20 (as above)
  • PaO2- arterial pressure of O2 (Obtained by ABG)
  • FiO2- Fraction of inspired oxygen (expressed as a decimal between 0.21 and 1.0)

PaO2/FiO2 ratio

SpO2/FiO2 (if SpO2 ≤ 97%) if intubated

PEEP or HFNC (Optiflow)
Mild 300-201 246-315 ≥5 or ≥30L
Moderate 200-101 149-245 ≥5 or ≥30L
Severe <100 <148 ≥5 or ≥30L

Management

  • Frequently requires intubation as non-invasive ventilation is not often effective.
  • Interventions with mortality benefit
    • Low tidal volume(Vt) ventilation (LTVV)
      • Goal Vt is 4-8 mL/kg of ideal body weight
      • Target plateau pressure of ≤ 30 cm H2O
      • Oxygenation goal: PaO2 55-80 mmHg or SpO2 88-95%
      • Slowly titrate PEEP and FiO2 concurrently to achieve oxygenation goals
      • pH goal: ≥ 7.20; “permissive hypercapnia”; titrate respiratory rate to target as normal pH as possible
      • Treat ventilator dyssynchrony
    • Proning
      • Can be considered if moderate to severe ARDS and oxygenation does not improve with LTVV
      • PROSEVA study (2013) demonstrated mortality benefit to early and prolonged proning (16 hours per day) in patients with moderate to severe ARDS (most significant benefit for severe)
  • Interventions that probably won’t hurt the patient but with less data to support them
    • Conservative fluid management with IV diuresis as needed (at risk for pulmonary edema d/t vascular leak); goal of net even or negative. FACTT study demonstrated 2 more ventilator-free days with aggressive diuresis, but no mortality difference
    • Glucocorticoids in ARDS d/t septic shock, COVID, CAP, steroid-responsive conditions; may shorten time on ventilator but may also result in some neuromuscular weakness
  • American Thoracic Society Guidelines of 2024 placed emphasis on early ECMO involvement and neuromuscular blockade in severe ARDS patients. Two trials on neuromuscular blockade (ACURASYS and ROSE) with conflicting results. EOLIA suggest benefit to ECMO in patients with P/F<80 or hypercapnic respiratory acidosis.
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