Acute Respiratory Distress Syndrome (ARDS)

Kira Duvall

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

  • Timing: Onset within one week of a known clinical insult or new/worsening respiratory symptoms.
  • Chest Imaging: Bilateral opacities on CXR/CT and/or consolidations on US. Not fully explained by effusions, atelectasis, or nodules/masses.
  • Origin of Edema: Respiratory failure not fully explained by cardiac failure or fluid overload.
  • 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, sometimes called lung protective ventilation).
      • 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.
  • Interventions that probably won’t hurt the patient but with less data to support them:
    • Conservative fluid management with IV diuresis as needed. 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.
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