Acute Respiratory Distress Syndrome

  • Pathophysiology: Insult causes diffuse alveolar damage, release of cytokines, and recruitment of neutrophils which damage capillary endothelium, increasing fluid in interstitium and alveoli. Loss of functional surfactant makes alveolar units prone to collapse
  • Causes: sepsis, aspiration, contusion, shock, inhalation, burns, pancreatitis, drugs (ASA, cocaine, TCA’s, opioids, contrast), possible genetic component
  • Berlin Definition
Timing Within 1 week of a known clinical insult or new worsening respiratory symptoms.
Chest Imaging Bilateral opacities not fully explained by effusions, lobar/lung collapse, or nodules.
Origin of Edema Respiratory failure not fully explained by cardiac failure or fluid overload.

Need objective assessment (e.g. echocardiography) to exclude hydrostatic edema if no risk factor present.

Oxygenation

Mild

Moderate

Severe

 

200 mmHg < PaO2/FiO2 </= 300 mmHg with PEEP or CPAP >/= 5 cm H2O

100 mmHg < PaO2/FiO2 </= 200 mmHg with PEEP >/= 5 cm H2O

PaO2/FiO2 </= 100 mmHg with PEEP >/= 5 cm H2O

  • ARDS Net Protocol
    • Avoid barotrauma (Ppl <30); Volutrauma (VT 6mL/kg IBW); alectatrauma (keep PEEP high before weaning FiO2)
    • Vent Setup: Calculate Ideal Body Weight; VT 6 mL/kg IBW; Adjust RR for goal minute ventilation. Keep checking at least q4 hours and adjust VT and RR to achieve goal pH (7.3 – 7.45) and Ppl < 30 cm H20
      • In patients with ARDS, lower tidal volume ventilation (6 mL/kg PBW) is associated with lower mortality and fewer days on the ventilator
    • Oxygenation goal: PaO2 55 – 80 mmHg or SpO2 88 – 95%

Higher PEEP/lower FiO2:

FiO2 0.3 0.3 0.3 0.3 0.3 0.4 0.4 0.5 0.5 0.5 -0.8 0.8 0.9 1.0
PEEP 5 8 10 12 14 14 16 16 18 20 22 22 22 – 24

 

  • “Dry Lungs are Happy Lungs”: Conservative fluid management (targeting CVP < 4 mmHg) improved lung function and decreased ventilator days and ICU days, but did not improve 60-day mortality. Conservative fluid strategy was NOT associated with increase in incidence of shock or need for dialysis at 60 days.
  • Paralyzing: Thought to improve patient-ventilator synchrony and decrease barotrauma. In RCT, patients with early (< 48h) severe (P/F < 150, PEEP at least 5) ARDS, who were paralyzed with cisatracurium for 48 hours had decreased 90-day mortality (32% vs 41%, NNT 11), reduced barotrauma, reduced pneumothorax, and no significant increase in paresis.
  • Proning: Improves V/Q mismatch by recruiting previously atelectatic dependent alveoli. In RCT, early (< 36h after intubation) and lengthy (16h/day) proning in severe ARDS patients (defined here as P/F < 150, FiO2 > 0.6 and PEEP at least 5 mm H2O) significantly decreased 28-day mortality (16% vs 33%, NNT 6).
  • Corticosteroids for Persistent ARDS: Methylpredinisolone did NOT improve mortality at 60 days, and was associated with increased mortality at 60 and 180 days if started at least 14 days after onset of ARDS
    • Did increase ventilator-free and shock-free days at 28 days, associated with improved oxygenation, lung compliance, and blood pressure (fewer days of vasopressor therapy)
    • Did not increase rate of infectious complications
    • Did increase rate of neuromuscular weakness