Acute Respiratory Distress Syndrome

Definition

Definition from Matthay et. al. 2023

ARDS is an acute, diffuse, inflammatory lung injury precipitated by a predisposing risk factor, such as pneumonia, nonpulmonary infection, trauma, transfusion, burn, aspiration, or shock. The resulting injury leads to increased pulmonary vascular and epithelial permeability, lung edema, and gravity-dependent atelectasis, all of which contribute to loss of aerated lung tissue. The clinical hallmarks are arterial hypoxemia and diffuse radiographic opacities associated with increased shunting, increased alveolar dead space, and decreased lung compliance. The clinical presentation is influenced by medical management (position, sedation, paralysis, positive end-expiratory airway pressure, and fluid balance). Histological findings vary and may include intraalveolar edema, inflammation, hyaline membrane formation, and alveolar hemorrhage.

Criteria

• Risk factors and origin of oedema:
  • Acute predisposing risk factor (e.g. pneumonia, nonpulmonary infection, trauma, transfusion, aspiration or shock)
  • Cardiogenic pulmonary oedema does not exclude ARDS but the cause of respiratory failure must primarily be non-cardiogenic
• Timing
  • Acute onset or worsening of hypoxemic respiratory failure within 1 week of the estimated onset of the predisposing risk factor or new or worsening respiratory symptoms.
• Chest imaging:
  • Bilateral opacities on chest radiography and CT or bilateral B lines and/or consolidations on ultrasound not fully explained by effusions, atelactasis or nodules/masses

Categorisation

• Intubated ARDS:
  • Mild:
    • $200<P_{\text{a}}\mathrm{O}{}_2:F_{\text{i}}\mathrm{O}{}_2<300$
    • $235<S_{\text{p}}\mathrm{O}{}_2:F_{\text{i}}\mathrm{O}{}_2<315$ (if $S_{\text{p}}\mathrm{O}{}_2<97\%$)
  • Moderate:
    • $100<P_{\text{a}}\mathrm{O}{}_2:F_{\text{i}}\mathrm{O}{}_2<200$
    • $148<S_{\text{p}}\mathrm{O}{}_2:F_{\text{i}}\mathrm{O}{}_2<235$ (if $S_{\text{p}}\mathrm{O}{}_2<97\%$)
  • Severe:
    • $P_{\text{a}}\mathrm{O}{}_2:F_{\text{i}}\mathrm{O}{}_2<100$
    • $S_{\text{p}}\mathrm{O}{}_2:F_{\text{i}}\mathrm{O}{}_2<148$ (if $S_{\text{p}}\mathrm{O}{}_2<97\%$)
• Nonintubated ARDS:
  • $P_{\text{a}}\mathrm{O}{}_2:F_{\text{i}}\mathrm{O}{}_2<300$
  • $S_{\text{p}}\mathrm{O}{}_2:F_{\text{i}}\mathrm{O}{}_2<315$ (if $S_{\text{p}}\mathrm{O}{}_2<97\%$) on HFNP with flow of > 30L/min or NIV/CPAP with at least 5 cm $\mathrm{H}{}_2\mathrm{O}$ of PEEP
• In resource-limited settings:
  • $S_{\text{p}}\mathrm{O}{}_2:F_{\text{i}}\mathrm{O}{}_2<315$ (if $S_{\text{p}}\mathrm{O}{}_2<97\%$)¹

Management

• Open lung model ventilation (i.e. low $V_{\text{T}}$ and high PEEP)

Sources

• Acute Respiratory Distress Syndrome Definitions • LITFL
• A New Global Definition of Acute Respiratory Distress Syndrome - PubMed
• IBCC: ARDS
• Mechanical Ventilation Explained Clearly by MedCram.com | 4 of 5 - YouTube

Footnotes

1. Neither PEEP nor a minimum oxygen flow rate are required for diagnosis in reosurce-limited settings ↩