BACKGROUND

Patients with multiple injuries are known to develop lung injury1 which can result in long-term disability or even death.2 A recent review of combat casualty deaths following admission to a hospital demonstrated that 8% of potentially preventable deaths are from multi-organ failure which includes ARDS.3 The purpose of this guideline is to review the diagnostic criteria for ARDS, to describe the frequency of this problem in combat casualties, and to recommend a series of management strategies to permit safe aeromedical evacuation of these patients.

Respiratory failure has been observed in combat casualties for over a century. Some degree of ARDS occurs in between 26% and 33% of combat casualties.1,4,5 In a review of the DoD Trauma Registry (DoDTR), 152 patients with ARDS were identified over a 7 year period.6 Independent risk factors for ARDS included female sex, shock or tachycardia on presentation, and severe injury (Military Injury Severity Score (mISS) ≥25). Patients with ARDS had a significantly increased risk of death as compared to intubated controls (12.8% vs. 5.9%, Odds Ratio 1.99, 95% confidence interval [1.12, 3.52], p=0.02). Further analysis of this population identified that increased crystalloid infusion and Fresh Frozen Plasma (FFP) transfusion independently predicted ARDS.7

DEFINITIONS

- ARDS develops as a result of both direct and indirect injury to the lungs. Common causes of ARDS following a direct injury include pneumonia or gastric aspiration. In combat casualties, direct insults such as pulmonary contusion, inhalation injury, and fat emboli may lead to ARDS. ARDS from indirect lung injury can occur in patients who receive multiple transfusions, who develop septic shock, or in those with severe acute pancreatitis.
- Cardiac failure or fluid overload must be ruled out when considering the diagnosis of ARDS. Several other disease processes can also mimic ARDS. Patients with these conditions will benefit from lung-protective ventilator management but may require disease-specific interventions as well. Examples include Acute Eosinophilic Pneumonia (AEP), Acute Interstitial Pneumonitis (AIP), Bronchiolitis Obliterans Organizing Pneumonia (BOOP), and Diffuse Alveolar Hemorrhage (DAH).8
- The definition of ARDS was updated in 2012. The new Berlin definition of ARDS reflects a range of severity from mild to moderate to severe, defines “acute onset” as within one week and specifies the need for Positive End Expiratory Pressure (PEEP) ≥5 cm H2O.9 The original American-European Consensus Conference (AECC) definition of ARDS which also included Acute Lung Injury (ALI)10 is less practical; so this CPG will refer to the new Berlin definition. Timing of ARDS must occur within one week of a known clinical insult described above, or must be in the context of new or worsening respiratory symptoms. On chest imaging (CXR or CT scan), bilateral opacities must be present which are not fully explained by pulmonary edema, effusions/hemothorax, lobar collapse, or pulmonary nodules. If the above criteria are met, the degree of hypoxemia with a PEEP or Continuous Positive Airway Pressure (CPAP) of at least 5 cm H2O determines the severity of ARDS.

1. Mild ARDS = PaO2 to FiO2 ratio (P:F) of > 200 and ≤ 300.

2. Moderate ARDS = P:F of >100 and ≤ 200.

3. Severe ARDS = P:F of ≤ 100.

- The diagnosis of ARDS is typically made in patients who have respiratory failure that requires intubation and mechanical ventilation.