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ARDS is characterized by diffuse injury to the pulmonary capillary endothelium and alveolar
endothelium that results in leakage of protein-rich fluid into the alveolar and interstitial
spaces. Protein, cellular debris and pulmonary surfactant in the edema fluid aggregate
within the alveolar spaces to form hyaline membranes that themselves predispose to
alveolar collapse and impaired pulmonary gas exchange. It is characterized by inflammation of the lung parenchyma leading to impaired gas exchange with concomitant systemic release of inflammatory mediators causing inflammation, hypoxemia and frequently resulting in multiple organ failure. This condition is often fatal, usually requiring mechanical ventilation and admission to an intensive care unit. A less severe form is called acute lung injury (ALI).
The formal definition of ARDS:
  • Acute onset
  • Bilateral infiltrates on chest radiograph sparing costophrenic angles
  • Pulmonary artery wedge pressure < 18 mmHg (obtained by pulmonary artery catheterization), if this information is available; if unavailable, then lack of clinical evidence of left ventricular failure suffices
  • if PaO2:FiO2 < 300 mmHg (40 kPa) acute lung injury (ALI) is considered to be present
  • if PaO2:FiO2 < 200 mmHg (26.7 kPa) acute respiratory distress syndrome (ARDS) is considered to be present

To summarize and simplify, ARDS is an acute (rapid onset) syndrome (collection of symptoms) that affects the lungs widely and results in a severe oxygenation defect, but is not heart failure

References:
Irwin RS, Rippe JM (2003). Irwin and Rippe's Intensive Care Medicine (5th ed.). Lippincott Williams & Wilkins.
Ashbaugh D, Bigelow D, Petty T, Levine B (1967). "Acute respiratory distress in adults". Lancet 2 (7511): 319–23.

Ventilator Management of ARDS

Ventilator Management of ARDS involves first intubation and mechanical
ventilation.  Initial ventilator settings may include an FiO2 of 70%,
tidal volume of 400 ml and respiratory rate of 14 / min. If the patient
remains hypoxemic (example pO2 < 60) and / or alkalotic (PCO2 < 40),
oxygenation may be improved by increasing either the FiO2 or by adding PEEP (positive end expiratory
pressure). Because high levels of FiO2 are contraindicated due to
the risk of pulmonary oxygen toxicity which may result in tissue damage
secondary to oxygen free radical creation, the goal in managing mechanically
ventilated patients should be to keep the FiO2 below 40%.  Adding PEEP
prevents alveolar collapse, directly counteracting the means by which ARDS
causes hypoxemia.  PEEP may also reopen some alveoli that have have
already collapsed.
- The foregoing is from #4 test 7481296

Difficulties in ARDS mangement result in part due to hypoxemia which
becomes refractory to high inspired oxygen concentrations. Adequated
oxygenation in ARDS often requires PEEP delivered via mechanical
ventilation. 
- The foregoing paragraph is from #5 Test Id 7481296.  You might
want to include it on the web project.
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