Acute Respiratory Distress Syndrome

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Summary 1) First Aid: USMLE Step 1 by Tao Le

Acute Respiratory Distress Syndrome is a syndrome that results from diffuse alveolar damage and leakage of fluid out of the pulmonary capillaries into the interstitium and alveolar spaces.

Pathophysiology 2) Wikipedia

ARDS Chest X-ray (University of Tokyo)

ARDS is an acute injury to the lungs that results in the flooding of the alveoli, which are the small air sacs within the lungs where the exchange of oxygen and carbon dioxide occurs, partial collapse of the lungs (atelectasis) and low levels of oxygen in the blood (hypoxemia). However, in ARDS, these changes are not due to heart failure. The clinical syndrome is associated with pathological findings including pneumonia, eosinophilic pneumonia, cryptogenic organizing pneumonia, acute fibrinous organizing pneumonia, and diffuse alveolar damage (DAD). Of these, the pathology most commonly associated with ARDS is DAD, which is characterized by a diffuse inflammation of lung tissue. The triggering insult to the tissue usually results in an initial release of chemical signals and other inflammatory mediators secreted by local epithelial and endothelial cells.
Neutrophils and some T-lymphocytes quickly migrate into the inflamed lung tissue and contribute in the amplification of the phenomenon. Typical histological presentation involves diffuse alveolar damage and hyaline membrane formation in alveolar walls. Although the triggering mechanisms are not completely understood, recent research has examined the role of inflammation and mechanical stress.

Causes 3) First Aid: USMLE Step 1 by Tao Le

  • Variety of disease processes – sepsis,Sepsis
  • infection, shock, traumaShock 
  • Trauma
  • aspiration, pancreatitis, DIG, hypersensitivity reactions, pneumonia, shock, aspiration of gastric contents, uremia, head trauma, multiple transfusions, DIC, and fat or amniotic fluid embolism 
  • Inhalation of irritants 
  • O2 toxicity
  • Heroin overdose

Symptoms 4) First Aid: USMLE Step 1 by Tao Le

  • Lung involvement – dyspnea and tachypnea

Alveoli of ARDS patient

Diagnosis 5)First Aid: USMLE Step 1 by Tao Le6)Robbins Basic Pathology by Vinay Kumar

  • Physical exam – crackles are often heard on auscultation 
  • Chest X-ray – may appear normal at onset, but eventually progresses to diffuse, symmetrical interstitial and alveolar edema, air bronchograms may be visible 
  • Pulmonary function testing restrictive pattern with a reduced Dlco 
  • Arterial blood gas testing – hypoxemia and hypocapnia, with a large AaDo2, supplemental O2 may not increase Po2 greatly due to shunt 
  • PO2/FiO2 < 200 mmHg 
  • Pulmonary capillary wedge pressure  normal 
  • Pathology – alveolar edema, epithelial necrosis, accumulation of neutrophils, presence of hyaline membranes lining the alveolar ducts.

Treatment

  • Intubation
  • Mechanical ventilation using low-volume ventilation

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