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Edema Caused by Microvascular InjuryThe second mechanism leading to pulmonary edema is injury to the capillaries of the alveolar septa. Here the pulmonary capillary hydrostatic pressure is usually not elevated, and hemodynamic factors play a secondary role. The edema results from primary injury to the vascular endothelium or damage to alveolar epithelial cells (with secondary microvascular injury). This results in leakage of fluids and proteins first into the interstitial space and, in more severe cases, into the alveoli. When the edema remains localized, as it does in most forms of pneumonia, it is overshadowed by the manifestations of infection. When diffuse, however, alveolar edema is an important contributor to a serious and often fatal condition, acute respiratory distress syndrome, discussed in the following section. ACUTE RESPIRATORY DISTRESS SYNDROME (DIFFUSE ALVEOLAR DAMAGE) Acute respiratory distress syndrome (ARDS) (synonyms include "shock lung," "diffuse alveolar damage," "acute alveolar injury," and "acute lung injury") is a clinical syndrome caused by diffuse alveolar capillary damage. It is characterized clinically by the rapid onset of severe life-threatening respiratory insufficiency, cyanosis, and severe arterial hypoxemia that is refractory to oxygen therapy and that may progress to extra-pulmonary multisystem organ failure. Chest radiographs show diffuse alveolar infiltration. Diffuse alveolar damage (DAD) is the histologic manifestation. ARDS is a well-recognized complication of numerous and diverse conditions, including both direct injuries to the lungs and systemic disorders ( Table 15-2 ). In many cases, a combination of predisposing conditions is present (e.g., shock, oxygen therapy, and sepsis). Morphology. In the acute stage, the lungs are heavy, firm, red, and boggy. They exhibit congestion, interstitial and intra-alveolar edema, inflammation, and fibrin deposition. The alveolar walls become lined with waxy hyaline membranes( Fig. 15-3 ) that are morphologically similar to those seen in hyaline membrane disease of neonates ( Chapter 10 ). Alveolar hyaline membranes consist of fibrin-rich edema fluid mixed with the cytoplasmic and lipid remnants of necrotic epithelial cells. In the organizing stage, type TABLE 15-2-- Conditions Associated with Development of Acute Respiratory Distress Syndrome Infection Sepsis * Diffuse pulmonary infections * ••Viral, Mycoplasma, and Pneumocystis pneumonia; miliary tuberculosis Gastric aspiration * Physical/Injury Mechanical trauma, including head injuries * Pulmonary contusions Near-drowning Fractures with fat embolism Burns Ionizing radiation Inhaled Irritants Oxygen toxicity Smoke Irritant gases and chemicals Chemical Injury Heroin or methadone overdose Acetylsalicylic acid Barbiturate overdose Paraquat Hematologic Conditions Multiple transfusions Disseminated intravascular coagulation Pancreatitis Uremia Date: 2016-04-22; view: 911
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