Edema Caused by Microvascular Injury

The 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: 917