Acute Respiratory Distress Syndrome
A form of pulmonary edema that causes acute respiratory failure, acute respiratory distress syndrome (ARDS, shock lung, stiff lung) results from increased permeability of the alveolocapillary membrane. Fluid accumulates in the lung interstitium, alveolar spaces, and small airways, causing the lung to stiffen. Effective ventilation is thus impaired, prohibiting adequate oxygenation of pulmonary capillary blood. Severe ARDS can cause intractable and fatal hypoxemia; however, patients who recover may have little or no permanent lung damage.
ARDS can result from any one of several respiratory and nonrespiratory causes:
aspiration of gastric contents
sepsis (primarily gram-negative), trauma (lung contusion, head injury, long bone fracture with fat emboli), or oxygen toxicity
viral, bacterial, or fungal pneumonia or microemboli (fat or air emboli or disseminated intravascular coagulation)
drug overdose (barbiturates, glutethimide, narcotics) or blood transfusion
smoke or chemical inhalation (nitrous oxide, chlorine, ammonia)
pancreatitis, hypertransfusion, cardiopulmonary bypass
Altered permeability of the alveolocapillary membranes causes fluid to accumulate in the interstitial space. If the pulmonary lymphatics can’t remove this fluid, interstitial edema develops. The fluid collects in the peribronchial and peribronchiolar spaces, producing bronchiolar narrowing.
Hypoxemia occurs as a result of fluid accumulation in alveoli and subsequent alveolar collapse, causing the shunting of blood through nonventilated lung regions. In addition, regional differences in compliance and airway narrowing cause regions of low ventilation and inadequate perfusion, which also contribute to hypoxemia.
Signs and symptoms
ARDS initially produces rapid, shallow breathing and dyspnea within hours to days of the initial injury (sometimes after the patient’s condition appears stable). Hypoxemia develops, causing an increased drive for ventilation. Because of the effort required to expand the stiff lung, intercostal and suprasternal retractions result. Fluid accumulation may produce crackles and rhonchi, and worsening hypoxemia causes restlessness, apprehension, mental sluggishness, motor dysfunction, and tachycardia (possibly with transient increased arterial blood pressure).
Severe ARDS causes overwhelming hypoxemia, which, if uncorrected, results in hypotension, decreasing urine output, respiratory and metabolic acidosis and, eventually, ventricular fibrillation or standstill.