critical care medicine
Acute Hypoxemic Respiratory Failure (AHRF, ARDS)
Pathophysiology of AHRF
ARDS is divided into 3 categories of severity: mild, moderate, and severe based on oxygenation defects and clinical criteria (see table Berlin Definition of ARDS ). The mild category corresponds to the previous category termed acute lung injury (ALI). In ARDS, pulmonary or systemic inflammation leads to release of cytokines and other proinflammatory molecules. The cytokines activate alveolar macrophages and recruit neutrophils to the lungs, which in turn release leukotrienes, oxidants, platelet-activating factor, and proteases. These substances damage capillary endothelium and alveolar epithelium, disrupting the barriers between capillaries and airspaces. Edema fluid, protein, and cellular debris flood the airspaces and interstitium, causing disruption of surfactant, airspace collapse, ventilation-perfusion mismatch, shunting, and Pulmonary Hypertension . The airspace collapse more commonly occurs in dependent lung zones. This early phase of ARDS is termed exudative. Later, there is proliferation of alveolar epithelium and fibrosis, constituting the fibro-proliferative phase. Causes of ARDS may involve direct or indirect lung injury. Common causes of direct lung injuryare Acid aspiration Overview of Pneumonia None Less common causes of direct lung injury are Amniotic fluid embolism None Diffuse Alveolar Hemorrhage None Drowning None Fat embolism None Lung and Heart-Lung Transplantation None Pulmonary Contusion None Irritant Gas Inhalation Injury None Common causes of indirect lung injuryinclude Sepsis and Septic Shock None Trauma with prolonged hypovolemic shock Less common causes of indirect lung injury include Hematopoietic Stem Cell Transplantation None Drug overdose (eg,aspirin,cocaine, opioids, phenothiazines, tricyclics) Burns None Cardiopulmonary bypass Complications of Transfusion ( Neurogenic pulmonary edema due to stroke, seizure, head trauma, anoxia Overview of Pancreatitis None Preeclampsia and Eclampsia None Radiographic contrast (rare) Septic Abortion None Sepsis and Septic Shock and Whatever the cause of airspace filling in AHRF, flooded or collapsed airspaces allow no inspired gas to enter, so the blood perfusing those alveoli remains at the mixed venous oxygen content no matter how high the fractional inspired oxygen (FIO2). This effect ensures constant admixture of deoxygenated blood into the pulmonary vein and hence arterial hypoxemia. In contrast, hypoxemia that results from ventilating alveoli that have less ventilation than perfusion (ie, low ventilation-to-perfusion ratios as occur in Asthma or 