Acute Respiratory Distress Syndrome (ARDS)



Pathophysiology

1) final common pathway of acute-onset progressive lung tissue damage after injury or physiologic insult Predisposing factors2) sepsis 3) shock 4) infection 5) massive trauma 6) massive blood transfusion 7) near drowning 8) aspiration 9) fat embolism 10) heroin overdose 11) radiation exposure 12) DIC 13) bleomycin 14) paraquat

Signs and Symptoms

1) hypoxemia 2) tachypnea 3) dyspnea 4) severe cases can result in lung fibrosis, respiratory insufficiency, and pulmonary hypertension

Characteristic Test Findings

Radiology1) bilateral diffuse infiltrates up to complete “white out” of lung fields on chest radiograph PFTs2) decreased lung compliance (“stiff lungs”) Ventilation status3) PaO2/FiO2 < 200 mm Hg (regardless of the positive end-expiratory level)

Histology/Gross Pathology

1) diffuse damage to endothelial cells with leakage of protein and fluids into interstitium and alveolar sacs 2) destruction of type 1 pneumocytes 3) proliferation of type 2 pneumocytes 4) formation of hyaline membranes in alveolar sace 5) accumulation of inflammatory cells in interstitium with deposition of collagen; can progress to a nonreversible fibrosis if severe enough

Associated Conditions

chronic alcoholism increases risk of ARDS with any predisposing cause

Biochemistry

1) activation of complement system with sequestration of neutrophils 2) ARDS is overexpression of the normal inflammatory cascade

Inheritance/Epidemiology

overall mortality is 50% and worsens as patient age increases

Treatment

1) mechanical ventilation to maintain a PaO2 > 60 mm Hg 2) APRV/bilevel being used more commonly in lieu of ARDSnet strategy of a low tidal volume and use of PEEP) 3) adequate intravascular fluid to perfuse the tissues 4) activated protein kinase C use has been largely invalidated 5) inhaled nitric oxide 6) partial liquid ventilation 7) turning patients from supine to prone increases PaO2 but does not improve overall mortality 8) reversed I:E ratio

Tips for USMLE

1) some patients with ARDS who recover regain completely normal lung function; others have residual scarring, “honeycomb lungs,” and respiratory insufficiency 2) if increased permeability in the alveolar-capillary membrane of lung is mentioned, think ARDS 3) think ARDS in the following situation: a 62 year-old alcoholic man sustains a gunshot wound to the abdomen and requires 25 units of PRBCs during resuscitation; postop he is initially well-oxygenated on low ventilatory settings, but 24 hours later he becomes increasingly hypoxic with a chest radiograph that shows diffuse bilateral white ares in the lung fields.

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