Pulmonology · Acute Respiratory Distress Syndrome
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The Berlin Definition requires acute onset within one week of a known clinical insult and bilateral opacities on chest imaging not fully explained by effusions or nodules.
Diagnosis requires a PaO2/FiO2 ratio ≤ 300 mmHg with a minimum of 5 cm H2O PEEP to confirm the severity of hypoxemia.
ARDS is characterized by non-cardiogenic pulmonary edema resulting from increased alveolar-capillary membrane permeability rather than elevated hydrostatic pressure.
The hallmark histological finding in the acute phase is the formation of hyaline membranes within the alveolar spaces.
Low tidal volume ventilation (6 mL/kg of predicted body weight) is the primary intervention to prevent ventilator-induced lung injury and reduce mortality.
Prone positioning is indicated in patients with severe ARDS (PaO2/FiO2 < 150 mmHg) to improve ventilation-perfusion matching and recruit dorsal lung segments.
The pulmonary capillary wedge pressure is typically ≤ 18 mmHg, which helps distinguish ARDS from cardiogenic pulmonary edema.
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A 54-year-old male is admitted to the ICU for severe sepsis secondary to pneumonia. On day 3 of admission, he develops acute worsening of respiratory status requiring increased oxygen support. Physical exam reveals diffuse crackles bilaterally, and a chest X-ray demonstrates patchy bilateral alveolar opacities. His arterial blood gas shows a PaO2/FiO2 ratio of 180 mmHg on 10 cm H2O of PEEP. An echocardiogram shows a normal ejection fraction with no evidence of left-sided heart failure.
What is the most appropriate initial ventilator strategy to improve this patient's survival?
Low tidal volume ventilation (6 mL/kg predicted body weight)
The patient meets the Berlin criteria for moderate ARDS; low tidal volume ventilation is the only ventilator strategy proven to reduce mortality by preventing barotrauma and volutrauma.
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Etiology / Epidemiology
Occurs in critically ill patients, most commonly due to sepsis, aspiration, or trauma.
Clinical Manifestations
Presents with refractory hypoxemia and diffuse bilateral infiltrates on imaging.
Diagnosis
Defined by Berlin criteria: PaO2/FiO2 ratio < 300 and non-cardiogenic pulmonary edema.
Treatment
Management requires low tidal volume ventilation (6 mL/kg) and avoiding high plateau pressures.
Prognosis
High mortality rate of 30-40%; survivors often face long-term pulmonary fibrosis.
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Epidemiology & Etiology
ARDS is a life-threatening form of respiratory failure triggered by direct or indirect lung injury. Sepsis remains the single most common cause. Other major triggers include pancreatitis, multiple blood transfusions, and pneumonia.
Pertinent Anatomy
The alveolar-capillary membrane is the primary site of injury. Damage to the type I and II pneumocytes leads to loss of barrier integrity and surfactant dysfunction. This results in alveolar collapse and atelectasis.
Pathophysiology
The process begins with an uncontrolled inflammatory response causing capillary leak and protein-rich fluid accumulation in the alveoli. This creates a shunt physiology where blood passes through non-ventilated areas. The resulting hyaline membrane formation impairs gas exchange, leading to severe hypoxemia.
Clinical Manifestations
Patients present with acute onset of dyspnea and tachypnea within 1 week of a known clinical insult. Physical exam reveals diffuse crackles and cyanosis. Red flags include rapid progression to respiratory failure requiring intubation and hemodynamic instability.
Diagnosis
Diagnosis is clinical using the Berlin criteria: acute onset, bilateral opacities on chest X-ray or CT not fully explained by effusions, and a PaO2/FiO2 ratio < 300 mmHg. It is critical to rule out cardiogenic edema; a pulmonary artery wedge pressure < 18 mmHg (if measured) confirms the non-cardiogenic nature.
Treatment
The cornerstone is lung-protective ventilation using low tidal volumes (6 mL/kg predicted body weight) to prevent ventilator-induced lung injury. Maintain plateau pressures < 30 cm H2O. Avoid high tidal volumes as they increase mortality. Prone positioning is indicated for severe cases (PaO2/FiO2 < 150).
Prognosis
Mortality is high, often driven by multi-organ failure rather than respiratory failure alone. Survivors frequently experience neuromuscular weakness and cognitive impairment. Long-term monitoring for pulmonary fibrosis is essential.
Differential Diagnosis
Congestive Heart Failure: elevated BNP and PCWP > 18 mmHg
Acute Interstitial Pneumonia: idiopathic, no identifiable trigger
Diffuse Alveolar Hemorrhage: associated with hemoptysis and falling hemoglobin
Pulmonary Embolism: normal chest X-ray despite severe hypoxemia
Severe Pneumonia: focal rather than diffuse infiltrates