Pulmonology Book

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Acute Respiratory Distress SyndromeAka: Adult Respiratory Distress Syndrome, Severe Acute Lung Injury, ARDS, Traumatic Wet Lung, Shock Lung, Congestive Atelectasis

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  1. See Also
    1. Acute Respiratory Failure
  2. Pathophysiology
    1. Neutrophil mediated endothelial damage
      1. Allows increased permeability to protein-rich fluid
      2. Results in low-pressure pulmonary edema
    2. Models of pathogenesis
      1. Surfactant disorder
        1. Neonatal Respiratory Distress Syndrome
      2. Fibrosis
        1. Idiopathic Pulmonary Fibrosis (chronic)
      3. Granulation
        1. Healing superficial skin wound
      4. Microatelectasis
  3. Risk factors
    1. Chronic lung disease
    2. Alcoholism
    3. Age over 65 years
  4. Causes
    1. Direct lung injury
      1. Pneumonia
      2. Gastric acid aspiration
      3. Pulmonary Contusion
      4. Fat embolism
      5. Toxic Inhalation injury
        1. Smoke inhalation
        2. Chlorine
        3. Nitrogen dioxide
        4. Phosgene
        5. Ammonia
        6. Cocaine
        7. Clove Cigarettes
      6. Near-drowning
      7. Severe pulmonary hemorrhage
      8. Oxygen Toxicity
    2. Indirect lung injury
      1. Sepsis (highest risk of ARDS)
      2. Multiple trauma
      3. Disseminated Intravascular Coagulation
      4. Cardiopulmonary bypass (CABG)
      5. Burn Injury
      6. Acute Pancreatitis
      7. Drug Overdose (Heroin, Cocaine)
      8. Transfusion reaction
      9. Ingestion
        1. Hydrocarbon ingestion
        2. Ethchlorvynol (Placidyl)
      10. Non-cardiac pulmonary edema
        1. High Altitude Pulmonary Edema
        2. Neurogenic pulmonary edema
        3. Heroin-induced pulmonary edema
      11. Infection (often in immunocompromised patients)
        1. Miliary Tuberculosis
        2. Pneumocystis carinii
        3. Diffuse fungal infection
          1. Histoplasmosis
          2. Blastomycosis
          3. Coccidioidomycosis
          4. Cryptococcosis
  5. Symptoms
    1. Onset within 24-72 hours of triggering event
    2. Progressive Dyspnea
  6. Signs
    1. Early
      1. Tachypnea
      2. Tachycardia
      3. Cyanosis
    2. Later
      1. Diffuse lung rhonchi
      2. Acute Respiratory Failure
  7. Labs: Arterial Blood Gas
    1. Critical for assessment
    2. Most sensitive for identifying ARDS early
  8. Radiology
    1. Chest XRay
      1. Early: Diffuse Interstitial Infiltrates
      2. Later: Diffuse fluffy infiltrates (pulmonary edema)
      3. No cardiomegaly or Pleural Effusions
    2. Chest CT
      1. Acute Phase
        1. Bilateral alveolar opacities
        2. Air Bronchograms
        3. Bullae
        4. Pleural Effusions
      2. Fibroproliferative stage
        1. Bilateral reticular opacities
        2. Decreased Lung Volume
        3. Large bullae
  9. Diagnosis
    1. Criteria
      1. Acute onset
      2. Identifiable Cause from above list
      3. Not due to Congestive Heart Failure
        1. Pulmonary artery wedge pressure <19 mmHg or
        2. No signs of left atrial Hypertension
      4. Hypoxemia despite Supplemental Oxygen
        1. See PaO2 to FIO2 ratios below
      5. Bilateral Pulmonary Infiltrates on Chest XRay
    2. Spectrum of lung injury based on PaO2/FIO2
      1. Interpretation regardless of PEEP
      2. Normal patient: 500 mmHg
      3. Acute lung injury: <300 mmHg
      4. Acute respiratory distress syndrome: <200 mmHg
    3. References
      1. Bernard (1994) Am J Respir Crit Care Med 149:818
  10. Management: General
    1. Identify and treat underlying cause
      1. Example: Treat site-specific infections
    2. Maximize nutritional status and fluid balance
      1. Eicosapentaenoic Acid (fish oil extract) effective
    3. Inotropic pressure support may be required
    4. Maintain adequate Sedation and analgesia
    5. Stress Ulcer prophylaxis
    6. Deep Vein Thrombosis Prevention
    7. Consider tracheostomy for prolonged intubation
    8. Pulmonary artery catheters are not routinely indicated
      1. Choose selectively in complicated fluid status
    9. Prone position reduces dependent consolidation
      1. Prone position requires adequate Sedation
      2. Prone position does not alter hemodynamic parameters
    10. Inhaled Beta Agonists appear effective
      1. Reduce Ventilatory pressures and increase oxygenation
    11. Measures not proven effective
      1. Inhaled nitric oxide
      2. Aerosolized surfactant replacement
      3. N-Acetylcysteine (Mucomyst)
      4. Vasodilators (e.g. Nitroprusside, Hydralazine)
      5. Corticosteroids
        1. Small trials with possible benefit later in course
      6. Prophylactic antibiotics
      7. Prophylactic Chest Tubes
    12. Experimental methods under current evaluation
      1. Liquid ventilation (lung filled with perfluorocarbon)
  11. Management: Lung Protective Ventilator Strategy
    1. See Mechanical Ventilation
    2. Overall strategy
      1. Limiting barotrauma decreases mortality in ARDS
    3. Set Tidal Volume to 6 ml/kg initially
      1. Much lower than Tidal Volume in other conditions
      2. Base Tidal Volume on Ideal Weight for height
    4. Lower FIO2 to avoid alveolar toxicity
      1. Titrate FIO2 down to 0.60 to keep O2 Sat >90%
    5. Set PEEP for maximal alveolar recruitment
      1. Monitor for reduced cardiac output (PEEP >12 mmHg)
    6. Allow some hypercapnia to reduce barotrauma risk
      1. Lower minute volumes (lower Tidal Volume and rate)
      2. Titrate to PaCO2 of 50 to 77 mmHg
      3. Titrate to pH of 7.20 to 7.30
    7. Maintain inspiratory pressures <30 mm hg
  12. Complications
    1. Nosocomial infection
    2. Pneumothorax (barotrauma related) in up to 41% of cases
    3. Gastrointestinal Bleeding (Stress Ulcer)
    4. Thromboembolism
  13. Course
    1. ARDS presents within 12-24 hours of antecedent event
    2. ARDS patients intubated within 72 hours in 90% cases
    3. High mortality rate (ICU: 37%, overall: 42%)
    4. Predictors of better prognosis
      1. Those who survive first 2 weeks have better prognosis
      2. Age under 55 years
      3. Trauma related ARDS
    5. Predictors of poor prognosis
      1. Elderly (especially over age 70 years)
      2. Immunocompromised patients
      3. Chronic Liver Disease
      4. Increased dead space fraction
  14. References
    1. Davies (1986) Acute Respiratory Failure, Cyberlog
    2. McIntyre (2000) Crit Care Med 28(9):3314
    3. Mortelliti (2002) Am Fam Physician 65(9):1823
    4. Udobi (2003) Am Fam Physician 67(2):315
    5. Valta (1999) Crit Care Med 27(11):2367
    6. Ware (2000) N Engl J Med 342:1334

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