Acute Respiratory Distress Syndrome

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Contents 1 Introduction 2 Etiology 3 Epidemiology 4 Pathology 5 Clinical-manifestations 6 Laboratory 7 Diagnostic-procedures 8 Radiology 9 Complications 10 Differential-diagnosis 11 Management 12 More General Terms 13 References

Introduction

• ARDS is the most severe form of acute pulmonary injury. It is a diagnosis of exclusion.

Etiology

• (predisposing conditions)
• pulmonary infection [2]
• aspiration of gastric contents
• sepsis, particularly from an abdominal source
• hemorrhagic shock
• drug overdose ( heroin, methadone, barbiturates)
• major trauma

• pulmonary contusion
• multiple fractures

• near drowning

Epidemiology

• incidence & mortality increases with age [6]

Pathology

• disruption of the alveolar capillary membrane
• increased vascular permeability
• accumulation of neutrophils & protein rich edema in the alveolar space
• up to 70% of alveoli may be non-functional
• may be component of alveolar dead space [6]
• a fibroproliferative stage may occur within 2 weeks of onset

• loss of alveolar structures
• obliteration of vasculature
• interstitial infliltration of chronic inflammatory cells
• deposition of type 1 collagen
• severity of fibroproliferative reaction correlates with increased mortality

• varying degrees of multiorgan failure accompany ARDS

Clinical-manifestations

• rapid onset

• < 48 hours after precipitating event [2]
• generally < 24 hours

• severe dyspnea
• tachycardia
• tachypnea
• diffuse bilateral crackles on chest auscultation

Laboratory

• arterial blood gas ( ABG)

• hypoxemia, pO2/ fiO2 < 200 mm Hg for ARDS
• pO2/ fiO2 < 300 mm Hg for acute lung injury

• pulse oximetry: SaO2 < 88%

Diagnostic-procedures

• hemodynamic monitoring

• distinguish cardiogenic pulmonary edema from ARDS
• pulmonary artery occlusion pressure < 18 mm Hg

• echocardiography

• distinguish cardiogenic pulmonary edema from ARDS

Radiology

• chest radiograph
• diffuse bilateral pulmonary infiltrates
• no pulmonary vascular redistribution (i.e. no heart failure)
• no cardiomegaly
• pleural effusions are rare in the absence of another condition predisposing to pleural effusion

Complications

• mortality is 50% when associated with sepsis
• multiple organ dysfunction & secondary pulmonary infections may occur if the patient survives the acute phase

Differential-diagnosis

• cardiogenic pulmonary edema
• diffuse alveolar hemorrhage
• acute eosinophilic pneumonia

Management

• oxygenation

• mechanical ventilation
• PEEP of 5-12 cm H2O

• 8 vs 13 cm H20 associated with similar outcomes [3]
• higher PEEP may reduce morbidity but not mortality [3]
• use lowest PEEP necessary to achieve SaO2 of 88% with FiO2 of < 60% [2]

• tidal volume 6 mL/kg IBW [2]
• peak inspiratory pressure < 30-35 mm Hg
• end- expiratory pressure < 30 cm Hg
• permissive hypercapnia

• pulmonary artery catheter of no benefit [8]
• intravascular volume management; maintain lowest possible intravascular volume compatible with adequate tissue perfusion
• intermittent ventilation in prone position (4-20 hours)
• corticosteroids may be of benefit for adrenal insufficiency

• hydrocortisone 50 mg IV every 6 hours + fludrocortisone 50 ug PO QD [5]

• unproven therapies [2]

• glucocorticoids (no adrenal insufficiency)

• may be of benefit in fibroproliferative phase
• methyprednisolone 1 mg/kg IV started early & tapered over 28 days of benefit [9]

• inotropic agents (unproven, controversial)
• nitric oxide:

• reduces pulmonary artery pressure & improves oxygenation
• effect on outcome unknown
• not recommended [2]

• ketoconazole not recommended [2]
• surfactant not recommended [2]

• conservative fluid management may have short-term benefit [7]
• any red cell transfusion may increase mortality [10]

• Prognosis:

• sepsis is most lethal form [6]
• most survivors discharged to long-term care facility [6]
• most survivors regain baseline lung function within 12 months

More General Terms

• respiratory distress syndrome (RDS)

References

1. Manual of Medical Therapeutics, 28th ed, Ewald & McKenzie (eds), Little, Brown & Co, Boston, 1995, pg 249-50
2. Medical Knowledge Self Assessment Program (MKSAP) 11, 14, 15, American College of Physicians, Philadelphia 1998, 2006, 2009
3. Brower RG, Lanken PN, MacIntyre N, Matthay MA, Morris A, Ancukiewicz M, Schoenfeld D, Thompson BT; National Heart, Lung, and Blood Institute ARDS Clinical Trials Network. Higher versus lower positive end-expiratory pressures in patients with the acute respiratory distress syndrome. N Engl J Med. 2004 Jul 22;351(4):327-36. PMID: [1]
   - Levy MM. PEEP in ARDS--how much is enough? N Engl J Med. 2004 Jul 22;351(4):389-91. No abstract available. PMID: [2]
   - Mercat A et al, Positive end-expiratory pressure setting in adults with acute lung injury and acute repsiratory distress syndrome: A randomized controlled trial. JAMA 2008, 299:646 PMID: [3]
   - Gattinoni L and Caironi P Refining ventilatory treatment for acute lung injury and acute respiratory distress syndrome. JAMA 2008, 299:691 PMID: [4]
4. Rubenfeld GD, Caldwell E, Peabody E, Weaver J, Martin DP, Neff M, Stern EJ, Hudson LD. Incidence and outcomes of acute lung injury. N Engl J Med. 2005 Oct 20;353(16):1685-93. PMID: [5]
5. Annane D et al, Effect of low doses of corticosteroids in septic shock patients with or without early acute respiratory distress syndrome Crit Care Med 2006; 34:22 PMID: [6]
6. Mather Matthay, Grand Rounds, UC Davis, Feb 16, 2006
7. Comparison of Two Fluid-Management Strategies in Acute Lung Injury. N Engl J Med. 2006 May 21; [Epub ahead of print] PMID: [7]
   - Rivers EP. Fluid-Management Strategies in Acute Lung Injury -- Liberal, Conservative, or Both? N Engl J Med. 2006 May 21; [Epub ahead of print] PMID: [8]
8. The National Heart, Lung, and Blood Institute Acute Respiratory Distress Syndrome (ARDS) Clincial Trials Network. Pulmonary- artery versus central venous catheter to guide treatment of acute lung injury. N Engl J Med 2006; 354:2213 PMID: [9]
   - Shore D Pulmonary-artery catheters - Peace at last? N Engl J Med 2006; 354:2773 PMID: [10]
9. Meduri GU, Golden E, Freire AX, Taylor E, Zaman M, Carson SJ, Gibson M, Umberger R. Methylprednisolone infusion in early severe ARDS: results of a randomized controlled trial. Chest. 2007 Apr;131(4):954-63. PMID: [11]
   - Annane D. Glucocorticoids for ARDS: Just Do It! Chest. 2007 Apr;131(4):945-6. No abstract available. PMID: [12]
10. Netzer G et al, Association of RBC transfusion with mortality in patients with acute lung injury, Chest 2007, 132:1116 PMID: [13]
11. What Is ARDS? [14]

acute respiratory distress syndrome; acute lung injury (ARDS, ALI)