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Acute respiratory distress syndrome

Summary

Acute respiratory distress syndrome (ARDS) is a severe inflammatory reaction of the lungs to pulmonary damage. While sepsis is the most common cause, a variety of systemic and pulmonary factors (e.g., pneumonia, aspiration) can lead to ARDS. Affected individuals initially present with acute-onset cyanosis, dyspnea, and tachypnea. The chief finding in ARDS is hypoxemic respiratory failure with decreased arterial oxygen pressure, which can progress to hypercapnic respiratory failure. Chest x-ray typically shows diffuse bilateral infiltrates. A defining laboratory feature of ARDS is a PaO2/FiO2 ratio ≤ 300 mm Hg. Management of ARDS is focused on maintaining adequate oxygenation, which often requires intubation and lung-protective mechanical ventilation. Moreover, any treatable causes of ARDS should be addressed. Even if adequate treatment is initiated, ARDS remains an acutely life-threatening disease with a high mortality rate. Most patients improve significantly in the weeks following the initial presentation, but some cases progress to pulmonary fibrosis, which prolongs hospital stays and delays the resolution of symptoms.

Definition

ARDS is a clinical syndrome of acute respiratory failure characterized by hypoxemia and bilateral pulmonary infiltrates that cannot be fully accounted for by heart failure or fluid overload. See the Berlin criteria for ARDS. [1][2]

Etiology

Systemic causes

Primary damage to the lungs


Sepsis is the most common cause of ARDS!

References:[3][4][5][6]

Pathophysiology

References:[7][5][8][9]

Clinical features

References:[10][11][12]

Diagnostics

Approach [2]

ARDS is a diagnosis of exclusion (see the Berlin criteria for ARDS). Consider ARDS in patients with rapid-onset respiratory failure and a potential trigger.

  • Order chest x-ray to evaluate for characteristic findings (bilateral infiltrates).
  • Perform an ABG and calculate the P/F ratio.
  • Consider additional testing in order to:
    • Identify triggers
    • Rule out differential diagnoses
    • Assess potential complications

Berlin criteria for ARDS

The Berlin criteria are the criteria most commonly used to define ARDS. All four of the following conditions must be met: [2][1]

ARDS diagnostic criteria include: Abnormal x-ray, Respiratory failure < 1 week after a known or suspected trigger, Decreased PaO2/FiO2, Should exclude CHF or fluid overload as a potential cause of respiratory distress.

Imaging

Chest x-ray is usually sufficient for diagnosis. However, distinguishing between ARDS and CHF can be challenging. In these cases, correlation with other tests (e.g., CT chest, lung ultrasound, echocardiogram) may be useful.

Chest x-ray [14][15]

  • Indications: all patients suspected of having ARDS
  • Acute findings (1–7 days)
    • Often normal in the first 24 hours
    • Diffuse bilateral symmetrical infiltrates
    • In severe cases: bilateral attenuations that make the lung appear white on x-ray (“white lung”)
    • Air bronchograms may be visible.
  • Intermediate (8–14 days) to late (> 15 days) findings
    • Typical course: Acute features remain stable, then resolve.
    • Fibrotic course: Reticular opacities begin to appear and may become permanent.
  • Findings supportive of ARDS rather than CHF

CT chest without contrast [14][15][16]

  • Indications: may be used if chest x-ray findings are insufficient or to further investigate for underlying causes or complications
  • Acute findings (1–7 days)
    • Symmetrical ground-glass opacities are the most important finding.
    • Gravity-dependent density gradient
      • The lungs may appear normal in nondependent regions.
      • Dense consolidation in dependent regions
    • Bronchial dilatation may be visible.
    • Additional findings may include small pleural effusions, air bronchograms (see “Chest x-ray” above).
  • Intermediate (8–14 days) to late (> 15 days) findings: a phase of stability is followed either by resolution or progressive development of fibrosis
    • Mixed findings may be seen.
    • Potential long-term persistence of ground-glass opacities
    • Cysts and bullae may develop.

Lung ultrasound [16]

  • Indications: may be helpful in differentiating between cardiogenic pulmonary edema and ARDS
  • Key findings
    • Bilateral B pattern
    • C pattern (consolidation)
    • Abnormal pleural line (thickening, irregular pattern, and/or alterations in lung sliding)

Laboratory studies [16]

Additional diagnostic studies [16]

  • ECG: Signs of STEMI, LVH, or cardiac arrhythmias may indicate CHF.
  • Echocardiography: to exclude or assess the degree of heart failure [15]
  • Bronchoscopy with bronchoalveolar lavage (BAL) [16]
    • Useful for infections that are hard to diagnose, inflammatory disease (e.g., vasculitis), and cancer
    • BAL samples can be tested with Giemsa/Gram staining as well as specialized cultures for intracellular bacteria, viruses, and fungi.
  • Right heart catheterization
    • To exclude CHF in the absence of any risk factors
    • PCWP > 18 mm Hg is considered to confirm the presence of cardiac insufficiency. [15]
  • Lung biopsy: consider in rare cases [16]
    • To evaluate the stage of lung fibrosis after a prolonged ARDS course and decide whether treatment with steroids may be indicated [16]
    • Indicated if other studies (e.g., BAL, blood cultures) are inconclusive

Differential diagnoses

The differential diagnoses listed here are not exhaustive.

Management

Approach

ARDS is a life-threatening condition that usually requires early lung-protective ventilation (i.e., with low tidal volumes and low plateau pressures) to prevent further lung damage.

All patients with ARDS [2][13][17][18][19][20]

The foundation of management in all patients with ARDS consists of treating hypoxemia, lung-protective ventilation (to minimize further lung damage), treatment of the underlying cause, and supportive care.

A low tidal volume and low plateau pressure are the principles of lung-protective ventilation!

Moderate to severe ARDS [17][18]

Prone positioning [24][25]

  • Effects
  • Indications [18]
  • Relative contraindications include: [26]
    • Hemodynamic instability
    • Severe trauma, surgery or spinal instability
    • ↑ ICP
    • Pregnancy
  • Duration: typically done for at least 12–16 hours/day
  • Complications include:
    • ET tube displacement or obstruction
    • Abnormal vital signs: SpO2, ↓ HR, ↓ BP
    • Hemoptysis
    • Pressure injuries
    • Other: facial edema, ocular injury , venous stasis

Lung recruitment maneuvers [18][27]

  • Definition: a series of treatment measures that increase the surface area of lung available for gas exchange
  • Methods
    • Sustained inflation techniques (e.g., increasing airway pressure to 30–40 cm H2O for 30–40 seconds followed by a decrease in PEEP)
    • Incremental PEEP increase: See ARDSnet protocol.
ARDSnet protocol for FiO2/PEEP titration [28][29][30]
Low PEEP/high FiO2 strategy
FiO2 (%) PEEP (cm H2O)

30

5
40 5
40 8
50 8
50 10
60 10
70 10
70 12
70 14
80 14
90 14
90 16
90 18
100 18–24
High PEEP/Low FiO2 strategy
30 5
30 8
30 10
30 12
30 14
40 14
40 16
50 16
50 18
50–80 20
80 22
90 22
100 22–24

Other considerations

Severe ARDS with persistent hypoxemia (rescue therapy) [13][2]

The following interventions should only be considered with expert consultation and when guideline-recommended treatments have failed.

Murray score for ARDS [38]
Clinical parameter Findings Points assigned
Alveolar consolidation on x-ray None 0
1 quadrant involved 1
2 quadrants involved 2
3 quadrants involved 3
4 quadrants involved 4
P/F ratio in mm Hg > 300 0
225–299 1
175–224 2
100–174 3
≤ 100 4
PEEP in cm H2O ≤ 5 0
6–8 1
9–11 2
12–14 3
> 15 4
Respiratory compliance in mL/cm H2O > 80 0
60–79 1
40–59 2
20–39 3
< 19 4

Interpretation: Add up the total points and divide the total by the number of parameters present.

Acute management checklist

All patients with ARDS [13]

Moderate or severe ARDS

Severe ARDS with persistent hypoxemia

  • Consider ECMO based on the Murray score for ARDS.
  • Expert consultation is required for further ventilator adjustment or experimental therapies.

Prognosis

  • Disease course
  • In patients with simultaneous multiorgan failure, the mortality rate is 30–50%. [41]

Related One-Minute Telegram

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last updated 09/18/2020
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