Acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is an acute, diffuse, inflammatory form of lung injury that is associated with a variety of aetiologies.

Pathophysiology

• Characterised by rapid onset of widespread lung inflammation
• Loss of aerated lung tissue:
  • Endothelial cell injury and pulmonary vascular permeability leading to flooding of alveoli
  • Lung oedema
  • Gravity-dependent atelectasis
• Outcomes:
  • Increased shunting
  • Increased alveolar dead space
  • Decreased lung compliance
• Stages
  • Early exudative stage
  • Fibroproliferative stage
  • Fibrotic stage

Diagnosis:

• Previously used Berlin criteria, new consensus guidelines 2024
• Suspect in patients with progressive dyspnoea, increasing O2 requirement, and bilateral alveolar infiltrates on chest imaging within one week of an inciting event.
• Chest signs - dyspnoea, tachycardia, diffuse crackles (severe: confusion, WOB, cyanosis, diaphoresis)
• ABG: hypoxaemia, with acute resp alkalosis, and elevated A-a gradient
  • Acute hypercapnoeic resp acidosis is ominous sign - pre-arrest
  • Other signs of underlying aetiology may be present
• Exclude acute cardiogenic pulmonary oedema - can base this off hx/ex/BNP (low BNP favours ARDS)

Ix:

• FBE/UEC/CMP/LFT/COAGS/ABG
• CXR/ECG
• MICRO

Severity:

• Mild ARDS: PaO2/FiO2 > 200 and <300 on vent (PEEP or CPAP >=5cm H20)
• Mod: PaO2/FiO2 100-200, PEEP >5cm H20
• Severe: PaO2/FiO2 < 100, PEEP >5cm H20

Can use SpO2 % if ABG unavailable

Management

• Oxygen via mechanical ventilation with PPV, initially lung-protective
• Conservative fluid balance
• Prone positioning for severe ARDS

Aetiology

ARDS has over 60 etiologies. This is an abbreviated list of the common causes of ARDS.

Etiology	Clinical features	Diagnostic tests
Sepsis	Fever hypotension, leukocytosis, lactic acidosis, infectious source	Appropriate clinical context and positive cultures
Aspiration pneumonitis	Witnessed or risk for aspiration, food, lipid laden macrophages, airway erythema on bronchoscopy	Presumptive diagnosis with negative cultures
Infectious pneumonia (including mycobacterial, viral, fungal, parasitic)	Productive cough, pleuritic pain, fever, leukocytosis, lobar consolidation or bilateral infiltrates in an immunosuppressed patient	Appropriate clinical context and positive respiratory cultures
Severe trauma and/or multiple fractures	History of trauma or fractures within the last week	Diagnosis is apparent
Pulmonary contusion	History of chest trauma (blunt or penetrating), chest pain	Presumptive diagnosis in the correct clinical context, negative cultures
Burns and smoke inhalation	Exposure to fire or smoke, cough, dyspnea, DIC, particulate matter on bronchoscopy, surface burns	Presumptive diagnosis in the correct clinical context, negative cultures
Transfusion related acute lung injury and massive transfusions	History of transfusion, dyspnea during or shortly after transfusion	Diagnosis of exclusion
HSCT¶	History of HSCT	Diagnosis of exclusion
Pancreatitis	Abdominal pain, vomiting, risk actors (eg, gallstones, alcohol, viral infection)	Elevated amylase and lipase, with or without abnormal imaging
Inhalation injures other than smoke (eg, near drowning, gases)	History of inhalation exposure (eg, chlorine gas)	Diagnosis of exclusion
Thoracic surgery (eg, post-cardiopulmonary bypass) or other major surgery	History of surgery, intraoperative ventilation, intraoperative transfusion	Diagnosis of exclusion
Drugs (chemotherapeutic agents, amiodarone, radiation)	New drugs or radiation exposure on history, lymphocytosis on lavage, lavage may have suggestive features of amiodarone toxicity ("foamy macrophages") but is nonspecific	Diagnosis of exclusion, lung biopsy occasionally helpful

Some patients remain ventilator-dependent during fibroproliferative phase

• Radiographically: progression from airspace opacification to coarser, reticular pattern of lung infiltration
• Persistent hypoxaemia, low lung compliance, high dead space, progressive pulmonary hypertension
• Differentiate from VAP or ventilator-induced lung injury

Complications:

• Barotrauma
  • PEEP stress
• Nosocomial infection
• Delirium
• VTE
• GI bleeding (stress ulcers)
• Poor nutrition