Acute Respiratory Distress Syndrome

Question 1

Define ARDS

Answer 1

A non-cardiogenic pulmonary oedema and diffuse lung inflammation syndrome with increased vascular permeability

Question 2

Criteria for diagnosis ARDS

Answer 2

The diagnosis is based on fulfilling 3 criteria:
- Acute onset (Within 1 week of predisposing condition)
- Bilateral opacities on CXR (infiltrates)
- PaO2/FiO2 (arterial to inspired oxygen) ratio of <300 on positive end-respiratory pressure (PPEP) or continuous airway pressure (CPAP) >5cm H2O (hypoxaemia)
- PCWP <19mmHg or a lack of clinical congestive heart failure

Question 3

Aetiology of ARDS

Answer 3

Can be caused by direct lung injury or occur secondary to severe systemic illness
Lung damage and release of inflammatory mediators → increased capillary permeability and pulmonary oedema → impaired gas exchange and reduced lung compliance
Damage to pneumocytes → loss of surfactant → alveoli collapse
Often accompanied by multi-organ failure

Question 4

Predisposing events to ARDS

Answer 4

Pulmonary:
- Pneumonia
- Gastric aspiration
- Inhalation of toxins e.g. smoke
- Drowning
- Injury/trauma
- Vasculitis
- Contusion

Extra-pulmonary
- Sepsis, shock
- Pancreatitis
- Burns
- Haemorrhage → hypovolaemia
- Multiple transfusions
- DIC, VTE, fat embolus
- Acute liver failure
- Head injury
- Malaria
- Obstetric events (eclampsia, amniotic fluid embolus)
- Drugs/toxins (aspirin, heroin, paraquat, barbiturates, NSAIDs, opiates)

Question 5

Symptoms and signs of ARDS

Answer 5

Following predisposing condition there is a rapid deterioration in respiratory function

Dyspnoea
Fever
Cough
Pleuritic chest pain

Obs: tachypnoea and tachycardia, low sats
General: Cyanosis, Peripheral vasodilation
Resp: Bilateral fine inspiratory crackles / pulmonary crepitations

Question 6

Investigations for ARDS

Answer 6

Sepsis 6: Blood cultures, ABG

Sputum culture

ABG: low PaO2
- PaO₂/FiO₂ (inspired oxygen) ratio of ≤300 on positive end-expiratory pressure (PEEP)
- (CPAP) ≥5 cm H₂O
BNP: exclude HF
FBC
U&Es
CRP/ESR
Cultures
Amylase/lipase
Glucose
Clotting
D-dimer
G&S

CXR: BILATERAL pulmonary infiltrates, alveolar and interstitial shadowing ((pulmonary oedema)
Pulmonary artery catheter: Pulmonary capillary wedge pressure (PCWP) <19mmHg
Echo: Exclude HF

Question 7

Management for ARDS

Answer 7

Usually requires ICU input or management
Treat the underlying cause e.g. antibiotics
Supportive:
- Oxygen - maintain high PEEP >5mmHg
- Central venous access → inotropes
- Proning - Prone positioning and muscle relaxation (ie paralysis) improve outcome sin ARDS
- Fluid balance
- Mechanical ventilation - Ventilate with low tidal volumes
- low tidal volume ventilation reduces mortality in ARDS
- ECMO, ECCO2R, Neuromuscular blocking agents
- Monitor pH instead of CO2

Non-shocked → sit upright
Shocked → colloid infusion

Question 8

Complications of ARDS

Answer 8

Death
Multiple organ failure
Pneumothorax
Persistent dyspnoea
Abnormal lung function
Hospital acquired pneumonia

Question 9

Prognosis for ARDS

Answer 9

30-50% mortality
Death often due to multiple organ failure rather than just resp. failure