Acute Respi Distress Syndrome

Question 1

What is the definition of ARDS + clinical hallmarks + pathological hallmarks?

Answer 1

ARDS represents the severe end of a spectrum of acute lung injury due to many different insults.

It is an acute, diffuse, inflammatory lung injury that leads to increased pulmonary vascular permeability, increased lung weight and a loss of aerated tissue

Within 1 week of known clinical insult or new or worsening respiratory symptoms.

Clinical hallmarks

• Hypoxemia despite positive end-expiratory pressure (PEEP)
• Bilateral extensive infiltrates/opacities on CXR not fully explained by effusions, lobar lung collapse, or nodules

Pathological hallmark: Diffuse alveolar damage

Question 2

What is the pathophysiology behind ARDS?

Answer 2

Inflammatory damage to the alveoli, either by locally produced pro-inflammatory mediators, or remotely produced and arriving via the pulmonary artery.

The change in pulmonary capillary permeability allows fluid and protein leakage in the alveolar spaces with pulmonary infiltrates. The alveolar surfactant becomes diluted with loss of its stabilizing effect, resulting in diffuse alveolar collapse and stiff lungs.

This leads to:

• Gross impairment of V/Q matching and shunting causing arterial hypoxia and very large A-a gradient. However, there is usually enough remaining functioning alveoli such that it maintains CO2 clearance, type ll respiratory failure rare in ARDS
• Pulmonary hypertension will develop secondary to the hypoxia but this may be helpful in aiding the V/Q mismatch rather than worsen it
• Reduced lung compliance due to loss of functioning alveoli (hence stiffness) and hyperinflation of the remaining alveoli to their limits of distension

Question 3

What are the causes of ARDs?

Answer 3

Pulmonary

• Pneumonia
• Aspiration (often gastric, even if on a PPI)
• Near drowning
• Contusion

Trauma, with or without pulmonary contusion

• Fractures, particularly multiple fractures and long bone fractures
• Burns

Others

• Sepsis
• Haemorrhage
• Transfusion-related acute lung injury (TRALI), usually within a few hours of transfusion
• Drug overdose (TCAs, opiates, cocaine, aspirin)
• Lung and hematopoietic stem cell transplant
• Acute pancreatitis
• Fat embolism
• Liver failure

Question 4

What is the clinical presentation of ARDS?

Answer 4

Typically within 6 to 72 hours of inciting event and worsens rapidly

Dyspnea ± dry cough

Cyanosis from hypoxemia

Diffuse crepitation

Signs of respiratory distress

• Tachypnea
• Tachycardia
• Diaphoresis
• Use of accessory muscles of respiration

Exclude signs of CCF

• Raised JVP
• Murmurs
• Pitting oedema

Question 5

What are the complications of ARDs?

Answer 5

• Barotrauma (from mechanical ventilation)
• Delirium (common)
• Nosocomial pneumonia (surveillance bronchoalveolar lavage important)
• Deep vein thrombosis
  Stress ulceration
• Catheter related infection

Question 6

What are the investigations for a patient with ARDS

Answer 6

Confirm ARDS
1) ABG!
- Hypoxaemia
- Respiratory alkalosis (hyperventilation)
- High alveolar-arterial oxygen gradient
PaO2/FiO2 < 300
2) CXR: Bilateral pulmonary infiltrates

Exclude CCF

1) BNP (should be <100 in ARDS)
2) ECG
3) Wedge Pressure less than 8mmHg

Exclude other causes of hypoxemic respiratory failure

1) Non-invasive respiratory sampling (tracheobronchial aspiration or mini BAL)
2) Gram stain, cytology, culture
3) Flexible bronchoscopy

Underlying Cause/Complications

1) FBC: signs of sepsis
2) LFTs
3) Renal panel: acute tubular necrosis usually follows ARDS
4) Amylase
5) CRP/Procalcitonin
6) PT/APTT
7) Blood cultures
8) Pulmonary artery catheter

Question 7

What is the management of ARDs?

Answer 7

Minority of patients die from ARDS alone and more commonly from secondary complications such as sepsis or multiorgan system failure, hence supportive care is crucial

Supportive Care

• Sedation & Analgesia (to improve tolerance of mechanical ventilation and reduce O2 consumption)
• Paralysis
• Hemodynamic monitoring
• Nutritional support
• Glucose control
• Nosocomial pneumonia
• DVT prophylaxis
• GI prophylaxis
• Venous access

Treat underlying cause!!

Management of Hypoxemia 
1) Supplemental oxygen
- Most patients require a high FiO2
- Therefore, most patients need intubation and mechanical ventilation - with PEEP
High inflation pressures may worsen ARDS directly, try to maintain plateau pressures <30mmHg
2) Fluid management
- Aim: conservative fluid management to reduce edema formation due to the increased pulmonary vascular permeability in ARDS
- Target CVP <4mmHg or PAOP <9mmHg
3) Ancillary measures 
- Done to avoid oxygen toxicity 
- Prone positioning 
- Increased oxygen delivery 
- Decreased oxygen consumption