Acute respiratory distress syndrome

Question 1

What is ARDS? !!! Characterised by? (x4)

Answer 1

Syndrome of acute and persistent lung inflammation with increased vascular permeability, characterised by (i) acute onset, (ii) bilateral infiltrates consistent with pulmonary oedema, (iii) hypoxaemia: PaO₂/FiO₂ (arterial to inspired oxygen) ratio of ≤300 on positive end-expiratory pressure (PEEP) or continuous positive airway pressure (CPAP) ≥5 cm H₂O, and (iv) no clinical evidence for raised left atrial pressure (pulmonary capillary wedge pressure below 18mmHg).

Question 2

What is PEEP?

Answer 2

Positive end-expiratory pressure (PEEP) is the pressure in the lungs above atmospheric pressure that exists at the end of expiration. The two types of PEEP are extrinsic PEEP (PEEP applied by a ventilator) and intrinsic PEEP (PEEP caused by an incomplete exhalation).

Question 3

What is the aetiology of ARDS? (x9)

Answer 3

• Sepsis
• Aspiration
• Pneumonia
• COVID-19
• Pancreatitis
• Trauma/burns
• Transfusion (massive, transfusion-related lung injury)
• Transplantation (bone marrow or lung)
• Drug overdose or reaction

Question 4

What is the pathophysiology of ARDS?

Answer 4

Severe insult to the lungs induces the release of inflammatory mediators, increased capillary permeability, pulmonary oedema, impaired gas exchange and decreased lung compliance. It is at the severe end of the spectrum of ‘acute lung injury’.

Question 5

What are the three stages of ARDS?

Answer 5

Patients progress through three pathological stages: exudative (inflammatory stage associated with pulmonary oedema), proliferative and fibrotic stage (characterised by continued respiratory failure, high minute ventilation, and poorly compliant lungs).

Question 6

What is the epidemiology of ARDS?

Answer 6

Incidence in UK is 1 in 6000

Question 7

What are the symptoms of ARDS?

Answer 7

Rapid deterioration of respiratory function, SOB, respiratory distress, cough (may have frothy sputum). Pleuritic chest pain is common when the aetiology of pneumonia.

Question 8

What are the signs of ARDS? (x4)

Answer 8

• Cyanosis
• Tachypnoea
• Bilateral widespread inspiratory crepitations
• Hypoxia refractory to oxygen treatment

Question 9

What does ARDS look like on CXR?

Answer 9

Bilateral alveolar and interstitial shadowing. May also see atelectasis. VERY NON-SPECIFIC.

Question 10

What are the other investigations for ARDS? (x6)

Answer 10

• PLASMA BNP: less than 100 pg/mL may distinguish ARDS from heart failure
• ABG: low partial pressure of oxygen; PaO2/FiO2 less than 300 on PEEP or CPAP
• SPUTUM CULTURE/AMYLASE/BLOOD CULTURE: indicative of aetiologies
• ECHOCARDIOGRAPHY: if there is severe aortic/mitral valve dysfunction, or decreased LVEF, this favours a haemodynamic aetiology over ARDS
• PULMONARY ARTERY CATHETERISATION: below 18 mmHg (pulmonary capillary wedge pressure aka pulmonary artery occlusion pressure; PAOP)
• BRONCHOSCOPY: exclude differentials such as diffuse alveolar haemorrhage, respiratory viruses (lavage fluid for microbiology) and cytology (eosinophils, viral inclusion bodies and cancer cells)