15.13 ARDS

Question 1

a) What features are required to meet the American-European criteria for a diagnosis of acute respiratory
distress syndrome (ARDS)? (20%) 4

Answer 1

ARDS is a multifactorial inflammatory process that may be triggered by a direct or indirect cause. There is a genetic component, as different individuals respond very differently to the same triggers. Direct causes
include aspiration, pneumonia, drowning, pulmonary embolus, pulmonary contusion, inhalational injury and reperfusion injury. Indirect causes include sepsis, blood transfusion, pancreatitis, trauma, burns and drugs

> > Onset within one week of known clinical insult.

> > Bilateral opacities on CXR or CT consistent with pulmonary oedema.

> > Respiratory failure not fully explained by cardiac failure or fluid overload.
Need to objectively assess this with e.g. echo if there is no clear ARDS trigger present.

> > Hypoxaemia with PEEP 5 cm H2O or more.

Question 2

b) List the clinical indices used to quantify and communicate problems with oxygenation in ARDS. (10%) 2

Answer 2

Mean arterial pressure (MAP),
fraction of inspired oxygen (FiO2),
and arterial partial pressure of oxygen (PaO2)

are used to calculate the oxygenation index:

Oxygenation index MAP x FiO2 x 100 / PaO2

Question 3

c) What tidal volume would you select for a patient that meets the criteria for ARDS? (10%) 2

Answer 3

1. Initiate ventilation at 8 ml/kg
   of predicted body weight,
2. Then reduce by 1 ml/kg every
   two hours or less until
   target of 6 ml/kg achieved.

Overinflation may directly damage
(volutrauma and barotrauma)
remaining healthy areas of lung
(as their compliance is greater than diseased areas).

Resulting cytokine release mediates “biotrauma” to lungs and distant organs.

Question 4

d) List the ventilatory (30%) measures that can be taken to improve
oxygenation or prevent further deterioration in a patient with ARDS.

Answer 4

Ventilatory: aim to maintain adequate gas exchange until cellular damage resolves without causing further lung injury due to baro-, volu-, atelec- and biotrauma:

> > PEEP.

> > Tidal volume 6 ml/kg.

> > Permissive hypercapnia.

> > Accept PaO2 sufficient to
adequately oxygenate tissues.

> > Keep peak pressure under 30 cm H2O.

> > Minimise the difference between
peak pressure and PEEP.

> > Recruitment manoeuvres:
no proven survival benefit.

> > Prone positioning.

Question 5

d) List non-ventilatory (30%) measures that can be taken to improve
oxygenation or prevent further deterioration in a patient with ARDS.

Answer 5

Non-ventilatory:

> > Management of any diagnosed underlying cause e.g. sepsis.

> > Conservative fluid management.

> > Physiotherapy: removal of secretions, improvement of gas exchange.

> > Early enteral feeding.

> > Ventilator care bundle: 30–45 degrees head-up, daily sedation hold, stress ulcer prophylaxis, venous thromboembolism prophylaxis.

> > ECMO.