FRCA Notes

Acute Respiratory Distress Syndrome

ARDS is probably all-too familiar in the wake of Covid-19.

It featured as an SAQ back in 2018, where the bulk of the marks were for knowing both ventilatory and non-ventilatory steps to improve oxygenation.

More recent CRQs have skirted around the topic, including ones on prone ventilation and APRV.

Resources

  • Acute Respiratory Distress Syndrome is diagnosed using the 4th Berlin Criteria (2012):
    1. Acute onset within 7 days
    2. Bilateral opacities on CXR
    3. PaO2/FiO2 ratio of <300mmHg (<39.9kPa)
    4. Not fully explained by fluid overload (inc. cardiac failure)

  • The Berlin definition of ARDS predicts mortality better than the previous AECC definition

  • The incidence of ARDS is difficult to establish, owing to difference in definitions and study design
  • The LUNG SAFE study (2016) prospectively assessed ICU admissions using the Berlin criteria, finding:
    • 10% of cases fulfilled ARDS criteria
    • 25% of those invasively ventilated fulfilled ARDS criteria

    Outcomes

  • Mortality is dependent on severity:
    • ARDS Severity P/F Ratio Mortality
    Mild Moderate Severe
    200 - 300mmHg 100 - 200mmHg <100mmHg
    27% 32% 45%

  • It's probably worth noting patients frequently die with hypoxaemia rather than necessarily from hypoxaemia
  • Survivors of ARDS are not significantly limited by chronic respiratory failure, but more by long-term muscle weakness and neuropsychiatric problems (BJA, 2022)

  • The Murray Lung Injury Score is used to assess severity of lung injury in ARDS
  • It uses four criteria:
    1. P/F ratio on 100% O2
    2. Degree of PEEP
    3. Lung compliance
    4. Number of CXR quadrants infiltrated
Score CXR consolidation P/F ratio (mmHg) PEEP (cmH2O) Compliance (ml/cmH2O)
0 None ≥300 ≤5 ≥80
1 1 quadrant 225 - 299 6 - 8 60 - 79
2 2 quadrant 175 - 224 9 - 11 40 - 59
3 3 quadrants 100 - 174 12 - 14 20 - 39
4 4 quadrants <100 ≥15 ≤19

  • A Murray Lung Injury score ≥3 represents severe respiratory failure and is a criterion for ECMO referral

  • A wide range of conditions may lead to ARDS
    • Predominantly respiratory: Pneumonia | Influenza | Covid - 19 | Aspiration of vomit | Inhaled toxic substance | TRALI
    • Sepsis
    • Trauma
    • Burns
    • Drowning
    • Pancreatitis

Differential diagnosis of bilateral pulmonary infiltrates on CXR

Vascular Infectious Traumatic Auto-immune Drug-induced Neoplastic
Pulmonary haemorrhage Bacterial Bilateral atelectasis Haemorrhagic Goodpasture's Eosinophilic pneumonitis Lymphangitis
Cardiogenic pulmonary oedema Viral Pulmonary contusions TRALI Methotrexate Infiltrative neoplasm
Cardiac failure Fungal Chemical pneumonitis Rheumatoid pneumonitis Amiodarone
PJP GVHD Alveolar haemorrhage


  • Acute inflammation of the alveolar-capillary membrane

  • Initially there is:
    • Increased membrane permeability
    • Recruitment of neutrophils and other mediators of acute inflammation into the airspace; BAL demonstrates raised IL-6 levels in ventilatory-associated lung injury
    • High-permeability pulmonary oedema
  • The inflammatory exudate inactivates surfactant leading to collapse and consolidation of distal airspaces
  • This causes progressive loss of the gas exchange surface area

  • The inflammatory process also paralyses the lung's means of controlling vascular tone
  • This prevents compensatory HPV in affected lung units
  • Deoxygenated blood is allowed to cross unventilated lung on the way to the left heart i.e. shunt

  • The combination of these two processes causes profound hypoxaemia and, eventually, hypercarbia too

  • Initially, use recruitment manoeuvres to improve oxygenation (although no mortality benefit)

Lung-protective ventilation

Factor Target
Tidal volumes 6ml/kg (4 - 8ml/kg) IBW (ARDSNet)
PEEP Optimised/high
Plateau pressure <30cmH2O
Driving pressure (Pplat - PEEP) <15cmH2O
Transpulmonary pressure <25cmH2O
pH (permissive hypercapnoea) >7.30

PEEP

  • PEEP has multiple beneficial effects:
    • ↓ shunt
    • ↓ HPV and therefore PVR
    • ↓ Pplat and therefore barotrauma
    • ↓ driving pressure and therefore shear force
  • It can be optimised in several ways:
    • The lower inflection point of the hysteresis curve i.e. the pressure-volume curve
    • As per ARDSnet tables
    • To driving pressure; can increase PEEP so long as driving pressure is decreasing

  • If a patient is already ventilated using a lung-protective strategy, maximal PEEP and has an FiO2 of 1.0, several ventilatory steps can be instigated to improve oxygenation

Refractory hypoxaemia

  • Use a 1:1 or even inverse I:E ratio

  • Sustained neuromuscular blockade e.g. cisatracurium infusion for 48hrs
  • Cisatracurium infusion associated with reduced mortality

  • Prone ventilation at least 12hrs/day and ideally 16hrs/day

  • Use alternative ventilatory modes e.g. APRV or other open lung strategies

  • Avoid positive fluid balance
    • Conservative fluid strategy is recommended as it may be beneficial with little harm
      • There are similar rates of AKI between conservative and liberal strategies
      • A conservative strategy is associated with reduced rate of RRT
    • ICS recommends a conservative strategy using restricted fluid use, diuretics and hyper-oncotic albumin

  • ECMO
    • Not recommended for all patients with ARDS as it carries a significant risk of bleeding (15% of patients)
    • Is recommended for patients with severe ARDS
    • May improve (CESAR) or not improve (EOLIA) survival

  • Steroids
    • Possibility of substantial benefit with little harm, but evidence of (very) low quality and from before a lung-protection era
    • From anecdotal evidence of speaking to ECMO centres, high-dose methylprednisolone is often recommended as a roll of the dice

  • High frequency oscillatory ventilation is not recommended owing to:
    • No evidence of benefit and potential increased mortality (OSCAR and OSCILLATE trials)
    • Some evidence in paediatrics and possibly bronchopleural fistulas

  • Inhaled pulmonary vasodilators such as nitric oxide (5 - 20ppm) are not associated with a mortality benefit and may be associated with renal dysfunction
    • Possibly useful in RV failure by causing vasodilation in viable lung units, reducing shunt fraction and PVR
    • May play a role for short-term use in patients awaiting instigation of ECMO
    • Other options include nebulised prostaglandins, nebulised milrinone

  • ECCOR: extra-corporeal carbon dioxide removal to facilitate use of lower tidal volumes has been proposed
    • The REST trial (JAMA, 2021) was terminated early due to high mortality in the ECCOR group

Inclusion criteria
Potentially reversible, severe respiratory failure:
P/F ratio <6.7kPa for ≥3hrs
P/F ratio <10kPa for ≥6hrs
Murray Lung Injury Score ≥3
Uncompensated hypercapnoea with pH <7.20 despite RR >35min-1
or due to life-threatening airway disease inc. asthma
Failed trial prone ventilation ≥6hrs
Failed optimal respiratory management with lung-protective ventilation

Exclusion criteria
Established / refractory multi-organ failure
Evidence of severe neurological injury
Prolonged cardiac arrest >15mins