Acute respiratory distress syndrome (ARDS). 

Berlin definition of ARDS:  

• Timing: respiratory failure within 1 week of a known insult or new and/or worsening respiratory symptoms 

• Origin: respiratory failure not fully explained by cardiac function or volume overload (need objective criterion such as echocardiography to exclude hydrostatic oedema if no risk factor is present) 

• Imaging: bilateral opacities on chest radiograph or CT not fully explained by effusion, collapse or nodules 

• Oxygenation: acute onset of hypoxaemia defined as PaO2/FiO2 <300 mmHg on at least PEEP 5 cmH2O 

  • PaO2/FiO2 of 200–300 (using mmHg) is mild ARDS 

  • PaO2/FiO2 of 100–200 (using mmHg) is moderate ARDS 

  • PaO2/FiO2 ≤100 (using mmHg) is severe ARDS 

1. Infectious: 

• Pneumonia: bacterial, viral or fungal 

• Extra-Pulmonary sources of sepsis 

• Aspiration of gastric contents 

2. Non-infectious: 

• Acute pancreatitis 

• Severe trauma 

• Transfusion of blood products 

• Drug overdose and drug-induced lung injury 

• Near drowning (inhalation of fresh/salt water) 

• Smoke inhalation/burns 

ARDS is often a clinical and radiographical diagnosis. Investigation is often based around identifying the precipitant (if unknown), excluding alternative diagnoses and initiating supportive and specific treatment. 

• Detailed history and physical examination 

• Radiology – CXR/CT Chest/Abdomen 

• Specific Laboratory tests e.g. Amylase 

• Infection Screen – Viral Swab/Blood/Urine/Sputum +/- BAL 

Following intubation and ventilation the patient was sedated and established on mechanical ventilation. The ventilator was set to Pressure Controlled Ventilation: Pins 22, PEEP 8, RR 14, I:E ratio 1:2. Tidal volumes at this stage is 290mls. 

The ARDSnet recommendations include: 

• Targeting tidal volume of 6 ml/kg of predicted body weight 

• Oxygenation goal: PaO2 8-10 kPa or SpO2 88-95% 

• Plateau pressure goal: ≤ 30 cm H2O 

• Permissive hypercapnia, allowing PaCO2 to rise in order to achieve lung protective ventilation (a pH of >7.20 is accepted) 

Following adjustment and titration of the ventilator settings the patient remains critically hypoxemic with a PaO2 8.3kPa on FiO2 1.0. He is fully sedated and an infusion of neuromuscular blockade is commenced which has minimal effect. A decision is made to prone the patient. 

• Optimisation of V/Q matching 

• Increase in FRC 

• Decreased atelectasis 

• Facilitates secretion drainage 

• Heart sits against the sternum (rather than left lung), therefore the lung is less compressed 

• Decreased transpleural pressure gradient between dependent and non-dependent lung in the prone position 

• Plateau pressure is more uniformly distributed when prone -> more uniform alveolar ventilation 

• Recruitment manoeuvres have been shown to be more effective in the prone position 

• Alterations in chest wall mechanics -> allowing lungs to inflate at lower pressures 

• Dorsoventral orientation of large airways 

There have been multiple conflicting RCTs but the weight of evidence now suggests prone ventilation is beneficial in selected severe ARDS patients. Gattinoni et al 2001 was the original study showing improvement in oxygenation of most patients with ALI /ARDS by proning. The PROSEVA trial by Guerin et al 2013 showed a marked mortality benefit (NNT = 6) for prone ventilation in severe ARDS (28-day mortality 16% prone versus 32.8% supine). 

At present there is limited evidence for mortality benefit with steroids in ARDS despite multiple studies into this. However, the RECOVERY trial (2020) demonstrated a mortality benefit with dexamethasone in patients with COVID-19 pneumonitis requiring either mechanical invasive ventilation or oxygen alone. 

After being proned for 18 hours, the patient has not responded to conventional treatment. His PaO2 is 7.7kPa on FiO2 1.0. He is fully sedated, paralysed with appropriate fluid balance. His ventilator settings have been maximally titrated. A referral for ECMO is made. 

ECMO is ExtraCorporeal Membrane Oxygenation. The extracorporeal circuit allows for the oxygenation and removal of carbon dioxide from blood and is used as a supportive strategy in patients who have a high risk of death despite conventional therapy. 

Indications include acute, severe, reversible respiratory failure with a high risk of death that is refractory to conventional management. 

Some contraindications of ECMO include: 

a) Absolute: 

• Progressive non-recoverable cardiac disease (not transplant candidate) 

• Progressive and non-recoverable respiratory disease (irrespective of transplant status) 

• Chronic severe pulmonary hypertension 

• Advanced malignancy 

• Graft versus Host disease 

• Weight >120kg 

• Unwitnessed cardiac arrest 

b) Relative: 

• Age > 75 years 

• Multi-trauma with multiple bleeding sites 

• CPR > 60 minutes 

• Multiple organ failure 

• CNS injury 

• The CESAR trial showed improved survival at 6 months (63% vs 47%) in adults with severe acute respiratory failure 

• EOLIA trial: early use of ECMO did not significantly improve mortality at 60 days in patients with severe ARDS, but when used as a rescue modality ECMO may help improve survival 

• Among neurologic, vascular, infectious and bleeding events that occurred during ECMO therapy, bleeding was the most frequent and had a significant impact on mortality. 

• In one study, the overall mortality was 37.3% (37.1% for the patients who underwent VA ECMO, and 37.7% for the patients who underwent VV ECMO). 

• Detailed history, examination and laboratory analysis along with low suspicion index are crucial for early identification of ARDS 

• Applying lung protective ventilation along with cautious fluid therapy can help reduce the mortality and long-term morbidity associated with ARDS  

• For rapidly failing with severe hypoxaemia cases, early consideration of prone positioning as well as referral for ECMO therapy should be considered 

ARDS (The Internet Book of Critical Care). Available at: Emcrit.org/ibcc/ards 

Aubron C, Cheng A AC, Pilcher D, et al. Factors associated with outcomes of patients on extracorporeal membrane oxygenation support: a 5-year cohort study. Crit Care 2013;17(2): R73. 

Howell MD, Davis AM. Management of ARDS in adults. JAMA 2018;319(7): 711-712. 

Matthay MA, Zemans RL, Zimmerman GA, et al. Acute respiratory distress syndrome. Nat Rev Dis Primers 2019;5(1): 18. 

Nickson C. Acute Respiratory Distress Syndrome – ARDS. Life in the Fast Lane 2020. Available at: https://litfl.com/acute-respiratory-distress-syndrome-ards/ 

Nickson C. Extracorporeal membrane oxygention. Life in the Fast Lane 2020. Available at: https://litfl.com/ecmo-extra-corporeal-membrane-oxygenation/