Acute Respiratory Distress Syndrome is characterised by widespread lung inflammation, with diffuse injury to alveolar cells, surfactant dysfunction, non-cardiogenic pulmonary oedema and activation of an immune response.
Pulmonary oedema results from increased permeability of airway membranes with recruitment of neutrophils and inflammatory mediators, which causes inactivation of surfactant leading to collapse (atelectasis) and consolidation of airways and loss of functional lung units available for gaseous exchange. This will lead to V/Q mismatch, as fewer areas are ventilating but remain perfused. This inflammatory process also inhibits hypoxaemic pulmonary vasoconstriction, allowing deoxygenated blood to meet non-ventilated lung tissue and again, worsening V/Q.
As a result of these processes, clinically ARDS presents as a type 1- hypoxaemic- respiratory failure with bilateral opacities seen on CXR, indicating heterogenous lung zones with some areas of aeration in contrast to areas of consolidation and inflammation. Lungs will tend to be stiff and non-compliant
x-ray showing ground glass opacities in severe ARDS.
As a syndrome rather than a singular disease, there is no one diagnostic tool or value that is used to make a diagnosis of ARDS. The 2012 Berlin definition outlines some criteria that must be present to make a diagnosis of ARDS, including-
presence of acute hypoxaemic respiratory failure
onset within 7 days of insult or worsening respiratory symptoms within 7 days
bilateral opacity on CXR/ CT not explained by effusions, lobar or lung collapse, or nodules
cardiac failure not the primary cause of acute respiratory failure
Outcomes/prognosis can vary widely dependent on patient factors including aetiology (which can include infection- Covid-19 and influenza-, sepsis, aspiration pneumonia, toxic inhalation and acute pancreatitis), co-morbidities and severity of illness.
Severity is grouped into mild, moderate and severe based on objective grading systems such as the Murray Score for lung injury (see the chart). In this system, a score of 0 indicates no lung injury, 1-2.5 indicates mild-moderate lung injury and >2.5 indicates severe injury.
The Berlin definition defines severity through PaO2/FiO2 (P/F ratio), calculated simply by dividing PaO2 from an ABG by FiO2 on the ventilator. A score of <300 indicates mild, <200 moderate and <100 severe ARDS.
Treatment focuses on supportive therapies (fluid, nutrition etc.) and treatment of underlying pathologies, and most commonly centres around mechanical ventilation (MV) which is likely to require significant sedation and potentially paralysis due to the non-compliance of the lungs.
The method of MV was overhauled in 2000 following publication of the ARDSnet ARMA trial.
Prior to this, patients were being ventilated with tidal volumes (TV) of 10-15mL/kg for ideal body weight (IBW). ARDSnet carried out a multicentre RCT with a total of 861 patients across 75 ICUs. They compared 12mL/kg TV with a plateau pressure of 50 cmH20 to 6mL/kg TV (which is closer to a physiologically normal TV, usually 400-500mL) and a plateau pressure of 30 cmH20.
The second group also had a co-intervention of higher PEEP to maintain oxygenation by splinting open alveoli and increasing the surface area available for gas exchange.
The trial actually ended early owing to the reduced mortality and reduced days on ventilation in the lower TV group, indicating clear benefits and signalling a change to standard practice. These findings have consistently been confirmed in further RCTs, and have led to widely accepted use of lung protective ventilation, with lower TV, lower pressures and permissive hypercapnia to allow for smaller TV.
Therefore in ARDS, a conservative TV target of 6mL/kg of IBW (or even as low as 4mL/kg) is recommended by the British Thoracic Society and the Faculty of Intensive Care Medicine as gold standard care, alongside a maximum plateau pressure of 30cmH2O. The ALVEOLI trial considering higher PEEP (as introduced in the low TV arm of the ARMA trial) found no mortality benefits for higher vs lower PEEP to maintain oxygenation, but an 'open lung' approach involving higher PEEP and alveolar recruitment manoeuvres may be of benefit in moderate-severe ARDS and is recommended by FICM/BTS.
Key points-
Target TV is always based on IBW rather than actual weight.
male = 50kg + 2.3 kg for each inch over 5 feet
female = 45.5 kg + 2.3 kg for each inch over 5 feet
Peak pressure alarm should be set at 30cmH2O
Higher TV and higher airway pressures can allow for better oxygenation and reduced atelectasis, but can also contribute to ventilator associated lung injuries (VALI). These include barotrauma (damage due to high pressure- leads to alveolar rupture and pneumothorax) and volutrauma (damage due to high volumes - leads to hyperinflation and shear injuries). Presence of VALI, understandably, contributes to increased morbidity and mortality. Having a maximum pressure of 30cmH2O may be just as important in avoiding further lung injury as having a suitable tidal volume.
Reduced TV, however, can lead to impaired clearance of CO2. This can lead to an increase in PCO2 and drop in pH (respiratory alkalosis- more on pH management here). If using lung protective ventilation with low tidal volumes, and bearing in mind that extracorporeal removal of carbon dioxide requires further research for use with ARDS according to FICM/ICS/BTS, a degree of permissive hypercapnia may be required. This allows the PCO2 to rise and pH to drop as low as 7.15 (check local guidance for acceptable limits) to facilitate lower TV. pH may be maintained around this level with bicarbonate infusions to allow for TVs of 4-6mL/kg without causing profound acidosis.
Other recommended interventions from FICM/BTS include use of ECMO, high PEEP and pronation for moderate-severe disease, alongside low TV lung protective ventilation and a conservative fluid balance strategy.
FICM/ICS/BTS 2018 guidance on ARDS
To conclude- ARDS is a syndrome involving widespread oedema, atelectasis and inflammation which is relatively poorly defined, but can be very severe with poor prognosis. Severity is usually defined by the Berlin definition dependent on P/F ratio, and treatment largely is mechanical ventilation and fixing the underlying cause. Mechanical ventilation follows a lung protective strategy, with low tidal volumes, potentially high PEEP and limited plateau pressures to avoid barotrauma and volutrauma. As a result, permissive hypercapnia is often indicated.
Thanks for reading, and good luck ventilating your next set of ARDSy, stiff, boggy lungs.
Love, Christie x
References-
ARDSNet (2000) 'Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome' in New England Journal of Medicine. 342(18), pp. 1301-1308. https://www.nejm.org/doi/full/10.1056/NEJM200005043421801
British Lung Foundation- 'Acute respiratory distress syndrome' https://www.blf.org.uk/support-for-you/ards
LITFL (2020) 'Protective lung ventilation' https://litfl.com/protective-lung-ventilation/
LITFL (2020) 'ARDSnet ventilation strategy' https://litfl.com/ardsnet-ventilation-strategy/
Raghavendran, K., Napolitano, L. (2011) 'ALI and ARDS: Challenges and advances' in Critical Care Clinics. 27(3), pp. 429-437. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3173767/
Pfeilsticker, F., Neto, A. (2017) ''Lung-protective' ventilation in acute respiratory distress syndrome: still a challenge?' in Journal of Thoracic Disease. 9(8), pp. 2238-2241. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5594148/#r19
Fuchs, H., Rossmann, N., Schmid, M., Hoenig, M., Thome, U., Mayer, B., Klotz, D., Hummler, H. (2017) 'Permissive hypercapnia for severe acute respiratory distress syndrome in immunocompromised children: a single centre experience' in PLoS One. 12(6). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5478142/
Faculty of Intensive Care Medicine (2018) 'Guidelines on the management of Acute Respiratory Distress Syndrome' https://www.ficm.ac.uk/sites/default/files/ficm_ics_ards_guideline_-_july_2018.pdf
Wikipedia (2021) 'Acute respiratory distress syndrome'
Lipes, J., Bojmehrani, A, Lellouche, F. (2012) 'Low tidal volume ventilation in patients without acute respiratory distress syndrome: a paradigm shift in mechanical ventilation' in Critical Care Research and Practice, 2012. https://www.hindawi.com/journals/ccrp/2012/416862/
LITFL (2020) 'Acute respiratory distress syndrome definitions' https://litfl.com/acute-respiratory-distress-syndrome-definitions/
ARDS Definition Taskforce (2012) 'Acute respiratory distress syndrome: the Berlin definition' in Journal of the American Medical Association. 307(23), pp. 2526-2533. https://pubmed.ncbi.nlm.nih.gov/22797452/
Kamo, T. et al. (2019) 'Prognostic values pf the Berlin definition criteria, blood lactate level and fibroproliferative changes on high resolution computer tomography in ARDS patients' in BMC Pulmonary Medicine. 19(37). https://bmcpulmmed.biomedcentral.com/articles/10.1186/s12890-019-0803-0
Morales-Quinteros, L., Camprubi-Rimblas, M., Bringue, J., Bos, L., Schultz, M., Artigas, A. (2019) 'The role of hypercapnia in acute respiratory failure' in Intensive Care Medicine Experimental. 7(39). https://icm-experimental.springeropen.com/articles/10.1186/s40635-019-0239-0