CARDS vs ARDS – implications for respiratory support
Based on a handful of early reports and anecdotal experience, experts hypothesised that severe COVID-19 pneumonia was clinically different from the more classical presentation of the acute respiratory distress syndrome (ARDS), despite fulfilling the Berlin definition. The most striking difference noted was the dissociation of the severity of hypoxaemia and the compliance of the respiratory system (Crs). It was proposed that patients were presenting along a time-related spectrum with two distinct phenotypes at either end. Initially, type ‘L” is characterised by low elastance (high Crs), low lung weight, low right-to-left shunt, and low lung recruitment potential. With time, patients would eventually become type “H” with high elastance (low Crs), high lung weight (oedema), high right-to-left shunt with greater potential for lung recruitment and thus resemble classical ARDS. Subsequently, numerous studies have examined the mechanics and gas exchange of COVID-19 patients and have found no consistent relationships between hypoxaemia, recruitability and compliance. There was no convincing evidence found of a time-related spectrum of disease. In conclusion, despite significant variability, COVID-19 produces a clinical picture largely consistent with classical ARDS. Furthermore, the outcomes using traditional lung protective strategies have been acceptable and do not warrant change at this stage.
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