Insights into COVID-19-associated critical illness: a narrative review.

Background and Objective: Since the outbreak of the 2019 novel coronavirus disease (COVID-19), acute respiratory distress syndrome (ARDS) and sepsis resulting from severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection have surged in intensive care units around the world. The heterogeneity of ARDS and sepsis has long been observed, and multiple subphenotypes and endotypes correlated with different outcomes and treatment response have been identified in the search for treatable traits. Despite their similarity to typical ARDS and sepsis, COVID-19-associated ARDS and sepsis harbor distinct features, raising the question as to whether they could be considered as subphenotypes or endotypes of the historical syndromes and, accordingly, benefit from specific therapeutic strategies. This review aimed to summarize and discuss the current knowledge of COVID-19-associated critical illness and the intrinsic subphenotypes or endotypes. Methods: Literature on the pathogenesis of COVID-19 and the subphenotyping of COVID-19-associated critical illness was derived from the PubMed database and reviewed. Key Content and Findings: Accumulating evidence, varying from clinical observation to basic research, has contributed to revealing the fundamental pathophysiological features of severe COVID-19 and has advanced our knowledge of the disease. COVID-19-associated ARDS and sepsis exhibit some distinctive features compared to the classic syndromes, including remarkable vascular abnormality and coagulopathy, and distinct respiratory mechanics and immune response. Some conventional subphenotypes derived from classic ARDS and sepsis have been validated in COVID-19, while novel subphenotypes and endotypes have also been identified in patients with this disease, who experience variable clinical outcomes and treatment responses. Conclusions: Subphenotyping of COVID-19-associated ARDS and sepsis can provide new insights into the development and management of these illnesses.