The SARS-CoV-2 pandemic has inspired new interest in understanding the fundamental pathology of acute respiratory distress syndrome (ARDS), which has been associated with severe coronavirus disease 2019 (Covid-19). ARDS has long been recognized to be remarkably heterogeneous, with not only a wide range of causes but also a broad spectrum of severity, abnormalities on imaging, and gas-exchange impairment.¹ The form of ARDS that is associated with Covid-19 is no different.² A long-standing goal³ has been to define endotypes that subdivide ARDS into groups on the basis of distinct biologic and pathologic processes in order to design higher-yield clinical trials and . . .