A doctor checking comatose COVID-19 patients for signs of a stroke instead stumbled onto a new clue about how the virus may harm the lungs—thanks to a test that used tiny air bubbles and a robot.

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Dr. Alexandra Reynolds, a neurologist at New York's Mount Sinai Health System, initially was baffled as she tracked "the cacophony of sound" made by those harmless bubbles passing through the bloodstream of patient after patient.

Yet the weird finding excited lung specialists who now are studying if it helps explain why often, the sickest coronavirus patients don't get enough oxygen despite being on ventilators.