From the Medscape report:
For their retrospective analysis, the researchers identified 174,180 patients from the National Inpatient Sample database, which is the largest all-payer inpatient healthcare database in the United States, and documents about 8 million hospital stays annually...
From 1996 to 2011, there was a 14.6% absolute reduction in mortality — from 46.8% to 32.2% (relative reduction, 31.0%). From 2000 to 2005, the absolute reduction was 8.9%.
However it is difficult to know how many of these patients actually met criteria for ARDS, as data capture was done using an ECD 9 code database.
Of note, the slope of the downward mortality curve was steeper around the time of introduction of low tidal volume ventilation.