…no temporal associations with mortality were demonstrated in RCTs (any time) or observational studies (after 1994). Pooled mortality from 1994 to 2006 was 44.0 (95% CI, 40.1–47.5) for observational studies, and 36.2% (95% CI, 32.1–40.5) for RCTs. Meta-regression identified study type (observational versus RCT, odds ratio, 1.36; 95% CI, 1.08–1.73) and patient age (odds ratio per additional 10 yr, 1.27; 95% CI, 1.07–1.50) as the only factors associated with mortality.
Why? In a Medscape report the authors speculate:
…that the failure to improve ARDS outcomes reflects the dearth of effective treatments and variable adherence to guidelines that recommend low tidal volume, early aggressive resuscitation, and appropriate antibiotic use and sedation protocols.
Studies included in the systematic review were published in a time period extending through 2006. That may not have been enough time for widespread adoption of low tidal volume ventilation, validated in the ARDSnet trial in 2000.