RecruitingDead Space and Inhaled Nitric Oxide in Pediatric Acute Respiratory Distress SyndromeThe goal of this observational study is to determine whether a marker of dead space (the end-tidal to alveolar dead space fraction \[AVDSf\]) is more strongly associated with mortality risk than markers of oxygenation abnormality (oxygenation index) and to determine whether dead space (AVDSf) is an important marker of heterogeneity in the inhaled nitric oxide (iNO) treatment effect for children with acute respiratory distress syndrome (ARDS). The study aims are: 1. To validate AVDSf for risk stratification of mortality in pediatric ARDS 2. To determine if there is heterogeneity in treatment effect for iNO defined by AVDSf 3. To detect the association between AVDSf and microvascular dysfunction trajectory and whether iNO therapy modifies this association This is a prospective, multicenter observational study of 1260 mechanically ventilated children with moderate to severe ARDS. In a subgroup of 450 children with severe ARDS, longitudinal blood samples will be obtained to measure plasma protein markers.