Definitions
The first qualifying arterial blood gas for the diagnosis of PARDS was used for the AVDSf calculation and was considered the onset of PARDS for purposes of this study. AVDSf was calculated using the Enghoff modification of the Bohr formula: AVDSf = (PaCO2 - PetCO2) / PaCO2. Echocardiograms were analyzed as previously described (Himebauch 2018). Echocardiographic measures of RV systolic function included: qualitative assessment (normal or mildly, moderately, or severely diminished), fractional area change (FAC), tricuspid annular plan systolic excursion (TAPSE), RV peak systolic global longitudinal strain (RV GLS), and RV peak systolic strain (also known as free wall strain). Abnormal FAC was defined as less than 35%. Abnormal TAPSE was defined as a value less than -2 standard deviations below published pediatric normal values for age14 and calculated through an online calculator (parameterz.blogspot.com). Abnormal RV GLS and strain were defined as values greater than -18% and greater than -21%, respectively, as less negative values reflect worse function. As pulmonary arterial catheters are rarely used in clinical practice, echocardiographic surrogates of pulmonary hypertension were defined as flattening or bowing of the septal position in systole in the parasternal short-axis view, tricuspid regurgitant velocity > 2.8 m/s2, or RV systolic pressure estimate greater than ½ systemic systolic pressure at the time of echocardiogram.