The authors compared ventilation parameter , including PaO2/FIO2 , A-a gradient , positive end-expiratory pressure , plateau pressure , dynamic compliance and hemodynamic parameters , including central venous pressure , mean arterial pressure , cardiac index , extravascular lung water index , fluid balance between early initiation ( within 12 hours after ARDS onset ) and late initiation of CRRT ( 48 hours after ARDS onset ) groups .