MMP9 and injury: Based on the previous evidence of predominant neutrophil recruitment and activation following CPB [6, 14], neutrophil-mediated MMP-9 activation [14, 20], and MMP-involved acute lung injury [7, 12], we hypothesized that MMP-9 activation may contribute to pulmonary dysfunction following CPB, which could be manifested by reduced PaO2/FiO2 ratio.