A method for determining the risk of developing hypoxemia, the method comprising - providing a biological sample from a subject, said subject having had open cardiac surgery with the use of cardiopulmonary bypass, wherein said sample has been provided from said subject between start of surgery and 72 hours after detachment from the CPB circuit; - determining the level of at least one biomarker selected from the group consisting of carnitine, isobutyrylglycine, N-Acetyl-Glucosamine, arachidonic and eicosapentanoic acid, isobutyric acid and citrate; and/or - determining the ratio between levels of biomarkers of at least one ratio selected from the group of ratios selected from citrate/phenylalanine, polyunsaturated fatty acids (PUFA)/phenylalanine, alanine/phenylalanine, citrate/ trimethylamine-N-oxide (TMAO), carnitine/TMAO, isobutyric acid/phenylalanine, and isobutyrylglycine/arginine; - comparing said levels of one or more biomarkers or ratios to one or more reference levels; - determining that said subject is at risk of developing hypoxemia if said level of one or more biomarkers are above the one or more reference levels, or determining that said subject is not at risk of developing hypoxemia, if said one or more levels are equal to or below the one or more reference levels; and/or - determining that said subject is at risk of developing hypoxemia if said one or more ratios between levels of biomarkers are above the one or more reference levels, or determining that said subject is not at risk of developing hypoxemia, if said level of one or more ratios between levels of biomarkers are equal to or below the one or more reference levels. - providing a biological sample from a subject, said subject having had open cardiac surgery with the use of cardiopulmonary bypass, wherein said sample has been provided from said subject between start of surgery and 72 hours after detachment from the CPB circuit; - determining the level of at least one biomarker selected from the group consisting of carnitine, isobutyrylglycine, N-Acetyl-Glucosamine, arachidonic and eicosapentanoic acid, isobutyric acid and citrate; and/or - determining the ratio between levels of biomarkers of at least one ratio selected from the group of ratios selected from citrate/phenylalanine, polyunsaturated fatty acids (PUFA)/phenylalanine, alanine/phenylalanine, citrate/ trimethylamine-N-oxide (TMAO), carnitine/TMAO, isobutyric acid/phenylalanine, and isobutyrylglycine/arginine; - comparing said levels of one or more biomarkers or ratios to one or more reference levels; - determining that said subject is at risk of developing hypoxemia if said level of one or more biomarkers are above the one or more reference levels, or determining that said subject is not at risk of developing hypoxemia, if said one or more levels are equal to or below the one or more reference levels; and/or - determining that said subject is at risk of developing hypoxemia if said one or more ratios between levels of biomarkers are above the one or more reference levels, or determining that said subject is not at risk of developing hypoxemia, if said level of one or more ratios between levels of biomarkers are equal to or below the one or more reference levels.