The incidence of postoperative delirium in patients after cardiovascular surgery is very high, which seriously affects the short-term and long-term prognosis of patients, but its mechanism is not clear. Recent studies have found that lactic acid participates in the process of sepsis by inducing histone lactatation in macrophages. histone H3 lysine 18 lactylation (H3K18la) is significantly associated with the severity of disease and inflammatory response. Inflammation plays an important role in postoperative delirium. We therefore hypothesized that macrophage H3K18la also plays a role in postoperative delirium. At the same time, the latest literature shows that the early markers of Alzheimer's disease (AD), such as Aβ, exosomes and phosphorylated tau, etc. [5], can also be detected in the serum of AD patients. Given the obvious correlation between Alzheimer's disease and postoperative delirium [6], we hypothesized that serum biomarkers of AD could also be used as serum biomarkers for postoperative delirium. The objectives of this study were to investigate the correlation between H3K18 in serum monocytes and postoperative delirium and its severity in patients undergoing cardiovascular surgery. Meanwhile, we also detect the serum levels of biomarkers of AD, such as Aβ, tau and exosomes etc., in patients undergoing major cardiac surgery. We aim to find the serum biomarkers of delirium after major cardiac surgery, so as to provide a more convenient diagnostic basis for the prevention of postoperative delirium.