Our study revealed that high postoperative BG level was associated with increased morbidity and mortality in non-diabetic RHD patients undergoing VRS, while preoperative BG level was not associated with those factors.
Rheumatic heart disease (RHD), a major burden in developing countries, causes enormous losses every year. Valve replacement surgery is an important treatment to improve the quality of life of patients, especially in elderly patients. Therefore, it is necessary to find effective predictors for these patients and pay more attention to improving postoperative survival. Glucose is the main source of energy for the body. Therefore, blood glucose levels are closely related to physical health, especially in critically ill patients.
Previous studies mostly focused on diabetes, or preoperative glucose. In fact, non-diabetic patients and postoperative glucose are also important. The mechanism of how an elevated postoperative BG level was associated with mortality has not been still fully understood. Thus, the following aspects should be taken into account.
Firstly, body glucose levels are regulated by the endocrine system. Postoperative acute hyperglycemia is regarded as the body's stress response, and stress hyperglycemia complicated by cardiac surgery is clinically common, especially after cardiopulmonary bypass, the incidence rate reaches 95%-100% . Gandhi et al. suggested that when the BG level was greater than 100 mg/dl, the risk of various complications could be increased by 30% for every 20 mg/dl increase. Thoracic surgery is a strong stimulus for the body, and when the variability of patient's BG level is considerable, it will aggravate the damage caused by such stress reaction, that is equivalent to a "second strike" effect .
On the one hand, hyperglycemia is always associated with hyperlactatemia [8, 9], increasing morbidity and mortality in acute critical illness [10, 11]. A previous confirmed that lactate levels might appropriately reflect the severity of disease and organ failure, and also were independently associated with short-term mortality in critically ill patients with liver cirrhosis . Although anaerobic glycolysis increases the substrate, however, suppressing hyperglycemia generating 2,3-diphosphoglycerate, absolute insulin secretion, or relatively insufficient may lead to an increase in plasma free fatty acid concentration, and fatty acid can increase the myocardial oxygen consumption.
On the other hand, BG concentration can influence the function of immune system. The inflammatory response is closely associated with the prognosis of surgery and has been demonstrated in a large number of studies [13, 14]. Studies have shown that inflammation affects wound healing and leads to an increase in postoperative mortality . A variety of inflammatory mediators have also been shown as predictors of postoperative risk . Researches have shown that plasma levels of interleukin 8 (IL-8) and C-reactive protein (CRP) are higher in patients with hyperglycemia than in patients with normal BG levels [17, 18], thereby reducing T cell expression and the body's immune response [19-21]. All the above-mentioned reasons may explain that the immune response may be the cause of acute hyperglycemia, leading to a poor prognosis in patients undergoing cardiac surgery.
This study had some limitations. First, as this was a retrospective analysis based on prospectively collected data, some confounding might have affected the results. Second, the primary endpoint was all-cause mortality except for suicide which reduced the ability to fully evaluate the causes of death and consequently accurately compare the ‘‘cause of death’’ outcomes from this study with those of other populations.