This review and meta-analysis provides rationale for the use of oral carbohydrate loading as preoperative fasting leads to metabolic stress and causes insulin resistance in patients undergoing cardiac surgery.
Preoperative fasting, defined as no solid food six hours prior to surgery and no clear liquids two hours prior to surgery, is the standard approach in elective surgery aimed at reducing the risk of aspiration during induction of anesthesia and intubation. On the other hand, evidence has shown that fasting not only contributes to catabolic state of stress response related to surgery, but also causes gastrointestinal (GI) problems after surgery and may lead to postoperative delirium and cognitive dysfunction. It is noteworthy that monitoring of GI function is very challenging in the ICU. Fasting times are often exceeded due to organizational issues; therefore, limiting the time without oral intake of food or liquids is of major importance in order to improve postoperative outcome and patient satisfaction.
Major cardiac surgery causes metabolic stress and insulin resistance that can be exacerbated by preoperative fasting . Insulin resistance may lead to hyperglycemia and decreased tissue responsiveness to the biological activity of insulin, a metabolic problem that induces catabolic state and may lead to increased morbidity, prolonged hospital and intensive care unit (ICU) stay and decreased survival . Stress hyperglycemia, even in non-diabetic patients, is a marker of stress response in critically ill patients and results from a release of contra-insulin hormones (i.e., glucocorticoids and catecholamines) . Hyperglycemia after cardiac surgery can be detrimental to the heart due to glucose toxicity that causes increased oxidative stress via the hexosamine metabolic pathway and by elevated levels of advanced glycation end-products .
One of the measures to improve post-operative outcome is oral carbohydrate loading (OCH) treatment, initiated to optimize the nutritional status of the patient prior to elective surgery as part of the Enhanced Recovery After Surgery (ERAS) pathway . ERAS is a multimodal, multidisciplinary initiative to improve perioperative care with the effect of substantial improvements in clinical outcomes and cost savings . It is especially relevant to cardiac surgery and includes issues related to human nutrition and metabolism during the preoperative preparation (fasting, preoperative carbohydrate treatment), intraoperative management (blood glucose monitoring and treatment) and postoperative approach (treatment of nausea and vomiting, early nutrition and gastrointestinal stimulation) .
Many studies have evaluated the effect of preoperative use of an oral drink of simple and/or other digestible carbohydrates on clinical and metabolic outcomes. Their results have shown that OCH decreases postoperative insulin resistance and improves glucose kinetics , facilitates return of bowel function , preserves skeletal muscle mass , modifies hormonal and metabolic response , prevents surgery-induced immunodepression , decreases surgical site infections , improves patients’ satisfaction  and lowers the total number of complications and the length of hospitalization time . It is important to limit the OCH use for patients without contraindications, including known gastroesophageal reflux disease, disorders of gastric motility or diabetes associated with diabetic gastroparesis .
Many studies regarding OCH have been performed involving a wide range of outcomes, but their quality is unsatisfactory, therefore the evidence that preoperative carbohydrate treatment reduces major endpoints—hospital stay and mortality—is still lacking. Despite the high importance of the problem, a meta-analysis of studies concentrating on the effects of preoperative carbohydrate treatment on clinical and metabolic endpoints in cardiac surgery has not been performed.
To our knowledge, this study is the first systematic review and meta-analysis of RCTs investigating the effect of oral carbohydrate treatment on outcome in patients undergoing cardiac surgery. This meta-analysis included 9 moderate randomized controlled trials, comprising 11 interventions, with a total number of 507 patients. The results show that oral preoperative carbohydrate treatment in patients undergoing elective cardiac surgery demonstrated a significant 20% reduction in the use of inotropic drugs, nearly 50% reduction of the length of ICU stay, a 28% decrease in the aortic clamping duration time and a 35% decrease of the postoperative insulin requirement in the cardiac ICU. The interventions differed between studies, yet preoperative OCH was safe (no occurrence of drink-related complications), associated with reduced development of postoperative insulin resistance, but the latter was not associated with any effect on surgical complications.
This meta-analysis has some limitations. The main limitation of this meta-analysis is the quality of included studies graded as moderate. The patient populations were small and significant heterogeneity in the study design occurred, such as inclusion and exclusion criteria, timing of randomization and the type and timing of the intervention. These limitations resulted in a small number of studies eligible for inclusion in this meta-analysis.
As the quality of evidence presented in this meta-analysis is moderate, it is obvious that further well-designed randomized trials including homogenous groups of patients are necessary to specifically examine the biochemical and clinical effects of preoperative carbohydrate loading in cardiac surgery. Further studies should be performed in patients with sufficiently long postoperative length of stay in the ICU and in the hospital.