In the direct preoperative period, patients with diabetes type 1 should follow their usual regimen, while patients with type 2 diabetes should be bridged to intensive insulin therapy (with the exception of patients successfully treated with diet together with metformin and on condition of minor procedures, such as tooth extraction, abscess incision, small amputation, cataract surgery). Oral hypoglycaemic agents (OHA) should be withdrawn 48 h before the surgery. Total daily intake (TDI) of insulin should be equal to 0.3–0.7 IU/kg. Long-acting insulin is expected to cover 40–50 % of daily dose (NPH injected twice daily at 8:00 a.m. and 10:00 p.m. or a single injection of long-acting analogue before sleep). Pre-prandial rapid-acting insulin is recommended to be given 3 times daily before meals according to proportions of 50–20–30 and should represent approximately 50–60 % of TDI [25].

The American Association of Clinical Endocrinologists and the American Diabetes Association 2009 consensus recommends that in the intensive care setting, target glucose level should be ≤180 mg/dL (10 mmol/L) and that glycaemia should be maintained in the range between 140 and 180 mg/dL (7.8–10 mmol/l). For surgical patients, a pre-prandial glucose concentration <140 mg/dL (7.8 mmol/L) and a random glucose concentration <180 mg/dL (10 mmol/L) are recommended [25]. The Society for Ambulatory Anesthesia Consensus Statement advocates to maintain intraoperative blood glucose levels between 100 and 180 mg/dL (5.5–10 mmol/L) [28]. The American College of Physicians 2014 updated guidelines for the management of inpatient hyperglycaemia recommend a target blood glucose level of 140–200 mg/dL (7.8–11.1 mmol/l) when insulin therapy is used in medical or surgical intensive care unit patients. Clinicians should avoid targets less than 140 mg/dL (<7.8 mmol/L) because harming risk increases with lower blood glucose targets. Moreover, they strongly recommend not using intensive insulin therapy to normalise blood glucose in patients with or without diabetes [29]. The Society of Thoracic Surgeons 2009 guidelines regarding blood glucose management in cardiac surgery recommend maintenance of blood glucose lower than 180 mg/dL (10 mmol/L) [30]. In patients who spend ≥3 days in ICU, require an intra-aortic balloon pump/inotropic/left ventricular assist device support, receive antiarrhythmic drugs or are on dialysis/continuous veno-venous hemofiltration, a blood glucose level of ≤150 mg/dL (8.3 mmol/L) is recommended [30].

Wilson et al. [31] reviewed and described 12 different insulin infusion protocols and found significant variations in initiation and titration of insulin, use of bolus dosing and calculations used for insulin dose adjustment. In clinical setting, however, two major well-recognised intraoperative algorithms of blood glucose control exist. The first algorithm is based on intravenous pump infusion of 50 IU of insulin dissolved in 50 mL 0.9 % saline and a separate infusion of 10 % glucose. In this protocol, 1 g of exogenous glucose is used every 0.3 IU of insulin. The rate of both simultaneous infusions is adjusted according to actual blood glucose level (Table 12.2). The second scheme is based on a single infusion drip with 500 mL of 5–10 % glucose containing approximately 8–16 IU of insulin and 10–20 mEq of potassium chloride administered at the rate of 80 mL/h. The amount of insulin in the solution should be higher (>20 IU) in case of obesity, cardiothoracic surgery, concomitant infection, hypothermia or initial glucose concentration >180 mg/dL. Conversely, the contents of insulin should be less than 12 IU in patients with low body mass index and previously treated with OHA. The amount of insulin in the solution should be increased by 2 IU for every 30 mg/dL increase of blood glucose above the threshold of 180 mg/dL and decreased by 4 IU if the blood glucose level falls to 100 mg/dL.

During intravenous administration of insulin, blood glucose level should be measured every 1 h using bedside or nearby stat laboratory monitoring. Of note, point-of-care testing can be disputed in the situation of hypoglycaemia, when it tends to overestimate blood glucose level [32]. Accordingly, higher alert value for hypoglycaemia (e.g. <70 mg/dL) should be implemented to trigger early glucose supplementation so as to allow time for prevention of symptomatic hypoglycaemia, which usually occurs at blood glucose levels of 45–55 mg/dL [33].

Insulin promotes intracellular potassium shift, possibly leading to hypokalaemia. Since perioperative IV insulin administration has a rapid onset of action, glucose and potassium levels must be strictly monitored.

Excessive doses of insulin can cause symptomatic hypoglycaemia (blood glucose level <45–55 mg/dL) manifested by sweating, tachycardia, mydriasis, pallor, weakness, nausea, confusion, aggressive behaviour, seizures, loss of consciousness, convulsions, brain damage and demise. Yet, this symptomatology is absent in patients under general anaesthesia, barring tachycardia and excessive sweating. This supports the need for hourly glucose monitoring.

Other side effects of insulin therapy include allergic reactions, lipodystrophy and weight gain.