Continuing at least part of the basal insulin is the reasonable, physiologic approach to controlling glucose levels before surgery in patients with diabetes. The process involves three basic steps:
- Ascertaining the type of diabetes
- Adjusting the basal insulin dosage
- Stopping the prandial insulin.
The steps are the same whether the surgery is major or minor. These recommendations are based on general principles of insulin action, data from large databases of surgical inpatients, and expert clinical experience translated into standardized protocols.1,2
WHY CONTINUE THE INSULIN?
Stopping or decreasing insulin because of a fear of hypoglycemia is not appropriate, as the resulting hyperglycemia can lead to delayed wound healing, wound infection, fluid and electrolyte shifts, diabetic ketoacidosis, and hyperosmolar states.
Insulin inhibits both gluconeogenesis and conversion of glycogen to glucose, processes that occur regardless of food intake. It also inhibits degradation of fats to fatty acids and of fatty acids to ketones. This is why inadequate insulin dosing can lead to uncontrolled hyperglycemia and even ketoacidosis, and thus why long-acting insulin is needed in a fasting state.
STEP 1: ASCERTAIN THE TYPE OF DIABETES
Does the patient have type 1 or type 2 diabetes, and does that even matter?
The type of diabetes should not matter, since ideally the insulin should be dosed the same for both types. However, the consequences of inappropriate insulin management may be different.
Usually, the type of diabetes can be ascertained by the history. If the patient was diagnosed at age 40 or later and was on oral medication for years before insulin was started, then he or she most likely has type 2. If the patient was younger than 40 at the time of diagnosis, was lean, and was started on insulin within a year of diagnosis, then he or she likely has type 1.
If this information is not available or is unreliable and the patient has been on insulin for many years, we recommend viewing the patient as being insulinopenic, ie, not producing enough insulin endogenously and thus requiring insulin at all times.
Though checking for antibody markers of type 1 diabetes might give a more definitive answer, it is not practical before surgery.
In the setting of surgical stress, withholding the basal insulin preoperatively and just giving a small dose of fast-acting (see Table 1 for the different classes of insulin) or shortacting insulin as part of a sliding scale (ie, insulin given only when the blood glucose reaches a certain high level) can send a patient with type 1 diabetes into diabetic ketoacidosis by the end of the day. This is less likely to occur in a patient with type 2 diabetes with some endogenous insulin secretion.
STEP 2: ADJUST THE BASAL INSULIN
Basal insulin is the insulin that the healthy person’s body produces when fasting. For a diabetic patient already on insulin, basal insulin is insulin injected to prevent ketogenesis, glycogenolysis, and gluconeogenesis in the fasting state.
If the basal insulin is long-acting
Long-acting insulins have a relatively peakless profile and, when properly dosed, should not result in hypoglycemia when a patient is fasting.
Preoperatively, the patient should take it as close as possible to the usual time of injection. This could be at home either at bedtime the night before surgery or the morning of surgery. If there is concern for hypoglycemia, the injection can be given when the patient is at the hospital.
- If the patient does not tend to have hypoglycemic episodes and the total daily basal insulin dose is roughly the same as the total daily mealtime (prandial) dose (eg, 50% basal, 50% prandial ratio), the full dose of basal insulin can be given.3
Example: If the patient is on insulin glargine 30 U at bedtime and insulin lispro 10 U with each meal and does not have hypoglycemic episodes, then insulin glargine 30 U should be taken at bedtime.
- If the patient has hypoglycemic episodes at home, then the basal insulin can be reduced by 25%.3
Example: If the patient is on insulin glargine 30 U at bedtime and insulin glulisine 10 U with each meal (appropriate proportion of doses, similar to the example above) but has hypoglycemic episodes at home on this regimen, then only 22 U of insulin glargine should be taken at bedtime.
- If the patient’s regimen has disproportionately more basal insulin than mealtime insulin, then the total daily doses can be added and half can be given as the basal insulin.
Example: If the patient is on insulin detemir 30 U every morning at 6 am and insulin aspart 6 U with each meal and has no hypoglycemic episodes, then 24 U of insulin detemir should be taken in the morning (ie, half of the total of 30 + 6 + 6 + 6).
- In the less common scenario of diabetes managed only with basal insulin (no other diabetes injections or oral agents), then half of the dose can be given.
- If the patient is on twice-daily long-acting basal insulin, then both the dose the night before surgery and the dose the morning of surgery should be adjusted.