Intravenous Infusion of Insulin in Glucose Solution
Another option is to treat type 1 diabetics undergoing surgery by giving an infusion of 5% or 10% glucose solution containing 5, 10, or even 15 units of insulin per liter, depending on the patient’s initial blood glucose concentration. At an infusion rate of 100 mL/h, the insulin is administered at a rate of 0.5, 1, or 1.5 units/h, respectively. In patients receiving corticosteroids, as much as 20 units per liter of insulin may be required.
There are a number of advantages to this regimen. First, the problem of absorption of insulin is avoided, since it is given intravenously. As a result, instead of an average 6-hour lag for maximal response to regular insulin, the effect starts within 10–15 minutes and is relatively constant. Second, unlike the fixed insulin dose with subcutaneous administration, the insulin infusion can be changed at any time in response to changes in blood glucose levels. Third, the dangers of hypo- and hyperglycemia are minimized, because if the intravenous solution is stopped (if the needle is inadvertently removed or the tubing clamped), both the glucose and the insulin are discontinued simultaneously. Since only about 10% of insulin adsorbs to glass or plastic, the resulting reduction in dosage is of little therapeutic importance. A similar continuous intravenous infusion of insulin has also become a common way to treat diabetic ketoacidosis.
Use of Insulin “Piggy-Backed” into the Glucose Infusion
Instead of mixing insulin in the same bottle as the glucose, an insulin solution is infused (“piggy-backed”) into the tubing delivering the 5% or 10% glucose. Generally, 50 units of regular insulin are mixed with 500 mL of normal saline—a solution containing 1 unit of insulin per 10 mL of solution. The glucose solution is given at a rate of 100 mL/h, the insulin infusion is adjusted (usually by IVAC pump) to deliver a total of 5 mL (0.5 units), 10 mL (1 unit), 30 mL (30 units) per hour, etc, depending on the results of blood glucose determinations obtained approximately hourly during the surgical procedure. Of the three techniques, this is the most flexible and allows the closest control of blood glucose levels. It requires careful monitoring of the pump delivery rate, because too rapid infusion of insulin will cause hypoglycemia. A number of simple algorithms have been recommended for adjusting the rate of insulin infusion according to the previous plasma glucose levels. This approach is especially useful during prolonged operations. The simplest and most practical procedure is to give no insulin if plasma glucose is less than 90 mg/dL. Above values of 90 mg/dL, the dosage of regular insulin in units per hour should equal 1% of the previous hour’s plasma glucose (mg/dL)—eg, at a glucose level of 200 mg/dL, give 2 units/h; at 300 mg/dL, give 3 units/h, etc.value exceeds 400 mg/dL.