What is the efficacy and safety of q 30 minutes vs. q 1hour glucose sampling and intervention for an intensive insulin protocol to achieve and maintain euglycemia in non-diabetic patients undergoing craniotomy? The investigators hypothesize that in non-diabetic patients undergoing craniotomy, monitoring glucose and modifying insulin infusions every 30 minutes compared to every hour will help them reach target glucose levels faster and maintain them more efficiently with the same insulin protocol.

Intraoperative blood glucose levels will be maintained in the target range of 90-110 mg/dL (euglycemia) using the protocol. The target serum glucose ranges chosen (90-110 mg/dL) was chosen to specifically target those levels associated with the best outcome in cerebral aneurysm surgery (serum glucose < 108 mg/dL) and aggressively avoid those levels associated with cognitive dysfunction (serum glucose < 128 mg/dL) and gross neurologic deficits (serum glucose < 152 mg/dL). Because the strength of these data, the investigators utilize an aggressive insulin protocol in their routine clinical care of patients undergoing craniotomies.

Each patient will be randomly assigned to one of two frequency of intervention groups-Group 30 will have the insulin dose adjusted every 30 minutes and Group 60 will have the insulin dose adjusted every 60 minutes. Blood glucose levels, insulin boluses and infusion rates, and dextrose boluses will be recorded on the respective Intraoperative Insulin & Glucose data sheet (Appendix 2 and 3 for Group 30 and Group 60, respectively).

Regardless of the protocol, glucose measurements will be conducted every 15 minutes but insulin intervention will be done at 30 minutes or one hour (per assigned treatment group). The interval data (15 minutes for the 30 min group and 15, 30, and 45 minutes for the 1 hour group) will be utilized to determine the pharmacodynamic effects of insulin (dose-response relationship). Interventions will only be performed at the assigned interval (30 vs 60 min) unless the glucose level is < 70 mg/dL, at which time the patient will receive 25 mL of 50% Dextrose solution. For these rescued patients, the protocol will be continued as scheduled.

On the day of surgery, after confirming entry into the study (i.e., confirmation of research consent), each patient will be randomly assigned to a specific protocol-q 30 minutes glucose measurements and intervention vs. q 60 minutes glucose measurements and intervention.Randomization will be performed in blocks of 10 using a computer generated random number assignment (odd numbers = q 30 minutes and even numbers = q 60 minutes)

Inclusion Criteria:

• All English speaking, non-diabetic, non-pregnant patients over the age of 18 undergoing open craniotomy for the surgical treatment of tumors or intracranial aneurysms.

Exclusion Criteria:

• Patients under 18 years of age, patients who are pregnant, patients with diabetes, BMI > 33 kg/m2.