The artificial endocrine pancreas was adapted to assist with the intraoperative localization of an insulin-secreting islet cell adenoma in a 23-year-old patient with a five-month history of hypoglycemic attacks. Glycemia was monitored in continuously withdrawn whole blood, and dextrose was infused to maintain euglycemia by an artificial endocrine pancreas with a closed loop that excluded its usual insulin delivery capability. The dextrose infusion rate was established in accordance with a control algorithm whose parameters were chosen to amplify changes in dextrose delivery rate for small alterations in the measured baseline glucose concentration. The dextrose infusion rate preoperatively was 155 mg. per minute and decreased to 100 mg. per minute with initiation of surgery. An area in the tail of the pancreas suspected of containing the insulinoma was excluded from the circulation by a noncrushing clamp. After 14 minutes the dextrose infusion progressively decreased to 27 mg. per minute reflecting a glycemic rise of 15 mg. per deciliter. These changes were taken to represent a fall in ambient insulin activity. This was subsequently confirmed directly by the demonstration of reduction in immunoreactive insulin and progressive increase in both plasma free fatty acid levels and postoperative glucose intolerance. Exclusion of the insulinoma from the circulation resulted also in a rapid decrease of circulating proinsulin concentration giving an estimated half-life of 25 minutes.

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