Open-Loop Intravenous Insulin Waveforms for Postprandial Exercise in Type I Diabetes

To obtain optimal metabolic control in diabetic individuals receiving open-loop insulin infusion, waveforms appropriate for exercise must be developed. The present study describes the responses of four nonobese type I diabetic individuals to a specific insulin waveform derived for postprandial exercise during intravenous open-loop insulin infusions. The open-loop system consisted of a silastic rubber reservoir, a programmable flow-rate controller, and a peristaltic pump connected by a silicone rubber catheter inserted into abranch of the external jugular vein. The basal insulin infusion rate (15 ± 0.5 mU/min) was adjusted to achieve fasting euglycemia (87 ± 7 mg/dl), and a three-step insulin infusion waveform to normalize the glycemic response to breakfast (peak glucose 127 ± 12 mg/dl at 45 min) was established. The three-step (each defined as S1, S2, S3) insulin waveform consisted of: S1, 30 ± 4 mU/min for 15 ± 3.5 min; S2, 58.7 ± 6.6 mU/min for 60 ± 4 min; S3, 33.7 ± 2.5 mU/min for 65 ± 12 min. When 45 min of moderate exercise (65 ± 6 W) wasperformed 30 min after the start of breakfast, symptomatic hypoglycemia occurred in all subjects (lowest mean glucose 49 ± 4 mg/dl) at varying times (65, 115, 180, and 180 min). The meal insulin waveform was then modified so that the infusion rate returnedto the premeal basal rate with the onset and for the duration of exercise, while at the conclusion of exercise, the remaining meal waveform was reinitiated. Under these circumstances, hypoglycemia was prevented and the meal glycemic excursion was entirely normalized.”Free“ immunoreactive insulin levels were significantly lower (P < 0.05) during exercise, consistent with the reduction in the infusion rate to basal with the modified meal insulin waveform. This insulin infusion waveform appears appropriate to normalize the glycemic response to postprandial exercise with open-loop intravenous insulin replacement.