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To evaluate the incidence and investigate determinants of insulin underdelivery events occurring with implanted pumps using peritoneal route from a 103 patient-year experiment.Of the MiniMed (MIP 2001) pumps implanted in 47 type I diabetic patients, 70 were refilled quarterly with successive batches (A, B, C, D) of U400 Hoechst 21 PH neutral insulin during a 3-year study period. Any reduction of insulin flow rate greater than 15 percent was considered as abnormal insulin delivery. Diagnosis of the cause of underdelivery was established according to the response to the following steps: 1) 0.01 mol/l NaOH rinse of pump circuits to solubilize insulin aggregates, 2) surgical examination and replacement of blocked catheters, and 3) postsurgical 0.01 mol/l NaOH rinse of pump. Step 2 was selected first if the increase of insulin requirements or reduction of flow rate were greater than 50 percent. Relative contributions of insulin and the implanted system to underlying events were analyzed.There were 76 episodes of insulin underlying that occurred during the study, resulting in an incidence of 74 events per 100 patient-years. Of 52 NaOH pump rinses, 30 restored normal flow rate. Surgery, performed after rinse failure (n = 22) or as the first step (n = 24), disclosed catheter blockages due to tip obstructions in 28 cases and omental encapsulations in 18 other cases. Five combined severe reductions of pump flow rate requiring pump replacements were diagnosed during surgery, and additional NaOH rinses had to be performed after catheter change in 12 other cases. Analysis of the incidence of underdelivery events indicated that both pump- and catheter-related problems were significantly increased while implanted systems infused batches B, C, and D versus batch A (P less than 0.01) whereas the duration of pump implantation had no significant influence.Underdelivery events constitute serious limiting obstacles to prolonged peritoneal insulin infusion from implanted pumps. Progress in insulin pump compatibility is expected to reduce their occurrence and, thus, to improve the feasibility of this treatment.