Effect of Infusion Rate and Indwelling Time on Tissue Resistance Pressure in Small-Volume Subcutaneous Infusion like in Continuous Subcutaneous Insulin Infusion

Background: To deliver exact volumes of liquid subcutaneously (e.g., during continuous subcutaneous insulin infusion [CSII]), the insulin pump has to overcome not only frictional losses of the mechanical drive and pressure losses in the tubing and infusion set, but also the tissue resistance pressure (TRP). Up to now, detailed information about the dependence of TRP on volumes and delivery rates common for CSII is missing. However, knowledge of the typical range of TRP during CSII is important to optimize occlusion detection and the design of insulin pumps. Materials and Methods: TRP was examined in 24 subjects (12 patients with type 1 diabetes mellitus and long-term CSII therapy and 12 subjects without diabetes) while subcutaneously infusing a liquid test solution via infusion sets with 8-mm steel cannulas using four different infusion rates (infused volume, 0.3 mL of saline solution). The primary objectives were to estimate the TRP and its dependence on the infusion rate, as well as the impact of the cannula indwelling time of roughly 80 h. Results: Stepwise increases in the infusion rate were associated with significant rises in median TRP: 0.01 mL/min (= 1 U/min for U100 insulin), 8.09 mbar; 0.05 mL/min, 18.28 mbar; 0.1 mL/min, 25.18 mbar; and 0.5 mL/min, 62.59 mbar. No statistically significant changes in TRP could be attributed to the catheter indwelling time of roughly 80 h. Conclusions: Median TRP increased significantly with higher infusion rates. Catheter indwelling time had no significant effect on the TRP. Occlusion detection may be improved by using rate-dependent detection thresholds.