Page 305 - Glucose Monitoring Devices

312 CHAPTER 15 Automated closed-loop insulin delivery

FIGURE 15.2
The glycemic risk index for adapting the penalty weight on the tracking error based on the
CGM measurement value.

set-point tracking error Q k ¼ QðGRI k Þ, with GRI k as the glycemic risk index value
at sampling instance k for a given glucose measurement reading. Manipulating the
penalty weights can thus inform the MPC of changing priorities and improve
glycemic control.

Quantifying plasma insulin concentrations
AP systems require a safety constraint to moderate the potential aggressive control
actions (insulin overdosing) and minimize the risk of hypoglycemia for significantly
improving the performance of the AP. Estimating the amount of available insulin in
the body is challenging because of the inter- and intrasubject variability attributed to
physiological differences and metabolic changes throughout the course of the day.
Despite the lack of direct measurement, maintaining the BGC in the target range
requires AP systems that are cognizant of the quantity of insulin previously admin-
istrated, which if not appropriately incorporated into the control algorithm may
cause overcorrection for the postprandial rise in BGC. Such excessive dosing in
either the bolus or basal insulin administered through CSII pumps can potentially
lead to hypoglycemia. Hence, in addition to the current and target BGC, a constraint
expressing an approximation of the insulin present in the body, such as the conven-
tional IOB estimates, is needed for insulin-dosing calculations.
The IOB is an estimate of the amount of insulin that is present in the blood and
the interstitial fluid cavity. It is typically determined through the approximation
of the insulin decay curves, which represent the amount of insulin still remaining
in the body due to the prior insulin infusions. Static approximations of the insulin
action curves are typically utilized in insulin pumps, with IOB calculations primar-
ily relying on basic insulin decay profiles. Furthermore, significant time-varying
delays induced by the absorption and utilization of the subcutaneously adminis-
trated insulin as well as diurnal variations in the metabolic state of individuals