Original articleThe control of blood glucose in the critical diabetic patient: a neuro-fuzzy method
Introduction
When a critical patient cannot meet his energy requirements, the parenteral administration of nutrients is needed. When the same patient is also diabetic, a good glycemic control is mandatory to avoid energy waste and optimize insulin treatment. The perioperative period is paradigmatic of such clinical situations and several algorithms to adjust insulin infusion rates have been proposed Alberti & Thomas, 1979, Hirsch et al., 1991, Pezzarossa et al., 1988, Watts et al., 1987. These algorithms operate through a feedback regulation Albisser, 1982, Westenskow, 1997 that follows the logic of an open-loop control system and adjust insulin administration rates according to a time schedule of blood glucose (BG) measurements. Since these conventional algorithms do not allow to reach and maintain near normal BG levels without increasing the frequency of BG measurements or the risk of hyper- or hypoglycemic events, target BG values are higher than desirable (between 8 and 11 mmol/l). Although, during the perioperative period, a near optimal BG levels have been advised, there is no clear-cut evidence to substantiate this approach in cases where the patient is allowed to be fed soon after surgery (Shade, 1988). Obviously, when the patient must be submitted to continuous assisted enteral or parenteral nutrition, BG should be maintained at near normal values to allow an efficient utilization of administered nutrients. To improve glycemic control, without increasing the number of BG determinations or the risk of hyper- or hypoglycemic events, we considered the last two BG determinations instead of the last one.
In this way, taking into account the degree and the direction of the glycemic variation, we could more accurately adjust the rate of insulin infusion. The many variables that characterize BG control in a critical diabetic patient make it very difficult to obtain a satisfying glycemic control. Since a mathematical model to manage such a complex system is not available, we appealed to fuzzy logic principles. Fuzzy logic, already used in the medical field Kosko, 1992, Kosko, 1993, allows to capture valuable information that could be lost if considered as distinctly categorized. A neural network (a software that simulates human neural processing capabilities) has been used to apply fuzzy logic theoretical principles and to extrapolate (based on previous knowledge) rules to be used to elaborate a nomogram for insulin infusion rates (neuro-fuzzy control system).
Section snippets
Fuzzy logic modelling
Glucose control can be conceived in terms of engineering control theory. Blood glucose level, the variable to be controlled, is the output of the controlled system, while the insulin infusion rate is the input to be adjusted to reach the desired BG value. In a conventional control system, observed and desired outputs are compared. The input is modified to reach the desired output according to a mathematical model that defines the relationships between the input and the output of the system.
Blood glucose
Mean starting BG levels were similar in the two groups (A: 15.1±0.8 vs. B: 15.8±0.8 mmol/l; P: ns) Fig. 2, Fig. 3. Subsequently, mean BG levels resulted lower when insulin infusion rates were adjusted with the neuro-fuzzy nomogram either (1) during the overall observation time (7.6.1±0.2 vs. 10.1±0.2 mmol/l (P<.00001); or (2) at the single time points (4 h: 11.1±0.6 vs. 14.8±0.8 mmol/l, P<.002; 8 h: 8.4±0.5 vs. 12.4±0.7 mmol/l, P<.001, 12 h: 7.0±0.5 vs. 10.1±0.6 mmol/l, P<.002; 16 h: 6.8±0.4
Sensitivity and effectiveness
The association between changes in BG values (Gt(n+1)−Gt(n)) and the consequent variations in insulin infusion rates (It(n+1)−It(n)) gives an evaluation of the sensitivity of control systems and results stronger in group A (r2=.60, P<.00001) than in group B (r2=.11, P<.00001). The association between the variations in insulin infusion rates (It(n+1)−It(n)) and the consequent variations of BG values (Gt(n+2)−Gt(n+1)) gives an estimate of the effectiveness of the control system and results
Discussion
When a critical diabetic patient is not permitted to eat and glucose and/or other nutrients are intravenously given, intravenous infusion of insulin is the best way to administer insulin Dunnet et al., 1988, Eldridge & Sear, 1996. A diabetic patient submitted to intravenous insulin infusion needs a continuous BG monitoring to control BG levels. Thus, in the past years, some algorithms that work with relatively few BG determinations were implemented to obtain an acceptable compromise between the
Acknowledgements
This research has been supported by 60% funds of MURST. The authors are indebted to Mr. Scott Hartman for his skillful revision of the manuscript.
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