Morning glucose correlates poorly with daily mean glucose in cardiac surgical ICU patients: impact of ultradian variation of glucose

Introduction

Treatment of hyperglycemia with an insulin infusion protocol (IIP) has improved outcomes in ICU and cardiac surgery patients [1]. Many studies have reported morning glucose values as representative of whole-day glucose control. We recently demonstrated that in a mixed ICU population this assumption does not hold [2]. Since it has been proposed that glucose control may be especially important in cardiac surgery ICU patients, we asked whether morning glucose accurately represented whole-day glucose and determined whether glucose varied over the course of the day in this patient group.

Methods

A prospective, observational single-center study in the cardiac surgical ICU of a university tertiary-care hospital. All glucose measurements in cardiac surgical ICU patients receiving an IIP targeting blood glucose 80 to 110 mg/dl were recorded from 1 June 2006 to 28 February 2007.

Results

We recorded 12,109 glucose measurements from 125 patients on IIPs. The glucose measurements were widely distributed with 1%, 7%, 47%, 33%, 9% and 2% in the ranges <60, 61 to 79, 80 to 110, 111 to 150, 151 to 200 and >200 mg/dl, respectively. After 8 hours of IIP, the proportion of values in the target range increased to 49%. The 06:00-hour glucose values were lower than other values (mean ± SD: 105 ± 24 mg/dl (n = 482) vs. 113 ± 33 mg/dl (n = 10,688); P < 0.0001). The 06:00 values were poorly correlated with the average glucose recorded for the remainder of the day (r² = 0.029), which was confirmed by Bland–Altman analysis. The time-averaged glucose data exhibited ultradian variation, peaking at 10:00 and 19:00 hours. The insulin dose varied with a similar pattern.

Conclusion

In cardiothoracic ICU patients receiving an IIP, glucose exhibited an ultradian pattern with peaks at 10:00 and 19:00 hours, and was lower in the early morning than during the remainder of the day. Consideration of this ultradian variation may avoid hypoglycemic and hyperglycemic episodes and so facilitate better glucose control with an IIP. Studies targeting control of hyperglycemia should report mean blood glucose values for the entire day rather than early morning values.