Page 135 - Glucose Monitoring Devices

136 CHAPTER 7 Clinical impact of CGM use

The turn of the 21st century saw the first clinically approved continuous glucose
monitoring (CGM) device using a subcutaneously sited sensor to measure changes
in interstitial fluid glucose concentrations. Glucose levels are derived from propor-
tionally produced electric currents and are automatically measured and accessible to
users at 5-min intervals. The evolution of CGM over the last decade has seen devices
become smaller with fewer required calibrations and longer sensor change intervals.
The addition of mobile Bluetooth technology has also allowed wireless data transfer
and integration of smartphone technology to display results on purpose-designed
applications. This chapter will address the current and future benefits, limitations,
and clinical applications of CGM technology.

Parameters of glucose control and risk association
HbA1c
The association between prolonged exposure to hyperglycemia and increased risk of
diabetes-associated micro- and macrovascular is well documented [1e4]. Signifi-
cant outcomes include the fourfold increased risk of coronary heart disease [5]
and cardiovascular disease being responsible for a quarter of diabetes deaths [6].
The Diabetes Control and Complications Trial (DCCT) and the United Kingdom
Prospective Diabetes Study revealed a reduced risk of microvascular complications
in participants with type 1 (T1DM) and type 2 (T2DM) diabetes following intensive
glycemic control, respectively [1,2]. These benefits extended long beyond the dura-
tion of the trial “legacy effect” with lower rates of microvascular disease persisting
despite HbA1c rebounding to levels seen in the standard care groups [7e10].
Furthermore, the curvilinear relationship between HbA1c and vascular risk suggests
that the greatest risk reduction is achieved when glycemic control is successfully
improved from hyper- to normoglycemia with minimal benefit to be gained from
further glucose reduction [3,11]. HbA1c levels above 6%e7% have been shown
to significantly increase microvascular and cardiovascular risk in diabetes popula-
tions. However, many trials including ACCORD and VADT have failed to identify
significant cardiovascular risk reduction with intensive glucose control and, in fact,
highlight the importance of balancing glucose reduction with the increased risk of
hypoglycemia.

Hypoglycemia
Despite pharmaceutical advances in the preparation and delivery of insulin
replacement, hypoglycemia remains a common complication among insulin-
requiring individuals with diabetes. Errors in mismatching insulin dose to ingested
carbohydrates and the nonphysiological pharmacokinetics and pharmacodynamics
of exogenous insulin are among various factors responsible for iatrogenic hypogly-
cemia. As established in the DCCT, hypoglycemia risk is further increased in
attempts to diminish vascular risk by implementing intensive insulin therapy.