Review
Impact of glycemic variability on cardiovascular outcomes beyond glycated hemoglobin. Evidence and clinical perspectives

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Abstract

Aims

The aim of this review is to focus on intra-day glucose variability (GV), specifically reviewing its correlation with HbA1c, the methods currently available to measure it, and finally the relationship between GV and cardiovascular outcomes, in type 1 and type 2 diabetic patients, and in the non-diabetic population.

Data synthesis

The term GV has been used in the literature to express many different concepts; in the present review, we focus our attention on intra-day GV. In particular, we try to assess whether GV provides additional information on glycemic control beyond HbA1c, since GV seems to be incompletely expressed by HbA1c, particularly in patients with good metabolic control. Many different indexes have been proposed to measure GV, however at the moment no “gold standard” procedure is available. Evidence in vitro, in experimental settings and in animal studies, shows that fluctuating glucose levels display a more deleterious effect than constantly high glucose exposure. However, these findings are not completely reproducible in human settings. Moreover, the relationship between GV and cardiovascular events is still controversial.

Conclusions

The term GV should be reserved to indicate intra-day variability and different indexes of GV should be used, depending on the metabolic profile of the population studied and the specific issue to be investigated. Self glucose monitoring or continuous glucose monitoring should be used for assessing glucose variability.

Introduction

In recent years, several papers and reviews have focused on the possible effects of glucose variability (GV) on cardiovascular diseases, and, in fact, when searching the term “glucose variability” on pubmed, 2891 results are found. However, going through the titles of these papers, it becomes immediately clear that the term “glucose variability” implies many different concepts.

The first concept relates to the day-to-day variability of fasting glucose, the second meaning refers to post-prandial spikes, the third to glycated hemoglobin (HbA1c) variability and, finally, the last concerns the intra-day GV. Further, in this last meaning, two different kinds of measurements are included: self-monitoring blood glucose (SMBG) or continuous glucose monitoring (CGM).

The labyrinth becomes even more intricate, when trying to address the impact of GV on cardiovascular mortality, as recently extensively discussed in the paper by Standl et al. [1], the only evidence being available for day-to-day variability of fasting blood glucose (FBG) [2] and for post-prandial glucose (PPG) [3], [4]. Only two papers investigated the relationship between HbA1c variability and cardiovascular mortality [5], [6]. Lastly, the role of intra-day GV on cardiovascular outcomes is still controversial [7], [8].

It should also be noticed that most of the available data have been collected in type 1 and type 2 diabetic patients, whilst in non-diabetic population, no data on the predictive role of GV on cardiovascular outcomes are available, possibly due to the paucity of data collected by CGM or SMBG in subjects without overt glycemic abnormalities [9].

The aim of this review is to focus on intra-day GV, specifically reviewing its correlation with HbA1c, the methods currently available to measure it, and finally the relationship between GV and cardiovascular outcomes, in type 1 and type 2 diabetic patients, and in non-diabetic population.

Section snippets

Relationship between glucose variability and HbA1c

Since evidence from the Diabetes Control and Complications Trial (DCCT) [10] has linked the reduction of HbA1c to lower incidence and progression of micro-vascular complications, current glycemic management mainly relies on HbA1c measurement. The DCCT investigators observed that ‘total glycemic exposure’ (HbA1c and duration of diabetes) only explained about 11% of the variation in retinopathy risk in the complete DCCT cohort, meaning that factors independent of HbA1c must presumably explain the

Methods to measure glucose variability

Many different indexes have been proposed to assess GV, however, at the moment no “gold standard” procedure is available. The introduction of CGM into clinical practice has resulted in a more accurate assessment of glycemic profile. Recently, Hill et al. [19] have proposed the normal reference ranges for mean blood glucose and GV derived from CGM for subjects without diabetes in different ethnic groups. The most commonly used indices of intra-day GV are hereinafter described.

  • a.

    Standard deviation

Evidence in vitro

Glycemic fluctuations are more deleterious for endothelial cells than constantly high glucose concentrations [30]. Apoptosis is significantly higher in human umbilical vein endothelial cells incubated for 14 days in media containing a daily alternating 5 or 20 mmol/l glucose versus stable high glucose. Moreover, Quagliaro et al. [31] showed that intermittent high glucose levels enhance, in endothelial cells, the formation of nitrotyrosine and 8-hydroxydeoxyguanosine (8-OHdG), markers of

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