Early adverse effect of abnormal glucose metabolism on arterial stiffness in drug naïve hypertensive patients

Study population

The study participants were 1375 subjects with never-treated essential hypertension recruited from the Hypertension and Peripheral Vessels Units of our Department. Office blood pressure (BP) was measured by a mercury sphygmomanometer, taking at least three measurements spaced by 1 min, allowing the patients to rest for 10 min before the examination. According to the guidelines of the European Society of Hypertension, measurements of systolic BP (SBP) ≥140 mm Hg and diastolic BP (DBP) ≥90 mm Hg were considered as systolic and diastolic arterial hypertension, respectively. Pulse Pressure (PP) was calculated as SBP minus DBP and mean arterial pressure (MAP) as DBP+1/3PP.

Hypertensive patients with glucose levels above 100 mg/dl were subject to a 2-h oral glucose tolerance test (OGTT) with 75 g of glucose. IFG was defined as fasting glucose levels between 100 and 125 mg/dl with normal OGTT (2-h postload glucose <140 mg/dl). Hypertensive subjects with 2-h postload glucose between 140 and 199 mg/dl were categorised as subjects with IGT, whereas DM 2 was defined as fasting glucose level ≥126 mg/dl in two different measurements or as 2-h postload glucose ≥200 mg/dl or use of anti-diabetic drugs. Accordingly, hypertensive patients were classified into four groups based on glucose metabolic status (criteria of World Health Organisation and American Diabetes Association 2003): (1) hypertensive patients with normal glucose (NG) regulation (70–99 mg/dl); (2) hypertensive patients with IFG; (3) hypertensive patients with IGT; and (4) hypertensive patients with DM 2.

Hypertensive subjects with NG were further divided into three groups according to glucose levels: (a) glucose concentration <80 mg/dl (N=103); (b) glucose concentration between 80–89 mg/dl (N=307) and (c) glucose concentration between 90–99 mg/dl (N=412).

Participants with coronary artery disease, severe heart valvular disease, cerebrovascular disease, peripheral artery disease and systemic inflammatory diseases were excluded from the study. Use of anti-inflammatory agents, including aspirin, statins and corticosteroids, as well as anticoagulants were also exclusion criteria for the study. Diabetics on insulin therapy were excluded from the study.

Body weight and height were measured in all subjects and body mass index (BMI) was calculated. Waist (W) and hip (H) circumference were measured with a tape and W/H ratio was calculated. Smoking status was assessed by recording the habit and duration of smoking (pack-years).

All participants gave their informed consent to participate in the study, which was approved by the Local Ethics Committee. Antihypertensive treatment started immediately after the completion of the full laboratory and arterial stiffness evaluation.

Arterial stiffness evaluation

Arterial stiffness was evaluated non-invasively by the measurement of carotid-femoral PWV. Carotid-femoral PWV, an established index of aortic stiffness, ⁴ was obtained using a validated non-invasive device (Complior^®, Artech Medical, Pantin, France), which allows online pulse wave recording and automatic calculation of PWV. Two different pressure-waveforms were obtained simultaneously at two sites: the right carotid and femoral artery. The time delay (t) between the feet of the recorded proximal and distal waves was automatically calculated. For the carotid-femoral segment, the distance (d) was estimated by subtracting the distance between carotid location and sternal notch from the distance between sternal notch and femoral site. The mean PWV of at least 10 consecutive pressure waveforms was calculated for further analysis.

All measurements were conducted in the morning between 9 and 11 am, with the individual in a supine position, in a quiet environment. Participants were requested to abstain from tobacco, coffee and food at least 5 h and from alcohol 12 h before the examination.

Laboratory measurements

Blood samples were collected between 8 and 9 am with individuals in the supine position after an overnight fast, centrifuged at 3000 g for 30 min at 3°C and stored at -70°C. High-sensitivity C-reactive protein (hsCRP) and fibrinogen were measured by immunonephelometry (Dade Behring, Marburg, Germany). The intra- and inter-assay coefficient of variation (CV) for hsCRP was <4.4% and <5.7%, respectively, and for fibrinogen was 2.7 and 2.6, respectively. Homocysteine and fasting insulin levels were also measured with enzyme immunoassay (ELISA, Abbott, Tokyo, Japan), and insulin resistance was estimated by applying the homeostasis model assessment (HOMA-IR). Glycated haemoglobin (HbA1c) was measured in all subjects by column chromatography. Biochemical variables such as plasma glucose, triglycerides, total cholesterol and high-density lipoprotein (HDL) cholesterol were measured using standard techniques.

Statistical analysis

All continuous variables are presented as mean value ± 1 standard deviation.

Difference in baseline characteristics and arterial stiffness across the four metabolic groups was studied using analysis of variance (ANOVA). The difference in PWV values between groups was further evaluated after adjustment for age, gender, smoking, BMI, total cholesterol, heart rate and MAP with post-hoc analysis. Similar analyses were performed for the subgroups of normal glucose regulation. Pearson correlation was used for the association of baseline characteristics with PWV. HsCRP was log-transformed due to its skewed distribution.

Stepwise linear regression analysis was applied to test the independent association of glucose metabolism with PWV (dependent variable) after adjustment for several confounders, including age, gender, smoking, BMI, W/H ratio, total cholesterol, hsCRP, heart rate and MAP; in order to assess the association of different descriptors of glucose metabolism with PWV, we constructed four different models where the main independent variable was glucose metabolism group (as categorical variable) or glucose concentration (as continuous variable) or insulin resistance (by HOMA) or HbA1c, respectively. Exact p values <0.05 were considered as statistically significant. Data analysis was performed with SPSS software, version 10.0 (Chicago, IL, USA).

Arterial stiffness and glucose metabolism

Many pathophysiological mechanisms might explain the results of our study. The constantly increased insulin concentration in conditions of abnormal glucose metabolism might contribute to the gradual increase in arterial stiffness. In healthy individuals, insulin amplifies endothelium-dependent vasodilation through increase in nitric oxide (NO) availability. ¹⁴ Moreover, experimental data on healthy subjects showed that insulin infusion diminishes wave reflections via arterial vasodilation. ¹⁵ However, the favourable effect of insulin on the vasculature may be abrogated in insulin resistance states, in which high circulating insulin levels are thought to enhance smooth muscle cell proliferation and endothelial dysfunction, ^16,17 thus accelerating the stiffening within the arterial wall. Although insulin resistance is often associated with abnormal lipid metabolism and hypertension, and one might argue that those two traditional cardiovascular risk factors may explain the increase in arterial stiffness associated with insulin resistance, the relationship of insulin with PWV in the present study was independent of those two potential confounders.

Another possible mechanism that may partly contribute to the increased arterial stiffness as glucose metabolism deteriorates is inflammation. The increase in the levels of all inflammatory markers measured in our study, for example hsCRP, fibrinogen and homocysteine followed the same trend as that of glucose levels across the four groups. Many studies, both in healthy individuals ¹⁸ and hypertensive subjects ¹⁹ have demonstrated an independent positive association of hsCRP with PWV, and an aetiological, through a cause-and-effect, relationship between inflammation and PWV has also been proposed. ²⁰ Pathophysiologically, hsCRP may unfavourably affect arterial function since it acts on smooth muscle cells by upregulating angiotensin type I receptor, stimulating the migration and proliferation of smooth muscle cells and enhancing the production of reactive oxygen species. ²¹ Nevertheless, in the present study, the relationship of glucose status with PWV was independent of hsCRP, a finding that supports the notion that inflammation may have a less pivotal role than hyperglycaemia per se on impaired arterial function. Future studies are needed to confirm this observation.

Clinical implications – limitations

The adverse prognostic role of arterial stiffness has been demonstrated by several longitudinal studies. ²² A stiff aorta increases left ventricular afterload, induces left ventricular hypertrophy and disturbs diastolic coronary perfusion, thus unbalancing coronary perfusion/myocardial demand equilibrium and enhancing cardiovascular disease. Given that the gradually deteriorating glucose metabolism results in gradual increase of arterial stiffness in hypertensive subjects, aims should be towards closer monitoring of glucose levels in this population. Whether intensive glucose treatment would benefit hypertensive patients with IFG or IGT is largely unknown. Similarly, as recent data in diabetic patients have shown, the optimum glucose level that should be achieved in order to prevent cardiovascular events is still unclear. ^3,23

The present study is a cross-sectional study, and thus no causal relationships between arterial stiffness and abnormal glucose metabolism can be inferred. Moreover, the lack of healthy controls might be considered as a possible limitation of the study.