Subjects

All patients with diabetes and prediabetes prospectively enrolled between November 2010 and February 2011 were admitted to diabetes and cardiology outpatient clinics. Categories of abnormal glucose metabolism were determined according to the 2003 guidelines of the American Diabetes Association. Prediabetic patients were classified into two groups on the basis of OGTT results: those with i-IFG and those with combined IFG and IGT (IFG+IGT).

The study population consisted of four groups: 48 patients with i-IFG, 44 patients with IFG+IGT, 48 patients with type 2 DM, and 60 subjects as age-matched and gender-matched controls.

CAD was excluded by negative results on dobutamine stress echocardiography, treadmill exercise tests, and coronary angiography.

At the beginning of the study, all prediabetic and diabetic patients were subjected to both dobutamine stress echocardiography and treadmill exercise tests. Coronary angiography was carried out when the results of one or both of the tests were positive and/or when patients described angina pectoris. As a result, 100 patients (30 from the DM group, 36 from the i-IFG group, and 34 from the IFG+IGT group) had negative result on both dobutamine stress echocardiography and treadmill exercise tests, and 40 patients (18 from the DM group, 12 from the i-IFG group, and 10 from the IFG+IGT group) had normal results on coronary angiography. In addition, on 24-hour Holter monitoring, no subjects showed any signs of myocardial ischemia.

On the basis of repeated blood pressure measurements, all patients and control subjects had normal arterial blood pressure (≤130/85 mm Hg or less) and were receiving no blood pressure–lowering medications. All were in sinus rhythm, with normal results on 12-lead electrocardiography, without signs of hypertrophy, bundle branch block, or pathologic Q waves. All had normal transthoracic echocardiographic results. Control subjects had normal results on physical examination; had no chronic cardiovascular illnesses, including hypertension and diabetes; had no symptoms or histories suggestive of cardiovascular disease; had normal results on dobutamine stress echocardiography and treadmill exercise tests; and were considered to have a low probability of coronary disease on clinical grounds.

Patients were excluded if they had diabetic complications such as retinopathy, clinical neuropathy, or macroalbuminuria; positive results for myocardial ischemia by dobutamine stress echocardiography or treadmill exercise testing; significant CAD (quantitative coronary angiographic stenosis diameter > 40%); regional wall motion abnormalities or impaired ejection fractions (<55%) by resting echocardiography; poor-quality echocardiographic imaging; valvular or congenital heart disease; previous surgical or percutaneous revascularization; or endocrine and systemic diseases other than DM. All other eligible patients were studied.

The study was conducted in accordance with the Helsinki II Declaration and approved by the local ethics committee. All participants provided written informed consent before entering the study.

Blood for biochemical analysis was obtained from fasting venous samples. Glucose, glycosylated hemoglobin (HbA _1c ), total cholesterol, high-density lipoprotein, and triglycerides were determined using standard laboratory techniques. Standard 75-g OGTTs were performed after an overnight fasting period of 10 hours.

Classification of IFG and IGT categories was made on the basis of American Diabetes Association criteria as follows: (1) for i-IFG, FPG of 100 to 125 mg/dL (5.6–6.9 mmol/L) and 2-hour postprandial plasma glucose <140 mg/dL (17.8 mmol/L); (2) for IFG+IGT, FPG of 100 to 125 mg/dL (5.6–6.9 mmol/L) and 2-hour postprandial plasma glucose of 140 to 199 mg/dL (7.8–11 mmol/L); and (3) for diabetes, FPG ≥ 126 mg/dL (7 mmol/L) and/or 2-hour postprandial plasma glucose ≥ 200 mg/dL (11.1 mmol/L).

Echocardiography

All subjects were examined in the left lateral decubitus position, using a System Five machine (GE Vingmed Ultrasound AS, Horten, Norway) with a 2.5-MHz multifrequency transducer. Electrocardiograms were recorded simultaneously. Echocardiographic data were taken during passively held end-expiration, which contained a minimum of three consecutive beats and were stored digitally. All measurements were analyzed by an experienced echocardiographer without knowledge of the clinical data.

Standard echocardiographic assessment included M-mode, cross-sectional, and Doppler blood flow measurements. LV volumes were calculated using the modified Simpson’s biplane method of disks, and LV mass was calculated using the Devereux-modified American Society of Echocardiography cube formula. LV volumes and LV mass were indexed for body surface area to obtain the LV end-diastolic volume index, LV end-systolic volume index, and LV mass index. LV ejection fraction was estimated using a modified Simpson’s biplane method. LV fractional shortening was calculated using a standard formula. Pulsed-wave Doppler of transmitral flow was carried out in the apical four-chamber view. The sample volume was placed at the tips of the mitral leaflets, and peak early diastolic mitral flow velocity (E), peak late diastolic mitral flow velocity (A), and deceleration time of early diastolic mitral flow were recorded. The E/A ratio was calculated.

Doppler tissue imaging was obtained from four-chamber and two-chamber apical views and the apical long-axis view during end-expiration apnea. Color tissue Doppler was superimposed on two-dimensional images. The width of the image sector and the depth of the imaging were adjusted to achieve a color Doppler frame scanning rate > 140 frames/sec. Pulse repetition frequency was set to the lowest possible value without aliasing. If found, increasing the pulse repetition frequency (typically between 1 and 2 kHz) eliminated the aliasing. The sampling window was positioned as parallel as possible to myocardial segment of interest to ensure the optimal angle of imaging. Myocardial tissue velocity, strain rate, and strain analysis were performed offline using customized computer software (EchoPAC version 6.3; GE Vingmed Ultrasound AS). Cine loops containing three consecutive cardiac cycles were analyzed, and all measurements were taken as the mean of the three consecutive cycles.

Myocardial tissue velocities were measured in the basal segments of the lateral, septal, anterior, inferior, posterior, and anteroseptal LV walls. Peak systolic velocity during ejection and the peak diastolic velocities during early filling from the waveforms were obtained. The average of these four basal velocities was used to calculate mean basal peak systolic velocity (Sm) and mean basal peak early diastolic velocity (Em). As a noninvasive estimate of LV end-diastolic pressure, the ratio of mitral inflow early diastolic velocity (E) to mean basal peak early diastolic velocity (Em) was determined.

Strain and strain rate curves were extracted from color tissue Doppler images by standard software (EchoPAC). Strain and strain rate were derived from strain and strain rate curves obtained by placing a sample volume (9.2 mm) in the basal and mid segments from the three apical views. The sample volume was located in the central part of each segment and tracked manually throughout the cardiac cycle to improve the accuracy of the technique. The peak systolic strain rate was assessed as an average of the values obtained from three consecutive heart cycles. The peak systolic strain rate in each segment was assessed as the lowest value between the R wave on the electrocardiogram and closure of the aortic valve, which was defined by a curved anatomic M-mode recording placed through the aortic valve leaflets in the apical long-axis view. We excluded data if we were unable to obtain a smooth strain curve or the angle between the scan line and wall was >20°. A total of 12 segments were assessed (mid and basal segments of the lateral, septal, anterior, inferior, posterior, and anteroseptal LV walls) in all study subjects, and mean peak systolic strain was calculated by dividing the sum of each longitudinal peak strain values to the number of used segments as global longitudinal LV systolic indices. In the same way, mean peak systolic strain rate and mean peak early diastolic strain rate, referred to as LV global systolic and diastolic indices, respectively, were computed. This methodology for obtaining global LV functional parameters is supported by a previous study, which demonstrated a strong association between the strain rate at the basal septum and peak elastance, which was the gold standard of global LV contractility. In another study, mean myocardial tissue Doppler velocities obtained from the basal segments of the left ventricle seemed to be the most feasible and accurate indices for routine assessment of diabetic myocardial disease.

To define the intraobserver and interobserver variability, the strain and strain rate measurements were repeated for 15 randomly selected patients after 4 weeks by the same observer on the same echocardiographic images and by a second independent observer. Intraobserver variability for the measurement of the peak systolic strain, peak systolic strain rate, and peak early diastolic strain rate was 4.4 ± 3.6%, 5.4 ± 3.7%, and 5.6 ± 3.3%, respectively. Interobserver variability for these same measurements was 4.9 ± 3.7%, 6.2 ± 3.1%, and 6.4 ± 2.8%, respectively ( Table 1 ).

Statistical Analyses

Statistical analyses were performed using SPSS version 15.0 (SPSS, Inc., Chicago, IL). Data are presented as mean ± SD, except for diabetes duration, which is presented as the geometric mean. Frequencies are expressed as percentages. To compare the variables derived from the clinical characteristics and standard echocardiographic examinations between the IFG, IGT, IFG+IGT, and normal groups, one-way analysis of variance and post hoc Bonferroni tests were used. Because body mass index (BMI) was significantly higher in the IFG+IGT and DM groups than in the control group, analysis of covariance was used to adjust for BMI to compare LV tissue Doppler parameters among groups. Student’s independent-samples t tests were used to compare differences between two groups. Categorical variables were compared using Fischer’s exact tests or χ ² tests as appropriate. Pearson’s correlation coefficient ( r ) was used to estimate correlations between individual parameters. Two-tailed P values < .05 were considered significant.