ORIGINAL ARTICLE
Triglyceride Glucose Index and Common Carotid Wall Shear Stress Cesare Tripolino, MD, Concetta Irace, MD, Faustina B. Scavelli, MD, Maria S. de Franceschi, MD, Teresa Esposito, MD, Claudio Carallo, MD, and Agostino Gnasso, MD
Objectives: Alterations in wall shear stress contribute to both clinical and subclinical atherosclerosis. Several conditions such as hypertension, diabetes, and obesity can impair shear stress, but the role of insulin resistance has never been investigated. The present study was designed to investigate whether insulin resistance assessed by TyG Index associates with wall shear stress in the common carotid artery. Methods: One hundred six individuals were enrolled. Blood pressure, lipids, glucose, and cigarette smoking were evaluated. TyG Index was calculated as log[fasting triglycerides fasting glucose / 2]. Subjects underwent blood viscosity measurement and echo-Doppler evaluation of carotid arteries to calculate wall shear stress. The association between TyG Index and carotid wall shear stress was assessed by simple and multiple regression analyses. Results: TyG Index was significantly and inversely associated with carotid wall shear stress both in simple (r = j0.44, P G 0.001) and multiple regression analyses accounting for age, sex, and major cardiovascular risk factors. The association was further confirmed after exclusion of subjects with diabetes, dyslipidemia, fasting blood glucose greater than 100 mg/dL, and triglycerides greater than 150 mg/dL. Conclusions: The present findings suggest that increasing insulin resistance, as assessed by TyG Index, associates with atherosclerosisprone shear stress reduction in the common carotid artery. Key Words: insulin resistance, wall shear stress, carotid artery (J Investig Med 2014;62: 340Y344)
W
all shear stress is the frictional force acting on the endothelial surface and contributes to both clinical and subclinical atherosclerosis,1,2 explaining the focal development of plaques despite the systemic nature of atherosclerosis.3 Wall shear stress can be evaluated noninvasively and reliably in vivo by ultrasound-based technique and magnetic resonance imaging.4,5 It has been demonstrated that different physiological and pathological conditions, including aging, inflammatory diseases, obesity, hypertension, and diabetes, can impair shear stress.6Y9 Insulin resistance (IR), a clue of metabolic syndrome, is the decreased sensitivity or responsiveness to the metabolic action of insulin. Actually, the hyperinsulinemic euglycemic clamp is considered the criterion standard for IR evaluation.10 In addition to this, a number of surrogate markers have been proposed, some derived from dynamic tests and others derived From the Department of Clinical and Experimental Medicine, ‘‘Magna Gr&cia’’ University, Catanzaro, Italy. Received July 18, 2013, and in revised form November 10, 2013. Accepted for publication November 19, 2013. Reprints: Agostino Gnasso, MD, Dipartimento di Medicina Sperimentale e Clinica ‘‘G. Salvatore’’, ‘‘Magna Gr&cia’’ University, University Campus, ‘‘S. Venuta’’, Germaneto, 88100, Catanzaro, Italy. E-mail: [email protected]. No competing financial interests exist. Copyright * 2014 by The American Federation for Medical Research ISSN: 1081-5589 DOI: 10.231/JIM.0000000000000043
340
from fasting steady-state conditions.11 In large observational studies, a direct and independent association between IR indexes and major cardiovascular events has been demonstrated.12 Recently, a simple index evaluating insulin sensitivity has been proposed: the product of fasting triglycerides and fasting glucose (TyG Index).13 TyG Index correlates with Homeostasis Model Assessment Index and IR evaluated by hyperglycemic clamp.14,15 Interestingly, TyG Index has been associated with symptomatic and asymptomatic atherosclerosis. Recently, the association between TyG Index and intima media thickness has been reported in a small sample of Brazilian population.14 In a previous article, we have described a strong association between TyG Index and carotid atherosclerosis.16 It is not known whether IR is associated with wall shear stress, and this hypothesis was tested in the present study. Insulin resistance was evaluated by TyG Index, and wall shear stress was measured in the common carotid artery.
SUBJECTS AND METHODS Subjects Subjects were recruited among participants to a cardiovascular disease prevention campaign. All participants were eligible unless they had one of the following conditions: arrhythmia at electrocardiography (ECG); flow disturbing stenoses of the carotid arteries; premenopausal women; use of diuretic and anticoagulant drugs; severe anemia (hemoglobin G10 g/dL); polycythemia (red blood cell count 96 106 cells/HL); severe renal, hepatic, or pulmonary disease; heart failure. All these clinical conditions can directly or indirectly, through medications, interfere with shear stress measurement. Those who signed the informed consent (n = 106) approved by our local ethical committee were recruited. Afterward, they were scheduled for a different day to undergo blood sample withdrawal and complete clinical examination and for an additional visit within 2 to 3 days, for the echo-Doppler examination.
Clinical Examination All subjects were examined in the morning in a room at 22-C, after overnight fasting. Well-trained personnel measured blood pressure, height, and weight by routine methods. The mean of 2 sitting blood pressure readings was used. Body mass index (BMI) was computed as weight (in kilograms) divided by height (in squared meters). A questionnaire was administered to evaluate smoking habit and drug use. Blood and plasma viscosity were measured within 2 hours from blood withdrawal; the blood specimen was added with heparin (35 IU/mL).
Cardiovascular Risk Factors Diabetes was defined as fasting blood glucose of 126 mg/dL or greater in 2 different occasions and/or use of antidiabetic agents. Hyperlipidemia was defined as total cholesterol and/or
Journal of Investigative Medicine
&
Volume 62, Number 2, February 2014
Copyright © 2014 American Federation for Medical Research. Unauthorized reproduction of this article is prohibited.
Journal of Investigative Medicine
&
Volume 62, Number 2, February 2014
triglycerides of 200 mg/dL or greater and/or use of lipid-lowering drugs. Hypertension was defined as systolic blood pressure/ diastolic blood pressure of 140/90 mm Hg or greater and/or use of antihypertensive agents. Subjects who smoked regularly during the previous 12 months were classified as smokers. Obesity was defined as BMI of 30 kg/m2 or greater.
Laboratory Measurements Blood lipids (total cholesterol, high-density lipoprotein [HDL] cholesterol, triglycerides) and fasting glucose were measured with commercially available kits. Hyperlipidemic and/or diabetic subjects who were assuming lipid- or glucose-lowering drugs were asked to take the last dose the day before examination. Low-density lipoprotein (LDL) cholesterol was computed according to Friedewald formula.
Insulin Resistance Assessment The TyG Index was calculated according to the following formula: Log½ fasting triglycerides ð mg=dLÞ fasting glucose ð mg=dLÞ=213 :
Blood Viscosity Measurement Viscosity measurement was performed at 37-C with a cone-plate viscometer (Wells-Brookfield DV-III, Stoughton, WI)
TyG Index and Shear Stress
equipped with a cp-40 spindle. Blood viscosity was measured at shear rate of 225 I sj1 in vitro.
Ultrasound Study Echo-Doppler examination for arterial diameter and blood flow velocity measurements was performed in a quiet room at 22-C, with an ECG-triggered high-resolution instrument (ATL HDI 3000; Philips; Advanced Technology Laboratories Inc, Oceanside, CA) equipped with a 5- to 10-MHz multifrequency linear probe. Internal diameter (ID) was defined as the distance between the leading edge of the echo produced by the intimalumen interface of the near wall and the leading edge of the echo produced by the lumen-intima interface of the far wall. Internal diameter was measured at the R (IDR) and T (IDT) waves of the ECG, representing the minimum and maximum carotid diameter, respectively. Arterial diameter was measured offline by a dedicated software as previously described.5 Blood flow velocity was detected with the sample volume reduced to the smallest possible size (1 mm) and placed in the centre of the vessel. The angle between the ultrasound beam and the longitudinal vessel axis (5) was kept between 44 and 56 degrees. The maximum Doppler frequency shift, that is, systolic peak velocity, and mean velocity (MV) were automatically recorded with autotracking as the mean of 3 cardiac cycles. We used peak velocity to calculate peak wall shear stress and time-averaged peak (TAP) to calculate mean wall shear stress. It has been suggested that MV of flow can be better estimated by TAP over an integral number of cardiac cycles. In our study, we evaluated TAP during 3 cardiac cycles.7
TABLE 1. Clinical and Biochemical Characteristics and Common Carotid Artery Parameters of Subjects Included in the Study (n = 106) and Divided According to Sex
n Clinical parameters Age, y SBP, mm Hg DBP, mm Hg BMI, kg/m2 Biochemical parameters Total cholesterol, mg/dL LDL cholesterol, mg/dL HDL cholesterol, mg/dL Triglycerides, mg/dL Fasting glucose, mg/dL Cardiovascular risk factors Hypertension, % Dyslipidemia, % Smoking, % Obesity, % Diabetes, % TyG Index Hemodynamic and hemorheologic parameters Carotid ID (R), mm Carotid ID (T), mm Carotid peak shear stress, dyn/cm2 Carotid mean shear stress, dyn/cm2 Viscosity, cP
Men
Women
73
33
52.3 T 17.3 128 T 14 79 T 8 27 T 4.2 178 T 39 107 T 36 49 T 13 114 T 50 104 T 40.4
63 T 10.3 144 T 21 84 T 12 28. T 4.4 205 T 61 122 T 59 57 T 16 124.4 T 38 107 T 35
P
0.001 0.000 0.015 NS 0.006 NS 0.014 NS NS
26 27 10 19 22 3.70 T 0.25
64 55 6 33 33 3.78 T 0.21
0.000 0.007 NS NS NS NS
6.0 T 0.8 6.5 T 0.9 22.5 T 9.0 10.7 T 4.0 4.8 T 0.5
5.6 T 0.7 6.0 T 0.7 18.9 T 5.3 10.0 T 3.4 4.6 T 0.7
0.001 G0.001 0.003 NS 0.004
DBP indicates diastolic blood pressure, NS, not statistically significant; SBP systolic blood pressure.
* 2014 The American Federation for Medical Research
Copyright © 2014 American Federation for Medical Research. Unauthorized reproduction of this article is prohibited.
341
Journal of Investigative Medicine
Tripolino et al
Peak (TP) and mean (TM ) wall shear stress were calculated according to the following formulas: TP ð dyn=cm2 Þ ¼ 4 K V SP = ID T TM ð dyn= cm2 Þ ¼ 4 K V M = ID R where V is expressed in cm/s, ID in cm, and K in poise.
Statistical Analysis Hemodynamic variables were calculated separately for the right and left side. For this reason, the number of observations is twice that of the patients studied. Statistical analysis was performed with SPSS 17.0 (SPSS, Inc., Chicago, IL). All variables (except triglycerides) showed a normal distribution. Triglycerides were log transformed. Continuous variables are reported as mean T SD, whereas dichotomous variables are reported as percentage. Student t test and W2 test were used where appropriate. The association between variables was evaluated using the simple regression analysis test. To evaluate the independent association between TyG Index and peak and mean shear stress, the stepwise regression analysis was applied. Age, sex, smoking habit, hypertension, type 2 diabetes, BMI, blood glucose, triglycerides, total cholesterol, LDL cholesterol, and HDL cholesterol were included in the model as independent variables in addition to TyG Index.
RESULTS Table 1 shows the clinical and biochemical characteristics of subjects enrolled in the study, according to sex. Age range was 25 to 70 years; 73 were male, and 33 were female. Men
&
Volume 62, Number 2, February 2014
were significantly younger than women and had a lower prevalence of hypertension, type 2 diabetes, obesity, and hyperlipidemia. TyG Index was comparable between sexes. Furthermore, men had significantly higher carotid artery diameter, systolic peak velocity, peak shear stress, and blood viscosity. Overall, 27 subjects had type 2 diabetes, and of these, 18 subjects were taking oral hypoglycemic agents, and 38 were hyperlipidemic, with only 15 taking a statin as a lipid-lowering drug. Figure 1 shows the association between shear stress (peak and mean) and TyG Index. Peak and mean shear stress were directly and strongly associated each other (r = 0.90, P G 0.001), and both were inversely associated with TyG Index. Furthermore, in simple regression analysis TyG Index was significantly and inversely associated with blood velocity (MV: r = j0.35; systolic peak velocity: r = j0.38, P G 0.001) and directly associated with arterial diameter (IDR: r = 0.31; IDT: r = 0.27, P G 0.001). In multiple linear regression analysis, TyG Index was significantly and independently associated with wall shear stress (Table 2). To further minimize the possible contribution of type 2 diabetes, hyperlipidemia, and thus the influence of medications on fasting blood glucose and triglycerides, we performed a second linear regression analysis excluding subjects with type 2 diabetes, dyslipidemia, fasting blood glucose of greater than 100 mg/dL, and triglycerides of greater than 150 mg/dL. Twelve women and 43 men were analyzed. The regression analysis confirmed the independent association between TyG Index and wall shear stress (Table 3). These same analyses were also performed in male subjects only. Peak shear stress was inversely related with age (r2 = 0.57, P G 0.0001) and TyG Index (r2 = 0.61, P G 0.0001), whereas
FIGURE 1. Association between wall shear stress (peak and mean) values and TyG Index values (n = 212 samples, P G 0.00001 for all coefficients).
342
* 2014 The American Federation for Medical Research
Copyright © 2014 American Federation for Medical Research. Unauthorized reproduction of this article is prohibited.
Journal of Investigative Medicine
&
Volume 62, Number 2, February 2014
mean shear stress is inversely related with age (r2 = 0.52, P G 0.0001), TyG Index (r2 = 0.56, P = 0.006), and BMI (r2 = 0.57, P = 0.01). Multiple linear regression analyses were also performed using either left or right common carotid artery peak shear stress as dependent variable, and the independent association between TyG Index and wall shear stress was confirmed (data not shown).
DISCUSSION The present investigation demonstrates that IR, evaluated as TyG Index, is significantly and independently associated with carotid wall shear stress. Type 2 diabetes has previously been reported to be associated with decreased wall shear stress.6 The data of the present study suggest that the reduced sensitivity to insulin, even long before the onset of type 2 diabetes, is associated with a reduction of the wall shear stress, thus predisposing to the development of atheromatous plaques. The finding is particularly important because it is confirmed in subjects with normal glucose and triglyceride levels. Therefore, the IR is not only at the base of metabolic abnormalities but is also associated with hemodynamic alterations that promote atherosclerosis. Overall, this could help to explain the frequent finding of macrovascular complications at onset of type 2 diabetes. These results are in line with those obtained in other studies in which endothelial function was assessed. Quinn et al.17 have shown a reduced flow-mediated vasodilatation (that is, dependent from the increase in shear stress) in subjects with IR compared with control subjects. In the same study, the authors have shown that the administration of an insulin-sensitizing agent such as pioglitazone, a drug that counteracts the IR especially in the muscles, improved endothelial function. The possible mechanisms by which IR causes hemodynamic changes can only be hypothesized. It is known that IR state, as well as aging or type 2 diabetes, is associated with an increase in vessel diameter. The reasons for this are numerous and have been widely discussed in the literature.6,18Y20 As a matter of fact, subjects with IR have arterial diameters frankly higher than those of healthy subjects and consequently also reduced blood velocity. Because blood velocity and diameter are the main determinants of wall shear stress, it is reasonable to expect a reduction of shear stress in IR state. In line with these concepts, in the present study, TyG Index was directly related with carotid ID and inversely with systolic peak velocity, whereas no association with blood viscosity was found. It is known that IR stimulates myocyte growth and interstitial fibrosis, leading to ventricular function impairment.21,22 Furthermore, IR compromises microvascular circulation, increasing peripheral resistance and then cardiac afterload.23 The combined action of these factors could impair cardiac contractile force, thereby reducing ejection fraction and consequently the blood flow velocity. Of
TABLE 2. Results of Multiple Stepwise Regression Analysis in the Whole Population Step 1 2 3
Variable
A Coefficient
R2
P
Age TyG Index BMI
j0.625 j0.163 j0.142
0.53 0.57 0.58
G0.001 0.001 0.004
Dependent variable: peak shear stress. Sex, diabetes mellitus, hypertension, cigarette smoking, triglycerides, glucose, total cholesterol, LDL cholesterol, and HDL cholesterol did not enter in the model.
TyG Index and Shear Stress
TABLE 3. Results of Multiple Stepwise Regression Analysis in Subjects Without Diabetes and Hyperlipidemia Step 1 2 3
Variable
A Coefficient
R2
P
Age TyG Index BMI
j0.680 j0.157 j0.143
0.62 0.65 0.66
G0.001 0.009 0.026
Dependent variable: peak shear stress. Sex, hypertension, cigarette smoking, triglycerides, glucose, total cholesterol, LDL cholesterol, and HDL cholesterol did not significantly improve the regression mode.
great importance is also the role of arterial diameter. In fact, as above reported, the increased vessel diameter,18Y20 typical of the IR, further reduces blood flow velocity, and both these factors determine the hemodynamic force derangement. Our data demonstrate that changes in blood velocity and diameter begin to occur very early and are strongly correlated with the presence of IR. The findings of the present study are intriguing, considering the significant role held by local hemodynamic forces in vascular physiology and pathophysiology. Wall shear stress, the frictional force that flowing blood generates tangentially to the endothelial surface, strongly influences arterial remodeling and the vessels’ health.1,24 In regions where low endothelial shear stress occurs, the atheroprotective genes are suppressed, whereas the proatherogenic genes are up-regulated, thereby promoting the atherosclerotic process.24 A particularly important point of the present study is whether the TyG Index truly reflects IR. The hyperinsulinemic euglycemic clamp is the criterion standard for IR evaluation, but it is difficult to use in daily clinical practice. On the other hand, TyG Index is simple to calculate based on the product of plasma triglycerides and glucose. It is based on the assumption that any increase in plasma triglycerides interferes with the normal metabolism of glucose in the muscle, thereby causing a reduced sensitivity to insulin.25 Even if this surrogate measure is not widely used yet, previous studies have demonstrated the high sensitivity and specificity of TyG Index and its high correlation with glucose clamp, and the Homeostasis Model Assessment of IR index. Furthermore, we have recently demonstrated that TyG Index is strongly related with carotid atherosclerosis in a large population-based study.16 Based on these previous studies, we believe that the TyG Index may represent a good approximation of the IR state and can usefully be used in large clinical trials.
CONCLUSIONS In conclusion, IR is a complex, systemic condition promoting the development of a wide spectrum of metabolic diseases such as hypertension, type 2 diabetes, dyslipidemia, and metabolic syndrome, conditions known to be associated with atherosclerosis and major cardiovascular event. The present study suggests that IR has probably important effects also on hemodynamic forces, favoring the expression of an atherosclerosis prone profile of the endothelial cells, at least in the common carotid arteries. REFERENCES 1. Irace C, Cortese C, Fiaschi E, et al. Wall shear stress is associated with intima-media thickness and carotid atherosclerosis in subjects at low coronary heart disease risk. Stroke. 2004;35:464Y468. 2. Jiang Y, Kohara K, Hiwada K. Association between risk factors for atherosclerosis and mechanical forces in carotid artery. Stroke. 2000;31:2319Y2324.
* 2014 The American Federation for Medical Research
Copyright © 2014 American Federation for Medical Research. Unauthorized reproduction of this article is prohibited.
343
Tripolino et al
Journal of Investigative Medicine
&
Volume 62, Number 2, February 2014
3. Stone PH, Coskun AU, Kinlay S, et al. Regions of low endothelial shear stress are the sites where coronary plaque progresses and vascular remodelling occurs in humans: an in vivo serial study. Eur Heart J. 2007;28:705Y710.
15. Guerrero-Romero F, Simental-Mendı´a LE, Gonza´lez-Ortiz M, et al. The product of triglycerides and glucose, a simple measure of insulin sensitivity. Comparison with the euglycemic-hyperinsulinemic clamp. J Clin Endocrinol Metab. 2010;95:3347Y3351.
4. Duivenvoorden R, Vanbavel E, de Groot E, et al. Endothelial shear stress: a critical determinant of arterial remodeling and arterial stiffness in humans-a carotid 3.0-T MRI study [published correction appears in Circ Cardiovasc Imaging 2011;1;4:e3]. Circ Cardiovasc Imaging. 2010;3:578Y585.
16. Irace C, Carallo C, Scavelli FB, et al. Markers of insulin resistance and carotid atherosclerosis. A comparison of the Homeostasis Model Assessment and Triglyceride Glucose Index. Int J Clin Pract. 2013;67:665Y672.
5. Gnasso A, Carallo C, Irace C, et al. Association between intima-media thickness and wall shear stress in common carotid arteries in healthy male subjects. Circulation. 1996;94:3257Y3262. 6. Irace C, Carallo C, Crescenzo A, et al. NIDDM is associated with lower wall shear stress of the common carotid artery. Diabetes. 1999;48:193Y197. 7. Irace C, Carallo C, De Franceschi MS, et al. Human common carotid wall shear stress as a function of age and gender: a 12-year follow-up study. Age (Dordr). 2011;34:1553Y1562. 8. Irace C, Mancuso G, Fiaschi E, et al. Effect of anti TNFalpha therapy on arterial diameter and wall shear stress and HDL cholesterol. Atherosclerosis. 2004;177:113Y118. 9. Simon AC, Pithois-Merli I, Levenson J. Physiopharmacological approach to mechanical factors of hypertension in the atherosclerotic process. J Hum Hypertens. 1991;5(suppl 1):15Y21. 10. DeFronzo RA, Tobin JD, Andres R. Glucose clamp technique: a method for quantifying insulin secretion and resistance. Am J Physiol. 1979;237:E214YE223.
17. Quinn CE, Lockhart CJ, Hamilton PK, et al. Effect of pioglitazone on endothelial function in impaired glucose tolerance. Diabetes Obes Metab. 2010;12:709Y715. 18. Crouse JR, Goldbourt U, Evans G, et al. for the ARIC Investigators: risk factors and segment-specific carotid arterial enlargement in the Atherosclerosis Risk in Communities (ARIC) Cohort. Stroke. 1996;27:69Y75. 19. Bonithon-Kopp C, Touboul PJ, Berr C, et al. Factors of carotid arterial enlargement in a population aged 59 to 71 years: the EVA Study. Stroke. 1999;27:654Y660. 20. Iannuzzi A, De Michele M, Bond MG, et al. Carotid artery remodeling in middle-aged women with the metabolic syndrome (from the ‘‘Progetto ATENA’’ study). Am J Cardiol. 2005;96:1162Y1165. 21. Kosmala W, Jedrzejuk D, Derzhko R, et al. Left ventricular function impairment in patients with normal-weight obesity: contribution of abdominal fat deposition, profibrotic state, reduced insulin sensitivity, and proinflammatory activation. Circ Cardiovasc Imaging. 2012;5:349Y356.
11. Muniyappa R, Lee S, Chen H, et al. Current approaches for assessing insulin sensitivity and resistance in vivo: advantages, limitations, and appropriate usage. Am J Physiol Endocrinol Metab. 2008;294:E15YE26.
22. Kosmala W, O’Moore-Sullivan TM, Plaksej R, et al. Subclinical impairment of left ventricular function in young obese women: contributions of polycystic ovary disease and insulin resistance. J Clin Endocrinol Metab. 2008;93:3748Y3754.
12. Wannamethee SG, Lowe GD, Shaper AG, et al. Insulin resistance, haemostatic and inflammatory markers and coronary heart disease risk factors in Type 2 diabetic men with and without coronary heart disease. Diabetologia. 2004;47:1557Y1565.
23. Clark MG, Wallis MG, Barrett EJ, et al. Blood flow and muscle metabolism: a focus on insulin action. Am J Physiol Endocrinol Metab. 2003;284:E241YE258.
13. Simental-Mendı`a LE, Rodrı`guez-Mora`an M, Guerrero-Romero F. The product of fasting glucose and triglycerides as surrogate for identifying insulin resistance in apparently healthy subjects. Metab Syndr Relat Disord. 2008;6:299Y304.
24. Chatzizisis YS, Coskun AU, Jonas M, et al. Role of endothelial shear stress in the natural history of coronary atherosclerosis and vascular remodeling: molecular, cellular, and vascular behavior. J Am Coll Cardiol. 2007;49:2379Y2393.
14. Vasques AC, Novaes FS, de Oliveira Mda S, et al. TyG Index performs better than HOMA in a Brazilian population: a hyperglycemic clamp validated study. Diabetes Res Clin Pract. 2011;93:e98Ye100.
25. Clerk LH, Rattigan S, Clark MG. Lipid infusion impairs physiologic insulin-mediated capillary recruitment and muscle glucose uptake in vivo. Diabetes. 2002;51:1138Y1145.
344
* 2014 The American Federation for Medical Research
Copyright © 2014 American Federation for Medical Research. Unauthorized reproduction of this article is prohibited.
Triglyceride Glucose Index and Common Carotid Wall Shear Stress Cesare Tripolino, MD, Concetta Irace, MD, Faustina B. Scavelli, MD, Maria S. de Franceschi, MD, Teresa Esposito, MD, Claudio Carallo, MD, and Agostino Gnasso, MD
Objectives: Alterations in wall shear stress contribute to both clinical and subclinical atherosclerosis. Several conditions such as hypertension, diabetes, and obesity can impair shear stress, but the role of insulin resistance has never been investigated. The present study was designed to investigate whether insulin resistance assessed by TyG Index associates with wall shear stress in the common carotid artery. Methods: One hundred six individuals were enrolled. Blood pressure, lipids, glucose, and cigarette smoking were evaluated. TyG Index was calculated as log[fasting triglycerides fasting glucose / 2]. Subjects underwent blood viscosity measurement and echo-Doppler evaluation of carotid arteries to calculate wall shear stress. The association between TyG Index and carotid wall shear stress was assessed by simple and multiple regression analyses. Results: TyG Index was significantly and inversely associated with carotid wall shear stress both in simple (r = j0.44, P G 0.001) and multiple regression analyses accounting for age, sex, and major cardiovascular risk factors. The association was further confirmed after exclusion of subjects with diabetes, dyslipidemia, fasting blood glucose greater than 100 mg/dL, and triglycerides greater than 150 mg/dL. Conclusions: The present findings suggest that increasing insulin resistance, as assessed by TyG Index, associates with atherosclerosisprone shear stress reduction in the common carotid artery. Key Words: insulin resistance, wall shear stress, carotid artery (J Investig Med 2014;62: 340Y344)
W
all shear stress is the frictional force acting on the endothelial surface and contributes to both clinical and subclinical atherosclerosis,1,2 explaining the focal development of plaques despite the systemic nature of atherosclerosis.3 Wall shear stress can be evaluated noninvasively and reliably in vivo by ultrasound-based technique and magnetic resonance imaging.4,5 It has been demonstrated that different physiological and pathological conditions, including aging, inflammatory diseases, obesity, hypertension, and diabetes, can impair shear stress.6Y9 Insulin resistance (IR), a clue of metabolic syndrome, is the decreased sensitivity or responsiveness to the metabolic action of insulin. Actually, the hyperinsulinemic euglycemic clamp is considered the criterion standard for IR evaluation.10 In addition to this, a number of surrogate markers have been proposed, some derived from dynamic tests and others derived From the Department of Clinical and Experimental Medicine, ‘‘Magna Gr&cia’’ University, Catanzaro, Italy. Received July 18, 2013, and in revised form November 10, 2013. Accepted for publication November 19, 2013. Reprints: Agostino Gnasso, MD, Dipartimento di Medicina Sperimentale e Clinica ‘‘G. Salvatore’’, ‘‘Magna Gr&cia’’ University, University Campus, ‘‘S. Venuta’’, Germaneto, 88100, Catanzaro, Italy. E-mail: [email protected]. No competing financial interests exist. Copyright * 2014 by The American Federation for Medical Research ISSN: 1081-5589 DOI: 10.231/JIM.0000000000000043
340
from fasting steady-state conditions.11 In large observational studies, a direct and independent association between IR indexes and major cardiovascular events has been demonstrated.12 Recently, a simple index evaluating insulin sensitivity has been proposed: the product of fasting triglycerides and fasting glucose (TyG Index).13 TyG Index correlates with Homeostasis Model Assessment Index and IR evaluated by hyperglycemic clamp.14,15 Interestingly, TyG Index has been associated with symptomatic and asymptomatic atherosclerosis. Recently, the association between TyG Index and intima media thickness has been reported in a small sample of Brazilian population.14 In a previous article, we have described a strong association between TyG Index and carotid atherosclerosis.16 It is not known whether IR is associated with wall shear stress, and this hypothesis was tested in the present study. Insulin resistance was evaluated by TyG Index, and wall shear stress was measured in the common carotid artery.
SUBJECTS AND METHODS Subjects Subjects were recruited among participants to a cardiovascular disease prevention campaign. All participants were eligible unless they had one of the following conditions: arrhythmia at electrocardiography (ECG); flow disturbing stenoses of the carotid arteries; premenopausal women; use of diuretic and anticoagulant drugs; severe anemia (hemoglobin G10 g/dL); polycythemia (red blood cell count 96 106 cells/HL); severe renal, hepatic, or pulmonary disease; heart failure. All these clinical conditions can directly or indirectly, through medications, interfere with shear stress measurement. Those who signed the informed consent (n = 106) approved by our local ethical committee were recruited. Afterward, they were scheduled for a different day to undergo blood sample withdrawal and complete clinical examination and for an additional visit within 2 to 3 days, for the echo-Doppler examination.
Clinical Examination All subjects were examined in the morning in a room at 22-C, after overnight fasting. Well-trained personnel measured blood pressure, height, and weight by routine methods. The mean of 2 sitting blood pressure readings was used. Body mass index (BMI) was computed as weight (in kilograms) divided by height (in squared meters). A questionnaire was administered to evaluate smoking habit and drug use. Blood and plasma viscosity were measured within 2 hours from blood withdrawal; the blood specimen was added with heparin (35 IU/mL).
Cardiovascular Risk Factors Diabetes was defined as fasting blood glucose of 126 mg/dL or greater in 2 different occasions and/or use of antidiabetic agents. Hyperlipidemia was defined as total cholesterol and/or
Journal of Investigative Medicine
&
Volume 62, Number 2, February 2014
Copyright © 2014 American Federation for Medical Research. Unauthorized reproduction of this article is prohibited.
Journal of Investigative Medicine
&
Volume 62, Number 2, February 2014
triglycerides of 200 mg/dL or greater and/or use of lipid-lowering drugs. Hypertension was defined as systolic blood pressure/ diastolic blood pressure of 140/90 mm Hg or greater and/or use of antihypertensive agents. Subjects who smoked regularly during the previous 12 months were classified as smokers. Obesity was defined as BMI of 30 kg/m2 or greater.
Laboratory Measurements Blood lipids (total cholesterol, high-density lipoprotein [HDL] cholesterol, triglycerides) and fasting glucose were measured with commercially available kits. Hyperlipidemic and/or diabetic subjects who were assuming lipid- or glucose-lowering drugs were asked to take the last dose the day before examination. Low-density lipoprotein (LDL) cholesterol was computed according to Friedewald formula.
Insulin Resistance Assessment The TyG Index was calculated according to the following formula: Log½ fasting triglycerides ð mg=dLÞ fasting glucose ð mg=dLÞ=213 :
Blood Viscosity Measurement Viscosity measurement was performed at 37-C with a cone-plate viscometer (Wells-Brookfield DV-III, Stoughton, WI)
TyG Index and Shear Stress
equipped with a cp-40 spindle. Blood viscosity was measured at shear rate of 225 I sj1 in vitro.
Ultrasound Study Echo-Doppler examination for arterial diameter and blood flow velocity measurements was performed in a quiet room at 22-C, with an ECG-triggered high-resolution instrument (ATL HDI 3000; Philips; Advanced Technology Laboratories Inc, Oceanside, CA) equipped with a 5- to 10-MHz multifrequency linear probe. Internal diameter (ID) was defined as the distance between the leading edge of the echo produced by the intimalumen interface of the near wall and the leading edge of the echo produced by the lumen-intima interface of the far wall. Internal diameter was measured at the R (IDR) and T (IDT) waves of the ECG, representing the minimum and maximum carotid diameter, respectively. Arterial diameter was measured offline by a dedicated software as previously described.5 Blood flow velocity was detected with the sample volume reduced to the smallest possible size (1 mm) and placed in the centre of the vessel. The angle between the ultrasound beam and the longitudinal vessel axis (5) was kept between 44 and 56 degrees. The maximum Doppler frequency shift, that is, systolic peak velocity, and mean velocity (MV) were automatically recorded with autotracking as the mean of 3 cardiac cycles. We used peak velocity to calculate peak wall shear stress and time-averaged peak (TAP) to calculate mean wall shear stress. It has been suggested that MV of flow can be better estimated by TAP over an integral number of cardiac cycles. In our study, we evaluated TAP during 3 cardiac cycles.7
TABLE 1. Clinical and Biochemical Characteristics and Common Carotid Artery Parameters of Subjects Included in the Study (n = 106) and Divided According to Sex
n Clinical parameters Age, y SBP, mm Hg DBP, mm Hg BMI, kg/m2 Biochemical parameters Total cholesterol, mg/dL LDL cholesterol, mg/dL HDL cholesterol, mg/dL Triglycerides, mg/dL Fasting glucose, mg/dL Cardiovascular risk factors Hypertension, % Dyslipidemia, % Smoking, % Obesity, % Diabetes, % TyG Index Hemodynamic and hemorheologic parameters Carotid ID (R), mm Carotid ID (T), mm Carotid peak shear stress, dyn/cm2 Carotid mean shear stress, dyn/cm2 Viscosity, cP
Men
Women
73
33
52.3 T 17.3 128 T 14 79 T 8 27 T 4.2 178 T 39 107 T 36 49 T 13 114 T 50 104 T 40.4
63 T 10.3 144 T 21 84 T 12 28. T 4.4 205 T 61 122 T 59 57 T 16 124.4 T 38 107 T 35
P
0.001 0.000 0.015 NS 0.006 NS 0.014 NS NS
26 27 10 19 22 3.70 T 0.25
64 55 6 33 33 3.78 T 0.21
0.000 0.007 NS NS NS NS
6.0 T 0.8 6.5 T 0.9 22.5 T 9.0 10.7 T 4.0 4.8 T 0.5
5.6 T 0.7 6.0 T 0.7 18.9 T 5.3 10.0 T 3.4 4.6 T 0.7
0.001 G0.001 0.003 NS 0.004
DBP indicates diastolic blood pressure, NS, not statistically significant; SBP systolic blood pressure.
* 2014 The American Federation for Medical Research
Copyright © 2014 American Federation for Medical Research. Unauthorized reproduction of this article is prohibited.
341
Journal of Investigative Medicine
Tripolino et al
Peak (TP) and mean (TM ) wall shear stress were calculated according to the following formulas: TP ð dyn=cm2 Þ ¼ 4 K V SP = ID T TM ð dyn= cm2 Þ ¼ 4 K V M = ID R where V is expressed in cm/s, ID in cm, and K in poise.
Statistical Analysis Hemodynamic variables were calculated separately for the right and left side. For this reason, the number of observations is twice that of the patients studied. Statistical analysis was performed with SPSS 17.0 (SPSS, Inc., Chicago, IL). All variables (except triglycerides) showed a normal distribution. Triglycerides were log transformed. Continuous variables are reported as mean T SD, whereas dichotomous variables are reported as percentage. Student t test and W2 test were used where appropriate. The association between variables was evaluated using the simple regression analysis test. To evaluate the independent association between TyG Index and peak and mean shear stress, the stepwise regression analysis was applied. Age, sex, smoking habit, hypertension, type 2 diabetes, BMI, blood glucose, triglycerides, total cholesterol, LDL cholesterol, and HDL cholesterol were included in the model as independent variables in addition to TyG Index.
RESULTS Table 1 shows the clinical and biochemical characteristics of subjects enrolled in the study, according to sex. Age range was 25 to 70 years; 73 were male, and 33 were female. Men
&
Volume 62, Number 2, February 2014
were significantly younger than women and had a lower prevalence of hypertension, type 2 diabetes, obesity, and hyperlipidemia. TyG Index was comparable between sexes. Furthermore, men had significantly higher carotid artery diameter, systolic peak velocity, peak shear stress, and blood viscosity. Overall, 27 subjects had type 2 diabetes, and of these, 18 subjects were taking oral hypoglycemic agents, and 38 were hyperlipidemic, with only 15 taking a statin as a lipid-lowering drug. Figure 1 shows the association between shear stress (peak and mean) and TyG Index. Peak and mean shear stress were directly and strongly associated each other (r = 0.90, P G 0.001), and both were inversely associated with TyG Index. Furthermore, in simple regression analysis TyG Index was significantly and inversely associated with blood velocity (MV: r = j0.35; systolic peak velocity: r = j0.38, P G 0.001) and directly associated with arterial diameter (IDR: r = 0.31; IDT: r = 0.27, P G 0.001). In multiple linear regression analysis, TyG Index was significantly and independently associated with wall shear stress (Table 2). To further minimize the possible contribution of type 2 diabetes, hyperlipidemia, and thus the influence of medications on fasting blood glucose and triglycerides, we performed a second linear regression analysis excluding subjects with type 2 diabetes, dyslipidemia, fasting blood glucose of greater than 100 mg/dL, and triglycerides of greater than 150 mg/dL. Twelve women and 43 men were analyzed. The regression analysis confirmed the independent association between TyG Index and wall shear stress (Table 3). These same analyses were also performed in male subjects only. Peak shear stress was inversely related with age (r2 = 0.57, P G 0.0001) and TyG Index (r2 = 0.61, P G 0.0001), whereas
FIGURE 1. Association between wall shear stress (peak and mean) values and TyG Index values (n = 212 samples, P G 0.00001 for all coefficients).
342
* 2014 The American Federation for Medical Research
Copyright © 2014 American Federation for Medical Research. Unauthorized reproduction of this article is prohibited.
Journal of Investigative Medicine
&
Volume 62, Number 2, February 2014
mean shear stress is inversely related with age (r2 = 0.52, P G 0.0001), TyG Index (r2 = 0.56, P = 0.006), and BMI (r2 = 0.57, P = 0.01). Multiple linear regression analyses were also performed using either left or right common carotid artery peak shear stress as dependent variable, and the independent association between TyG Index and wall shear stress was confirmed (data not shown).
DISCUSSION The present investigation demonstrates that IR, evaluated as TyG Index, is significantly and independently associated with carotid wall shear stress. Type 2 diabetes has previously been reported to be associated with decreased wall shear stress.6 The data of the present study suggest that the reduced sensitivity to insulin, even long before the onset of type 2 diabetes, is associated with a reduction of the wall shear stress, thus predisposing to the development of atheromatous plaques. The finding is particularly important because it is confirmed in subjects with normal glucose and triglyceride levels. Therefore, the IR is not only at the base of metabolic abnormalities but is also associated with hemodynamic alterations that promote atherosclerosis. Overall, this could help to explain the frequent finding of macrovascular complications at onset of type 2 diabetes. These results are in line with those obtained in other studies in which endothelial function was assessed. Quinn et al.17 have shown a reduced flow-mediated vasodilatation (that is, dependent from the increase in shear stress) in subjects with IR compared with control subjects. In the same study, the authors have shown that the administration of an insulin-sensitizing agent such as pioglitazone, a drug that counteracts the IR especially in the muscles, improved endothelial function. The possible mechanisms by which IR causes hemodynamic changes can only be hypothesized. It is known that IR state, as well as aging or type 2 diabetes, is associated with an increase in vessel diameter. The reasons for this are numerous and have been widely discussed in the literature.6,18Y20 As a matter of fact, subjects with IR have arterial diameters frankly higher than those of healthy subjects and consequently also reduced blood velocity. Because blood velocity and diameter are the main determinants of wall shear stress, it is reasonable to expect a reduction of shear stress in IR state. In line with these concepts, in the present study, TyG Index was directly related with carotid ID and inversely with systolic peak velocity, whereas no association with blood viscosity was found. It is known that IR stimulates myocyte growth and interstitial fibrosis, leading to ventricular function impairment.21,22 Furthermore, IR compromises microvascular circulation, increasing peripheral resistance and then cardiac afterload.23 The combined action of these factors could impair cardiac contractile force, thereby reducing ejection fraction and consequently the blood flow velocity. Of
TABLE 2. Results of Multiple Stepwise Regression Analysis in the Whole Population Step 1 2 3
Variable
A Coefficient
R2
P
Age TyG Index BMI
j0.625 j0.163 j0.142
0.53 0.57 0.58
G0.001 0.001 0.004
Dependent variable: peak shear stress. Sex, diabetes mellitus, hypertension, cigarette smoking, triglycerides, glucose, total cholesterol, LDL cholesterol, and HDL cholesterol did not enter in the model.
TyG Index and Shear Stress
TABLE 3. Results of Multiple Stepwise Regression Analysis in Subjects Without Diabetes and Hyperlipidemia Step 1 2 3
Variable
A Coefficient
R2
P
Age TyG Index BMI
j0.680 j0.157 j0.143
0.62 0.65 0.66
G0.001 0.009 0.026
Dependent variable: peak shear stress. Sex, hypertension, cigarette smoking, triglycerides, glucose, total cholesterol, LDL cholesterol, and HDL cholesterol did not significantly improve the regression mode.
great importance is also the role of arterial diameter. In fact, as above reported, the increased vessel diameter,18Y20 typical of the IR, further reduces blood flow velocity, and both these factors determine the hemodynamic force derangement. Our data demonstrate that changes in blood velocity and diameter begin to occur very early and are strongly correlated with the presence of IR. The findings of the present study are intriguing, considering the significant role held by local hemodynamic forces in vascular physiology and pathophysiology. Wall shear stress, the frictional force that flowing blood generates tangentially to the endothelial surface, strongly influences arterial remodeling and the vessels’ health.1,24 In regions where low endothelial shear stress occurs, the atheroprotective genes are suppressed, whereas the proatherogenic genes are up-regulated, thereby promoting the atherosclerotic process.24 A particularly important point of the present study is whether the TyG Index truly reflects IR. The hyperinsulinemic euglycemic clamp is the criterion standard for IR evaluation, but it is difficult to use in daily clinical practice. On the other hand, TyG Index is simple to calculate based on the product of plasma triglycerides and glucose. It is based on the assumption that any increase in plasma triglycerides interferes with the normal metabolism of glucose in the muscle, thereby causing a reduced sensitivity to insulin.25 Even if this surrogate measure is not widely used yet, previous studies have demonstrated the high sensitivity and specificity of TyG Index and its high correlation with glucose clamp, and the Homeostasis Model Assessment of IR index. Furthermore, we have recently demonstrated that TyG Index is strongly related with carotid atherosclerosis in a large population-based study.16 Based on these previous studies, we believe that the TyG Index may represent a good approximation of the IR state and can usefully be used in large clinical trials.
CONCLUSIONS In conclusion, IR is a complex, systemic condition promoting the development of a wide spectrum of metabolic diseases such as hypertension, type 2 diabetes, dyslipidemia, and metabolic syndrome, conditions known to be associated with atherosclerosis and major cardiovascular event. The present study suggests that IR has probably important effects also on hemodynamic forces, favoring the expression of an atherosclerosis prone profile of the endothelial cells, at least in the common carotid arteries. REFERENCES 1. Irace C, Cortese C, Fiaschi E, et al. Wall shear stress is associated with intima-media thickness and carotid atherosclerosis in subjects at low coronary heart disease risk. Stroke. 2004;35:464Y468. 2. Jiang Y, Kohara K, Hiwada K. Association between risk factors for atherosclerosis and mechanical forces in carotid artery. Stroke. 2000;31:2319Y2324.
* 2014 The American Federation for Medical Research
Copyright © 2014 American Federation for Medical Research. Unauthorized reproduction of this article is prohibited.
343
Tripolino et al
Journal of Investigative Medicine
&
Volume 62, Number 2, February 2014
3. Stone PH, Coskun AU, Kinlay S, et al. Regions of low endothelial shear stress are the sites where coronary plaque progresses and vascular remodelling occurs in humans: an in vivo serial study. Eur Heart J. 2007;28:705Y710.
15. Guerrero-Romero F, Simental-Mendı´a LE, Gonza´lez-Ortiz M, et al. The product of triglycerides and glucose, a simple measure of insulin sensitivity. Comparison with the euglycemic-hyperinsulinemic clamp. J Clin Endocrinol Metab. 2010;95:3347Y3351.
4. Duivenvoorden R, Vanbavel E, de Groot E, et al. Endothelial shear stress: a critical determinant of arterial remodeling and arterial stiffness in humans-a carotid 3.0-T MRI study [published correction appears in Circ Cardiovasc Imaging 2011;1;4:e3]. Circ Cardiovasc Imaging. 2010;3:578Y585.
16. Irace C, Carallo C, Scavelli FB, et al. Markers of insulin resistance and carotid atherosclerosis. A comparison of the Homeostasis Model Assessment and Triglyceride Glucose Index. Int J Clin Pract. 2013;67:665Y672.
5. Gnasso A, Carallo C, Irace C, et al. Association between intima-media thickness and wall shear stress in common carotid arteries in healthy male subjects. Circulation. 1996;94:3257Y3262. 6. Irace C, Carallo C, Crescenzo A, et al. NIDDM is associated with lower wall shear stress of the common carotid artery. Diabetes. 1999;48:193Y197. 7. Irace C, Carallo C, De Franceschi MS, et al. Human common carotid wall shear stress as a function of age and gender: a 12-year follow-up study. Age (Dordr). 2011;34:1553Y1562. 8. Irace C, Mancuso G, Fiaschi E, et al. Effect of anti TNFalpha therapy on arterial diameter and wall shear stress and HDL cholesterol. Atherosclerosis. 2004;177:113Y118. 9. Simon AC, Pithois-Merli I, Levenson J. Physiopharmacological approach to mechanical factors of hypertension in the atherosclerotic process. J Hum Hypertens. 1991;5(suppl 1):15Y21. 10. DeFronzo RA, Tobin JD, Andres R. Glucose clamp technique: a method for quantifying insulin secretion and resistance. Am J Physiol. 1979;237:E214YE223.
17. Quinn CE, Lockhart CJ, Hamilton PK, et al. Effect of pioglitazone on endothelial function in impaired glucose tolerance. Diabetes Obes Metab. 2010;12:709Y715. 18. Crouse JR, Goldbourt U, Evans G, et al. for the ARIC Investigators: risk factors and segment-specific carotid arterial enlargement in the Atherosclerosis Risk in Communities (ARIC) Cohort. Stroke. 1996;27:69Y75. 19. Bonithon-Kopp C, Touboul PJ, Berr C, et al. Factors of carotid arterial enlargement in a population aged 59 to 71 years: the EVA Study. Stroke. 1999;27:654Y660. 20. Iannuzzi A, De Michele M, Bond MG, et al. Carotid artery remodeling in middle-aged women with the metabolic syndrome (from the ‘‘Progetto ATENA’’ study). Am J Cardiol. 2005;96:1162Y1165. 21. Kosmala W, Jedrzejuk D, Derzhko R, et al. Left ventricular function impairment in patients with normal-weight obesity: contribution of abdominal fat deposition, profibrotic state, reduced insulin sensitivity, and proinflammatory activation. Circ Cardiovasc Imaging. 2012;5:349Y356.
11. Muniyappa R, Lee S, Chen H, et al. Current approaches for assessing insulin sensitivity and resistance in vivo: advantages, limitations, and appropriate usage. Am J Physiol Endocrinol Metab. 2008;294:E15YE26.
22. Kosmala W, O’Moore-Sullivan TM, Plaksej R, et al. Subclinical impairment of left ventricular function in young obese women: contributions of polycystic ovary disease and insulin resistance. J Clin Endocrinol Metab. 2008;93:3748Y3754.
12. Wannamethee SG, Lowe GD, Shaper AG, et al. Insulin resistance, haemostatic and inflammatory markers and coronary heart disease risk factors in Type 2 diabetic men with and without coronary heart disease. Diabetologia. 2004;47:1557Y1565.
23. Clark MG, Wallis MG, Barrett EJ, et al. Blood flow and muscle metabolism: a focus on insulin action. Am J Physiol Endocrinol Metab. 2003;284:E241YE258.
13. Simental-Mendı`a LE, Rodrı`guez-Mora`an M, Guerrero-Romero F. The product of fasting glucose and triglycerides as surrogate for identifying insulin resistance in apparently healthy subjects. Metab Syndr Relat Disord. 2008;6:299Y304.
24. Chatzizisis YS, Coskun AU, Jonas M, et al. Role of endothelial shear stress in the natural history of coronary atherosclerosis and vascular remodeling: molecular, cellular, and vascular behavior. J Am Coll Cardiol. 2007;49:2379Y2393.
14. Vasques AC, Novaes FS, de Oliveira Mda S, et al. TyG Index performs better than HOMA in a Brazilian population: a hyperglycemic clamp validated study. Diabetes Res Clin Pract. 2011;93:e98Ye100.
25. Clerk LH, Rattigan S, Clark MG. Lipid infusion impairs physiologic insulin-mediated capillary recruitment and muscle glucose uptake in vivo. Diabetes. 2002;51:1138Y1145.
344
* 2014 The American Federation for Medical Research
Copyright © 2014 American Federation for Medical Research. Unauthorized reproduction of this article is prohibited.
