Evaluation of local carotid stiffness and inflammatory biomarkers in stable angina pectoris

Introduction

There is a variety of evidence on the crucial role of pre-clinical markers in determining the risk of cardiovascular (CV) morbidity and mortality in individuals. Arterial stiffness (AS) parameters are among these markers that are commonly used to identify structural and functional changes in the arteries during the atherosclerotic process [1, 2]. Diverse clinical studies have demonstrated the independent predictive value of AS for CV events in patients with hypertension (HT), diabetes mellitus (DM), end-stage renal disease, in elderly patients and in the general population [1, 3–6]. Based on this evidence, AS is considered a useful marker in the assessment of CV risk and diagnosis of subclinical CV disease [7]. Arterial stiffness can be measured locally in the common carotid artery (CCA) through carotid pulse wave velocity (PWV). Measurement of PWV is the simplest non-invasive, rapid and widely used method in the evaluation of large AS [8]. The localized PWV of an artery has a direct association with elastic modulus [9]. Recently, several methods have emerged for the estimation of PWV using ultrasound principles [1]. Echo-tracking systems (ETS) – high-resolution wall-tracking systems that analyze the raw radiofrequency (RF) signals – have been developed [10]. They provide optimal conditions for the precise determination of local AS, which is directly determined, requiring no assumption from models of circulation [1]. These systems are able to track the arterial wall movement in real time and to measure the carotid arterial distension with an error less than 5 μm [11]. They also have the advantage of automatic measurement of carotid intima-media thickness (IMT) simultaneously with carotid PWV. Carotid IMT reflects the presence of atherosclerotic plaques and the general atherosclerotic burden [12, 13]. Carotid IMT evaluation by ultrasound allows an easy, safe, non-invasive and cost-effective method to detect subclinical atherosclerosis and has been shown to be a prognosticator of CV events [14].

With the recognition of the crucial link between arterial damage, inflammation and coronary atherosclerosis, inflammatory biomarkers such as high-sensitivity C-reactive protein (hsCRP) and fibrinogen estimation have assumed a vital role in cardiac risk assessment. There is an increasing amount of evidence that hsCRP is increased in patients with CV disease, and elevated levels of this marker usually precede the development of CV disease [15]. Among patients with stable angina pectoris (SAP), hsCRP levels have been related to an increased vascular event rate [16]. Fibrinogen is also an important inflammatory biomarker in addition to its crucial role in coagulation. Previous studies have found that the incidence of coronary artery atherosclerosis and CAD increases significantly as the level of fibrinogen becomes elevated [17, 18].

Aim

The present study aimed to study local CS that was measured by a high-spatial resolution ETS (MyLab25; ESAOTE, Genoa, Italy) in addition to hsCRP and fibrinogen levels among patients presenting with SAP. This is the first study evaluating the association between local CS parameters and fibrinogen level in SAP patients.

Material and methods

Assessment of local carotid arterial stiffness

For the evaluation, subjects lay down in the supine position in a quiet and temperature-controlled room (23°C) for 10 min. An independent radiologist trained and certified in vascular echography performed blood pressure (BP) and arterial measurements. Before the ultrasound examination, BP measurements (both systolic blood pressure (SBP) and diastolic blood pressure (DBP)) of the patients were taken from the dominant arm by an aneroid sphygmomanometer at 5-minute intervals for 15 min and the average was taken as the casual BP level. Mean blood pressure (MBP) was calculated as: MBP = DBP + [(SBP – DBP)/3]. Brachial pulse pressure (PP) was calculated as: PP = SBP – DBP.

All patients underwent common carotid artery (CCA) measurements with a novel system (MyLab25 QAS machine; ESAOTE, Genoa, Italy) based on an automatic high resolution echo-tracking technology (WallTrack system) that utilized a 7.5–12 MHz linear array transducer. The systolic and diastolic carotid diameters were automatically measured on the distal wall of the left and right CCA 2 cm below the bifurcation. Concisely, variations in momentary arterial diameter during the cardiac cycles were displayed in real time on a graphic giving the operator visual feedback to adjust the quality of the image before measurement acquisition (Figure 1). Carotid diameter was calculated as the distance between media-adventitia interfaces. Systolic and diastolic values were determined for each cardiac cycle and carotid distension was calculated as the mean value of their difference in the last 8 s of the image sequences. The cross-sectional distensibility coefficient (DC) was estimated through the variations in arterial cross-sectional area and blood pressure during systole as described previously [19]. Distensibility coefficient was calculated as DC = ΔA/(A × ΔP), where A is the diastolic lumen area, ΔA is the stroke change in lumen area, and ΔP is local PP. Lumen area and stroke change in lumen area were calculated from diameter and distension values, assuming the cross section of the artery to be circular. Finally, DC was converted into carotid stiffness (CS) using the Moens-Korteweg equation, PWV = [(PP × A)/(ΔA × ρ)]^1/2, where ρ is the blood density. This formula gives CS = (ρ × DC)^–1/2, expressed in m/s, allowing a direct comparison with carotid-femoral PWV. Common carotid IMT was automatically and simultaneously measured as the mean of the relative values of the last 8 s on the same image sequences. Carotid IMT was measured in the far wall of the CCA, 1 cm proximal to the carotid bulb. Parameters are indicated as the mean of the right and left CCA value.

Figure 1

Measurement of carotid intima-media thickness by Esaote wall tracking (QAS, MyLab) system. The movement of carotid walls is tracked in the entire region of interest (green rectangle) composed of 32 scanning lines. Continuous red lines indicate the automatic positioning of wall-tracking points at the media-adventitia interface. Continuous green lines show dynamically the amplified vessel wall movement. The values of carotid intima-media thickness (QIMT) and minimum diameter (D) are displayed on the screen, and the average value (AVG) over the last six beats and SD are continuously calculated.

Results

Discussion

This study evaluated AS parameters using carotid PWV by the local measurement method on the carotid arterial wall in SAP patients. Additionally, hsCRP and fibrinogen levels were measured to investigate the effect of inflammation on CS parameters and the severity and extent of CAD. As a result, a positive association between carotid PWV, intima-media thickness, hsCRP and fibrinogen levels and the extent of CAD was found. In addition, a significant positive correlation was detected between CS, carotid IMT, fibrinogen and hsCRP levels.

The pathophysiology of the coronary atherosclerotic process is complex, involving endothelial cell injury, lipid infiltration and conversion of macrophages into lipid-laden foam cells which closely correlate with inflammation [20]. The inflammatory response is a crucial point in the occurrence, development and progression of CAD. Therefore, serum levels of inflammatory markers can give a clue about the disease severity and may be used to predict acute coronary events [21]. In the present study, we found that the serum hsCRP and fibrinogen levels were related to CAD severity as higher serum levels of fibrinogen and hsCRP were determined in patients with single-, double- and triple-vessel disease. Liang et al. [22] also observed significantly higher hsCRP levels in patients with double- and triple-vessel disease. Current studies suggest that fibrinogen similar to hsCRP is an important inflammatory factor [23]. Therefore, an increase in fibrinogen level enhances the risks of atherosclerosis and CV events [17, 18]. The Atherosclerosis Risk in Communities Study showed that elevated levels of fibrinogen are a risk factor in CAD. Karahan et al. [24] also showed serum fibrinogen level as a significant inflammatory marker for predicting the severity of CAD. Similar to these studies, plasma fibrinogen levels also demonstrated a stepwise increase from the control group to the triple-vessel group in our study. Moreover, a positive correlation between fibrinogen and hsCRP levels and carotid PWV was detected. These data indicate that AS is correlated with systemic inflammation and the inflammation plays an important role in the development of arterial wall stiffness. Increased AS is considered as one of the key factors related to CV disease [25, 26]. It causes an increase in cardiac ventricular load, thereby reducing cardiac systolic performance and increasing myocardial oxygen demand [27]. This may lead to the progression of atherosclerosis, likely mediated by increased cycle stress resulting in arterial wall thickening. Previous studies have shown that increased AS prevents the hemodynamic buffering effect of the CV system, contributing to elevated SBP and PP, CAD and left ventricular hypertrophy [25, 28]. The PWV is defined as the velocity at which the pressure waves, generated by the systolic contraction of the heart, propagate along the arterial tree. The evaluation of PWV provides complementary information about the elastic properties of the arterial system. The higher PWV corresponds to lower vessel distensibility and compliance and, therefore, to higher AS. Increased PWV is associated with a significant CV risk in the general population [29, 30]. It is an important marker of AS and is now widely accepted as the gold standard measure of AS [1]. There are various invasive and non-invasive methods in determining PWV. The non-invasive measurement of AS is usually accomplished by a set of devices that measure PWV and that provide the capability to perform pulse pressure waveform analysis. Because, the local PWV measurement is made on a short segment of an artery and represents an early diagnostic tool able to identify the local stiffness of the arterial wall [31]. A non-invasive color Doppler ultrasound machine was used in this study to measure local CS. The CS is usually determined cross-sectionally from local changes in pressure and artery diameter [32]. On clinical application, in the early stage of atherosclerotic disease, the elastic properties will be affected locally and thus the functional properties of the local large arteries can be analyzed with local assessment of PWV [33]. Gaszner et al. [34] reported that carotid PWV in CAD patients was significantly higher than the control group with normal coronary arteries (7.4 ±1.3 vs. 6.5 ±1.1 m/s, p < 0.01). Similar findings were detected in the present study.

Another finding of this study concerns the observation that carotid IMT, measured simultaneously on the same image sequence and on the same tract of CCA, was significantly greater in patients with triple-vessel disease as compared to other groups. There are many reports determining the association between carotid IMT and atherosclerotic lesions in a single arterial territory such as coronary, renal or lower limb arteries [35–37]. In another study, the mean carotid IMT was detected to be increased with greater atherosclerosis extent, and it may be translated into the number of territories with significant stenosis [12]. Additionally, there was a significant positive correlation between carotid IMT and hsCRP level and carotid PWV. Kablak-Ziembicka et al. [38] investigated the relationship between carotid IMT, inflammatory biomarkers and atherosclerotic extent. Similarly to our results, they found a significant relation between carotid IMT value and atherosclerosis extent. Moreover, they observed more advanced atherosclerosis with a higher baseline hsCRP level. The results of the Three-City Study documented the importance of the simultaneous measurement of both CS and carotid IMT in the same vessel [39]. According to this study, an association between CS and carotid IMT was detected, similarly to our study. Carotid IMT and CS provide structural and functional data, respectively, and assessment of both parameters gives complementary information on the carotid vessel.

There was not a significant relationship of PWV with the lipid profile of the patients including TC, LDL cholesterol, HDL cholesterol and TG in this study. Although positive and negative relations of AS with cholesterol level have been reported using different methods, most studies using PWV have determined no correlation with TC [28, 40]. Our findings were in agreement with a recent large population-based study by Amar et al. [28]. They detected no significant association between PWV and TC, TG and HDL cholesterol as well.

On the other hand, a significantly negative correlation was determined between PWV and eGFR. AS is a highly prevalent condition in patients with chronic kidney disease, and recent studies showed the independent impact of AS on mortality and morbidity in this patient population [41]. Sedaghat et al. studied 3666 patients and also obtained results similar to our study that higher indices of AS which was evaluated using PWV are related to a steeper decline in kidney function [42].

All these results are in line with the previous data. However, to the best of our knowledge, this is the first study evaluating the association between fibrinogen level and local CS parameters in SAP patients. Also, AS parameters were obtained from a novel RF-based technique requiring specific and sophisticated ultrasound equipment. A standard high definition ultrasound machine that automatically measures carotid PWV and IMT, both of which are pre-clinical indicators of atherosclerosis, was used for early prediction of CAD. The RF-based ETS implemented in standard ultrasound equipment may provide a quick and simple tool to evaluate local AS during routine measurements of IMT. Such a combined evaluation of local arterial structure and function may further improve the assessment of vascular aging and early atherosclerotic injury.

There are some limitations of this study. First, our study groups were composed of a relatively small number of individuals. Second, we used brachial BP measurement for the calibration of carotid diameter changes. Brachial pressure usually overestimates the central pressure, especially in young subjects [43]. Third, carotid artery elastic parameters were measured locally at a specific site; therefore stiffening of the carotid arterial wall can be attributed to an anatomically defined segment of the arterial tree. Hence, further large-scale studies are required to generalize the clinical significance of our findings and to use ETS in routine clinical practice for the assessment of vascular function and CAD risk.

Conclusions

Carotid stiffness and carotid IMT measured using the echo-tracking technique in addition to fibrinogen and hsCRP levels were well correlated with the presence and extent of CAD in patients with SAP. Thus, local carotid arterial stiffness and function measurements together with fibrinogen and hsCRP level may provide additional benefit in clinical practice for the evaluation of patients suspected to have CAD. Therefore, this novel method together with measurement of the above inflammatory biomarkers may be used to detect high-risk SAP patients and accordingly to plan the clinical and medical follow-up.

Conflict of interest

The authors declare no conflict of interest.