The relationship between the existence of angiographic coronary artery calcification and the severity of coronary artery disease

Introduction

Cardiovascular disease is the leading cause of death in the western world and in Turkey [1, 2]. Coronary artery calcification (CAC) is the accumulation of calcification in atherosclerotic plaques, and is an indicator of coronary atherosclerosis [3-7]. It is also a subclinical predictor of future coronary events [8, 9]. It can be detected by fluoroscopy, multi-slice computed tomography (CT), and intravascular sonography [10]. Risk factors for CAC are similar to those for coronary artery disease (CAD), and include male sex, older age, chronic kidney disease, and low educational status [11, 12]. Many studies have investigated the relationship between CAC and the percentage of coronary artery stenosis; however, no research has been done to determine if any relationship exists between CAC and either the severity or extent of CAD.

Aim

In this study, we primarily aimed to explore whether there are any such associations.

Material and methods

Patients and study design



After the study had been approved by the local ethics committee of Harran University School of Medicine, 282 consecutive CAD patients who had been referred to our clinic for coronary angiography were recruited. All patients provided written informed consent. Patient characteristics and baseline data were recorded. Patients were classified as having hypertension if their blood pressure was over 90 mm Hg or systolic blood pressure over 140 mm Hg; if they were already using hypertensive medication; or if hypertension had already been diagnosed, as indicated in their previous medical records. Diabetes mellitus (DM) was diagnosed in accordance with the ADA criteria, of if the patient was already on diabetes medication. Height, weight and waist circumferences were measured according to a standardized protocol. All patients underwent coronary angiography. We then allocated the patients to one of two groups, according to the presence or absence of CAC. The CAC(+) group consisted of 126 patients (mean age = 63.18 ±9.8 years; 45 women, 81 men), the CAC(–) group

156 patients (59.48 ±11.0 years; 58 women, 98 men). Patients without angiographic lesions were considered to be without CAD or normal The severity and extent of coronary atherosclerosis were calculated using Gensini scores. Exclusion criteria were impaired renal function, unsatisfactory visualization of the coronary arteries, calcification of the aortic valve or mitral annuli, constrictive pericardial disease, and pleural calcification. Type 1 diabetes patients were excluded because of inadequate sample size

(5 patients).



Patient catheterization and determining CAC calcification



All patients were catheterized percutaneously via either the right femoral artery using the standard Judkins technique, or the right brachial artery by the Sones technique. Before we administered opaque material, throughout the coronary artery, trace CAC deposits were identified by visual examination during coronary angiography. Working together, two experienced physicians interpreted each coronary angiogram.



Assigning a Gensini score



A Gensini score was assigned to each patient as per the published protocol. When generating a Gensini score, as a first step, narrowing of the lumen of any coronary artery is assigned a grade of ‘1’ for 1-25% narrowing, ‘2’ for 26-50% narrowing, ‘4’ for 51-75% narrowing, ‘8’ for 76-90% narrowing, ‘16’ for 91-99% narrowing, and ‘32’ for total occlusion. This score then is multiplied by a factor that takes into account the importance of the lesion’s position in the coronary arterial tree. The position score that is assigned is ‘5’ for the left main coronary artery; ‘2.5’ for the proximal left anterior descending (LAD) and proximal left circumflex (LCX; or 3.5 if the LCX is dominant); ‘1.5’ for the mid-region of the LAD; ‘1’ for the distal LAD, the first diagonal, the proximal right coronary artery (RCA), the mid RCA, the distal RCA, the posterior descending, the distal LCX, the mid LCX (‘2’ if the LCX is dominant), and the obtuse margin; and ‘0.5’ for the second diagonal and the posterolateral branch. The final Gensini score is expressed as the sum of all the individual coronary artery scores (14); for example, if a patient had 30% narrowing of the left main coronary artery, 60% narrowing of the proximal LAD and 20% narrowing of the distal LCX, that patient’s summation score would be calculated as (2 × 5) + (4 × 2.5) +

(1 × 1) = 21. To aid in the generation of Gensini scores, at least 5 different planes of view were obtained for each patient (right anterior oblique caudal, right anterior oblique cranial, left anterior oblique cranial, left anterior oblique caudal, and antero-posterior cranial).



Statistical analysis



All data were analyzed using SPSS version 11.5 (SPSS Inc., Chicago, IL, USA). Continuous variables were expressed as means ± SD, and categorical variables as percentages. Between-group analyses were assessed by independent Student’s t-tests for continuous variables and by Pearson’s 2 analysis for categorical variables. Multivariate logistic regression analysis was performed to identify independent predictors of significant CAC. For all statistical analyses, a p-value of < 0.05 was considered statistically significant.

Results

Clinical characteristics, laboratory variables, and the Gensini scores for the CAC(+) and CAC(–) groups are presented in Table 1. Age, hypertension, cigarette smoking, body mass index (BMI), waist circumference, systolic blood pressure, fasting blood glucose, urea, hemoglobin, and leukocyte and platelet counts were not significantly different between the CAC(+) and CAC(–) groups. Serum levels of triglyceride (155.72 mg/dl vs. 194.42 mg/dl; p < 0.028) and VLDL cholesterol (31.00 mg/dl vs. 40.31 mg/dl;

p < 0.023) were significantly lower in the CAC(+) group than in the CAC(–) group. Conversely, diastolic blood pressure levels were higher in the CAC(+) group (83.06 mm Hg vs. 79.52 mm Hg; p = 0.038) (Table 1), and DM was seen more frequently (42.0% vs. 30.5%; p = 0.026) (Table 1). Upon logistic regression analysis, the Gensini score was found to be an independent determinant of CAC ( = –0.017,

2 = 4.35, p = 0.007) (Table 2).

Discussion

Coronary artery calcification begins shortly after fatty streak formation [13], a finding that should be detectable by microscopic methods [14]. The CAC consists of small aggregates of crystalline calcium amid a core of lipid particles in the atherosclerotic plaque [13-15]. Calcification in the atherosclerotic plaque is an organized, regulated process, similar to bone formation. Non-hepatic Gla-containing proteins, such as osteocalcin, which are actively involved in the transport of calcium out of vessel walls, are suspected to have key roles in the pathogenesis of CAC [7]. Known to be involved in bone mineralization, osteopontin and its mRNA have been identified in calcified atherosclerotic lesions [7]. Calcified human atherosclerotic plaque also contains protein-2a, a potent factor for osteoblastic differentiation, as well as cells that are capable of osteoblastic differentiation. These cells may be those from which vascular calcifying cells are derived. These and other recent findings indicate that calcification is an active process and not simply the passive precipitation of calcium phosphate crystals, as once thought. Thus, it has become well known that accelerated active calcification in the coronary arteries is an indicator of atherosclerosis, and that CAC does not exist on normal vessel walls [16]. However, a clinically more important point pertains to the severity and extent of the atherosclerosis. Consequently, we focused on whether coronary calcification is also an indicator of the severity and extent of atherosclerosis, as well as its presence.

Screening for evidence of CAC is done to evaluate patients with chest pain, to screen asymptomatic subjects, and to follow the progression of coronary atherosclerosis [7]. Coronary artery calcification can be identified using either fluoroscopy or computed tomography. Of these, fluoroscopy is the more frequently used, as we did in this study. The sensitivity of fluoroscopy in determining significant stenosis (greater than 50% occlusion) in patients with CAC has ranged from 40% to 79%, and its specificity from 52% to 95% [7, 17, 18].

The results of this study provide several new insights. First, to our knowledge, this is the first study to compare coronary calcification in type 2 diabetics versus non-diabetics. We identified a significant difference between diabetic and non-diabetic patients in terms of CAC, but there was no gender effect, the calcification ratio being no different between males and females. Colhoun et al. previously reported a significant difference between the genders; however, their study was of type 1, not type 2, diabetics. Their coronary calcification rates were 21% in non-diabetic females and 47% in diabetic females, a difference that was not apparent in males (55% vs. 52%) [19]. In truth, this result was unexpected and still appears somewhat conflicting. In our study, we found no difference between diabetic and non-diabetic males, or between diabe­tic and non-diabetic females. However, we studied type 2 diabetes patients. Consequently, the contradictory results between the two studies likely arose because of the different patient populations that were studied.

Another finding of our study was that the frequency of coronary calcification was no different in patients with hypertension or dyslipidemia, or in smokers compared to non-smokers. No other studies have compared CAC in hypertensive and non-hypertensive patients. In two previous studies, coronary calcification was found to be associated with hypertension, but the subjects in both studies were chronic renal failure patients, in whom widespread calcification is not an unusual finding [20, 21]. In terms of dyslipidemia, the results of some studies suggest that higher non-HDL cholesterol is associated with higher, and higher HDL-C with lower electron beam computed tomography (ECBT) CAC scores [22, 23]. In our study, triglyceride (TG) and VLDL levels were elevated in CAC patients, but regression analysis failed to reveal a significant correlation.

Third, when we entered variables that were statistically different between CAC(+) and CAC(–) patients (age, diastolic blood pressure, triglyceride, VLDL, diabetes mellitus, and Gensini score) into a regression model, we found that only the Gensini score (extensiveness of coronary artery disease) remained as an independent predictor of CAC. Diabetes mellitus was not found to be an independent determinant of CAC, despite CAC being prominent in diabetes patients. What this tells us is that these other variables exert whatever influence they have through diabetes, rather than directly.

As a fourth finding, though previous studies have shown that calcific deposits are more prevalent and present in greater amounts in elderly individuals and those with more advanced atherosclerotic lesions [15, 16], we identified no such association between the presence or amount of calcific deposits and age.

Finally, perhaps the most important and pioneering finding of this study pertained to the Gensini score, a known determinant of both the severity and extent of coronary artery disease. In this study, the first to have sought any relationship between this score and CAC, we found that it was independently related to CAC.

In conclusion, coronary calcification appears to be more prevalent in type 2 diabetes mellitus patients than in the normal population, though no gender difference exists. Other risk factors for CAD – such as smoking, hypertension, and dyslipidemia – seemed to be unrelated to CAC. Previous studies have demonstrated that the amount of CAC is correlated with the total plaque burden in coronary arteries [4, 16, 24-27]. Our data show that the coronary artery calcification detected by coronary angiography is indeed related to CAD location and severity, through the Gensini score. In other words, the Gensini score (the extensiveness of coronary artery disease) is independently associated with the presence of CAC. Clearly, the usefulness and application of this relationship warrant further clinical investigation.

Acknowledgments

The authors do not report any conflict of interest regarding this work. The poster of this article was awarded as the best poster presentation at the 25th National Cardiology Congress which was held during 22-25 October, 2009 in Istanbul, Turkey. This poster was published in the congress book. This congress was credited by EBAC (European Board for Accreditation in Cardiology).

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