a Department of Epidemiology & Biostatistics, Erasmus MC, Rotterdam, The Netherlands
b Department of Radiology, State University Groningen/Academic Hospital Groningen, Groningen, The Netherlands
c Department of Radiology, Daniel den Hoed Clinic, Erasmus MC, Rotterdam, The Netherlands
* Correspondence to: Dr J. C. M. Witteman, Department of Epidemiology & Biostatistics, Erasmus MC, P.O. Box 1738, 3000 DR Rotterdam, The Netherlands. Tel: +31 10 4087365; Fax: +31 10 4089382
E-mail address: j.witteman{at}erasmusmc.nl
Received 27 May 2003; revised 3 October 2003; accepted 16 October 2003
Abstract
Aims We examined associations between cardiovascular risk factors and coronary calcification assessed by electron-beam tomography (EBT) in an unselected population of older subjects.
Methods and results The Rotterdam Coronary Calcification Study is a population-based study in subjects >=55 years. Participants underwent EBT scanning. Coronary calcification was quantified according to the Agatston score. Cardiovascular risk factors were assessed 7 years before and concurrently to scanning. We used the first 2013 participants for the present analyses. Risk factors assessed 7 years before scanning were strongly associated with calcium score. Associations with blood pressure and cholesterol attenuated when measured concurrently to scanning. Although the number of risk factors was strongly associated with a high calcium score in asymptomatic subjects, 29% of the men and 15% of the women without risk factors had a high calcium score.
Conclusions This population-based study in older subjects shows that cardiovascular risk factors are associated with coronary calcification. Associations were stronger for risk factors measured at earlier age. Almost 30% of the men and 15% of the women without risk factors had extensive coronary calcification.
Key Words: Atherosclerosis Calcium Tomography Coronary disease Risk factors
1. Introduction
Cardiovascular risk factors like obesity, hypertension, hypercholesterolaemia, diabetes and smoking are associated with atherosclerosis at different sites17and with an increased risk of coronary heart disease.812Several population-based studies have investigated the association between cardiovascular risk factors and coronary calcification. In asymptomatic adults, these cardiovascular risk factors were strongly associated with the amount of coronary calcification.1324However, only one population-based study has been performed in older subjects (mean age 80 years).25In the latter study, only smoking and triglycerides were associated with coronary calcification.
We investigated the associations of cardiovascular risk factors and coronary calcification in a population of older subjects. Risk factors were measured on average 7 years before and at the time of electron-beam tomography (EBT) scanning.
2. Methods
2.1. Study population
The Rotterdam Coronary Calcification Study is designed to study determinants and consequences of coronary calcification,detected by EBT. The study is embedded in the Rotterdam Study. The Rotterdam Study is a population-based study, which started with a baseline visit between 1990 and 1993. All inhabitants of a suburb of Rotterdam, aged 55 years and over, were invited (response 78%). The rationale and design of the Rotterdam Study have been described elsewhere.26Follow-up visits took place in 1993 to 1994 and 1997 to 1999. From 1999 onwards the study population is extended with a second cohort comprising inhabitants who reached the age of 55 years after the baseline examination in 1990 to 1993 and subjects aged 55 years and over who migrated into the research area.
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2.2. Coronary calcification
We assessed coronary calcification in the epicardial coronary arteries detected on EBT scans. Imaging was performed with a C-150 Imatron scanner (Imatron, South San Francisco, California, U.S.A.). Before the subjects were scanned, they exercised breath holding. From the level of the root of the aorta through the heart, 38 images were obtained with 100ms scan time and 3mm slice thickness. We acquired images at 80% of the cardiac cycle, using electrocardiogram triggering, during a single breath-hold. The scanner was calibrated on a daily basis using a water phantom. Quantification of coronary calcification was performed with AccuImage software (AccuImage Diagnostics Corporation, South San Francisco, California, U.S.A.) displaying all pixels with a density of over 130 Hounsfield Units. Trained scan readers were blinded to the clinical data of the participants. The presence of calcification was defined as a minimum of two adjacent pixels (area=0.52mm2) with a density over 130 Hounsfield Units. We placed a region of interest around each high-density lesion in the epicardial coronary arteries. The peak density in Hounsfield Units and the area in mm2of the individual coronary calcifications were calculated. A calcium score was obtained by multiplying each area of interest with a factor indicating peak density within the individual area, as proposed by Agatston et al.27We added the scores for individual calcifications, resulting in a calcium score for the entire epicardial coronary system.
2.3. Cardiovascular risk factors
The Rotterdam Coronary Calcification Study is embedded in the ongoing Rotterdam Study. Therefore, information was available on risk factors assessed 7 years before EBT scanning (19901993) and concurrent to scanning (19972000). Apart from blood sampling methods, protocols for the interview and clinical examination were identical at both examinations. Information on smoking and medication was obtained during the home interview of the Rotterdam Study and the number of packyears of smoking was computed. Clinical measures were obtained during a visit at the Rotterdam Study research centre. Height and weight were measured and body mass index (BMI) was calculated (weight (kg)/height (m)2). We defined obesity as a BMI>=30. Blood pressure was measured at the right brachial artery using a random-zero sphygmomanometer with the participant in sitting position. The mean of two consecutive measurements was used in the analyses. We defined hypertensionas a systolic blood pressure>=160 and/or a diastolic bloodpressure>=100 and/or the use of blood pressure loweringmedication with the indication hypertension.
In 19901993 non-fasting blood samples were obtained while in 19972000 blood samples were obtained after an overnight of fasting. Between 1990 and 1993, serum total cholesterol was determined by an enzymatic procedure and high-density lipoprotein (HDL) was measured similarly after precipitation of the non-HDL fraction.28Between 1997 and 2000, serum total cholesterol was determined by an automated enzymatic procedure using Roche CHOD-PAP reagent kit and high-density lipoproteins (HDL) were measured with the Roche direct HDL cholesterolassay using PEG-modified enzymes and dextran sulphate. We defined hypercholesterolemia as a total cholesterol level>=6.2mmol/l and/or the use of cholesterol lowering medication. Glucose was determined enzymatically by the Hexokinase method. Diabetes was defined as the use of anti-diabetic medication and/or non-fasting glucose levels =11.1mmol/l (19901993) and/or fasting glucose levels =7.0mmol/l (19972000). Impaired glucose tolerance was defined as no use of anti-diabetic medication and/or non-fasting glucose levels of 7.811.0mmol/l (19901993) and/or fasting glucose levels of 6.17.0mmol/l (19972000).29
2.4. Statistical analysis
Median calcium scores were computed for age categories. The distribution of the residuals was highly skewed when we used the total calcium score for linear regression analysis. After log-transformation of the calcium score, the residuals were normally distributed with a constant variance. Therefore, log (total calcium score +1) was used for linear regression analysis. We used age and the cardiovascular risk factors as independent variables and the log calcium score as dependent variable. We computed the increase in log calcium score per standard deviation increase of the independent continuous variables. Since calcium scores were much higher in men than in women, all analyses were performed in men and women separately. In a multivariate model we entered age and all cardiovascular risk factors except diastolic blood pressure and computed regression coefficients for age and all entered cardiovascular risk factors. Regression coefficients for diastolic blood pressure were computed by entering age and all cardiovascular risk factors except systolic blood pressure. Regression analyses were repeated after exclusion of subjects with a history of coronary artery disease (myocardial infarction, percutaneous transluminal coronary angioplasty (PTCA), coronary artery bypass graft (CABG) and angina pectoris on the Rose questionnaire).30
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We added the number of risk factors (0, 1, 2, >=3). After exclusion of subjects with coronary artery disease, logistic regression analysis with age and dummy variables for 1, 2 and >=3 risk factors in the model was used to compute the percentage of subjects with a calcium score >400 (high calcium score according to Rumberger)31for the number of risk factors. Risk factors were defined as follows: (1) obesity, (2) hypertension, (3) hypercholesterolaemia, (4) diabetes and (5) current smoking. Median calcium scores were computed for subjects with and without a history of coronary artery disease. SPSS 11.0 for Windows (SPSS, Inc., Chicago, Illinois) was used for data analysis.
3. Results
Table 1shows the baseline characteristics of the study population. Electron-beam tomography scans wereobtained in 2063 subjects. Subjects undergoing an EBT scan had approximately the same levels of cardiovascular risk factors as the non-responders. There were slight differences between responders and non-responders in age (70.6 vs 72.4 years), sex (46% vs 38% male), BMI (27.0 vs 26.7kg/m2) and ever smoking (90% vs 86% for men, 53% vs 49% for women). Median calcium scores increased from 81 in men aged 6064 to 548 in men aged 8085. In women, median calcium scores increased from 8 to 135. Men had much higher calcium scores than women in all age categories: median calcium scores in men were on average five times higher than in women (Fig. 1).
Table 2shows that BMI, systolic blood pressure, diastolic blood pressure (not significant), total cholesterol, diabetes and smoking measured 7 years before scanning were positively associated with the calcium score while HDL-cholesterol was inversely associated with the calcium score. When we measured risk factors concurrently to EBT scanning, the strength of the association did not change for BMI. However, systolic and diastolic blood pressure were not associated with the calcium score when measured concurrently. After exclusion of subjects with blood pressure lowering medication, systolic blood pressure was positively associated with the calcium score although this did not reach significance in men while no association was present between diastolic blood pressure and the calcium score (data not shown). The association of total cholesterol and the calcium score disappeared when we measured cholesterol concurrently to coronary calcification. After exclusion of subjects with lipid lowering medication, total cholesterol was strongly associated with the calcium score in women while no association was present in men (data not shown). HDL-cholesterol measured at the time of scanning was only in women associated with the calcium score; in men no association was present. In women, the association between diabetes and coronary calcification was stronger when measured at the time of scanning. In men, the strength of the association between diabetes and coronary calcification slightly decreased. Associations for smoking were slightly stronger when measured at the time of scanning. Results slightly changed after exclusion of subjects with a history of coronary artery disease (Table 3). In men, body mass index measured 7 years before EBT scanning was not significantly associated with the calcium score and systolic blood pressure measured at the time of scanning was positively associated with the calcium score. In women, cholesterol measured at the time of scanning was positively associated with the calcium score.
Fig. 2shows geometric mean calcium scores for categories of risk factors measured 7 years before scanning. While the calcium score is only gradually increasing in the higher categories of BMI and smoking, a gradual increase in calcium score was seen for systolic blood pressure, diastolic blood pressure and total cholesterol. HDL-cholesterol was inversely associated with the calcium score. Compared to subjects with normal glucose tolerance, calcium scores were elevated in subjects with diabetes but not in subjects with an impaired glucose tolerance.
Fig. 3shows the percentage of subjects with a high calcium score according to the number of risk factors in subjects without a history of coronary artery disease. The number of risk factors measured concurrently to scanning was strongly associated with a high calcium score in both men and women: while 29% of the men without risk factor had a high calcium score, 52% of the men with three or more risk factors had a high calcium score. The corresponding percentages in women were 15% and 26%. When we measured risk factors 7 years before scanning, strength of the association slightly attenuated in both men and women.
At the time of scanning, 24% of the men and 12% of the women had a history of coronary artery disease. Corresponding percentages for 7 years before scanning were 18% and 10%. Fig. 4shows that subjects with a history of coronary artery disease had a five times higher calcium score than subjects without coronary arterydisease.
4. Discussion
The present population-based study shows that age and male sex are the most important risk factors for coronary calcification. Cardiovascular risk factors measured 7 years before EBT scanning were strongly associated with the amount of coronary calcification. Associations for blood pressure and cholesterol attenuated or even disappeared when measured concurrently to EBT scanning. Although the number of cardiovascular risk factors was strongly associated with a high calcium score, 29% of the men and 15% of the women without risk factors had a high calcium score.
Age and sex are strongly associated with the amount of coronary calcification. Age had a strong and graded association with the calcium score in this population of older subjects: a six-fold increase in calcium score was seen in men and a ten-fold increase in women. Men had calcium scores that were five times higher than in women.
The present study showed that cardiovascular risk factors measured 7 years before EBT scanning were strongly associated with the amount of coronary calcification while associations were weaker for blood pressure and cholesterol when measured at the time of EBT scanning. Several causes should be considered. Firstly, the observation of weaker associations for blood pressure and cholesterol in 7 year older subjects is in line with the observation that the predictive value of cardiovascular risk factors attenuates with increasing age.3234This hypothesis is supported by a recent study in older adults with a mean age of 80 years finding no association of BMI, hypertension, total cholesterol, HDL-cholesterol, diabetes with coronary calcification. In the latter study, only the number of packyears smoked and triglycerides were associated with coronary calcification.25Conversely, studies in young and middle-aged adults found that BMI, systolic blood pressure, diastolic blood pressure, total cholesterol, diabetes and smoking were positively associated with the amount of coronary calcification while an inverse association with HDL-cholesterol was observed.1319,23Secondly, at the time of EBT scanning more subjects were treated with blood pressure lowering medication (39% vs 23%) and with cholesterol lowering medication (16% vs 3%) than 7 years before EBT scanning. Misclassification of risk factors due to treatment will lead to an underestimation of the strength of the associations. The stronger associations for systolic blood pressure and cholesterol (women) after exclusion of subjects with medication use support this hypothesis.
It has been suggested that the decision to perform EBT scanning in asymptomatic subjects should be based on risk factor assessment.35In asymptomatic subjects, 29% of the men and 15% of the women without risk factors had extensive coronary calcification. This should be taken into account when performing EBT scanning based on cardiovascular risk factor assessment.
In conclusion, this population-based study shows that age and male sex are the most important risk factorsfor coronary calcification. Cardiovascular risk factors assessed 7 years before EBT scanning are strongly associated with coronary calcification. Associations of blood pressure and cholesterol with the calcium score attenuated when risk factors were measured concurrently to EBT scanning. Although cardiovascular risk factors are strongly associated with the amount of coronary calcification in asymptomatic subjects, almost 30% of the men and 15% of the women without cardiovascular risk factors have extensive coronary calcification.
Acknowledgments
Supported by a grant from the Health Research and Development Council, grant numbers 28-2975 and 21000022.
References