Population

MESA is composed of a multiethnic population of 6,814 men and women aged 45 to 84 years without histories of clinical cardiovascular disease at baseline enrolled between July 2000 and August 2002 at six sites in the United States. The MESA cohort includes white, African American, Hispanic, and Chinese participants. Participants were excluded at baseline if they had physician diagnoses of heart attack, stroke, transient ischemic attack, heart failure, angina, or atrial fibrillation or histories of any cardiovascular procedures. Participants with weights > 300 lb, pregnancy, or any medical conditions that would prevent long-term participation were also excluded. MESA protocols and all studies described herein were approved by the institutional review boards of all collaborating institutions. All participants gave informed consent.

Risk Factors and Anthropomorphic Variables

Age, gender, race or ethnicity, and medical history were self-reported. The use of antihypertensive and lipid-lowering medications (statins) was also recorded. Current smoking was defined as self-report of one or more cigarettes in the past 30 days and prior smoking as any smoking history before this interval. Body mass index was calculated from the participant’s weight in kilograms divided by the square of height in meters. Resting systolic and diastolic blood pressures were measured three times in the seated position using a Dinamap Pro 100 automated oscillometric sphygmomanometer (Critikon, Tampa, FL).

Glucose and lipids were measured after a 12-hour fast. Serum glucose was measured using rate reflectance spectrophotometry on the Vitros analyzer (Johnson & Johnson Clinical Diagnostics, Inc., Rochester, NY). Diabetes mellitus was determined using the 2003 American Diabetes Association’s fasting criteria algorithm. Total cholesterol was measured using a cholesterol oxidase method (Roche Diagnostics), as was high-density lipoprotein after precipitation of non–high-density lipoprotein cholesterol with magnesium/dextran and triglycerides using Triglyceride GB reagent (Roche Diagnostics GmbH, Mannheim, Germany).

Carotid Artery Measures

The patients were supine, with their heads rotated 45° toward the side opposite to the side being imaged. A transverse sweep was recorded from the low neck through the carotid artery bifurcation into the internal carotid artery (ICA). Doppler velocity measurements were made at the site of any bulb or proximal ICA lesion or in the proximal ICA if no lesions were seen. A common carotid artery was then imaged at 45° from the vertical with the beginning of the bulb shown on the image (to the left). Three views centered on the ICA bulb were taken: one anterior, one lateral (at 45°), and one posterior. Images were acquired on the right and on the left. A matrix array probe (M12L; GE Medical Systems, Milwaukee, WI) was used with the frequency set at 13 MHz for the common carotid artery and 9 MHz for the ICA and with two focal zones at a frame rate of 32 frames/sec.

All carotid artery measurements were blinded and made at the ultrasound reading center in Boston. The two common and six ICA images were used to measure the intima-media thickness (IMT) of both the near and far walls. Common carotid artery IMT was calculated as the mean of the maximum IMT on each of the two common carotid artery images. Maximum ICA IMT was calculated as the maximum wall thickness seen on any of the ICA images. Images of the ICA (three per side) and the videotaped transverse sweep were further reviewed to determine the presence and severity of any lesion in the ICA, either near or far walls. These lesions were graded on a semiquantitative scale of 0% (absent lesion) and lesions causing relative narrowing of 1% to 24% and >25%. Doppler velocity measurements at the site of the lesions with peak systolic velocities ≥ 125 cm/sec were considered to indicate >50% diameter narrowing. The 43 lesions detected in this fashion were included in the >25% group. The greater of the right and left sides was used for final grading of lesion severity. Readers were reviewed every 2 to 3 weeks during sessions to establish uniformity of interpretations by having the principal investigator (J.F.P.) review selected studies. All carotid artery interpretations were blinded to demographics.

Blinded replicate scans were performed and read on 112 participants. Agreement for the presence of plaque was measured as a κ coefficient of 0.92 (standard error, 0.03). The correlation coefficient for replicate ICA IMT measurements was 0.88 ( n = 110) and 0.91 for common carotid artery IMT measurements ( n = 112).

Outcomes

Two separate coronary artery studies were acquired on each participant at baseline (2000–2002) with either electrocardiographically gated electron-beam or multidetector computed tomography during a breath hold. The presence of CAC was determined using the Agatston method. Scans were read centrally at the Los Angeles Biomedical Research Institute at Harbor-UCLA Medical Center. Calcification was considered as positive when the Agatston score was >0.

Separate coronary artery scans were repeated on a randomly selected half of the cohort at a second cohort examination (September 2002 to January 2004) and on the other half at the third examination (March 2004 to July 2005). The median interval between visits for the whole cohort was 2.4 years after the baseline examination. All CAC scoring was blinded to demographics.

We restricted our analyses to individuals with complete risk factors. Of the original 6,814 participants, information was missing on risk factors in 157, and five had baseline cardiovascular disease. Further information was missing on plaque in 11, common carotid IMT in 83, and ICA IMT in 106.

All of these 6,452 participants underwent baseline CAC studies. Follow-up CAC studies were available in 5,445. Of the 1,007 participants without CAC follow-up studies, follow-up was maintained in 607 through phone interviews or clinic visits, while follow-up was lost for 400 participants.

The development of a CAC score > 0 was deemed a positive outcome in individuals without positive calcium scores at baseline ( n = 2,837). Change in calcium score was considered the outcome in individuals with calcium scores > 0 at baseline ( n = 2,708).

Statistical Analyses

The mean (and standard deviation) values of continuous variables and the distribution of dichotomous variables as a percentage in each group are shown.

Analysis of variance and χ ² tests were used to compare individuals with repeat CAC examinations to those without CAC examinations, stratified as individuals with follow-up after the scheduled clinic visit as well as individuals lost to follow-up.

We compared the demographics of individuals with positive calcium scores with those with calcium scores of 0 at baseline using χ ² and Student’s t tests.

The rate of developing a positive calcium score has been reported at approximately 16% in MESA. Because the odds ratios derived from logistic regression models tend to overestimate the relative risk estimates in situations in which the event rates are high, we used relative risk regression models. We used a modified Poisson approach to estimate the relative risk and confidence intervals (CIs). Cardiovascular risk factors, the time interval between the two CAC studies, and common carotid artery IMT were predictor variables in multivariate risk regression models to which were added measurements of plaque.

In subjects with positive calcium scores at baseline, CAC change over time was used as the outcome in multivariate linear regression models that included the same risk factors as the multivariate risk regression models.

Figures were generated showing the prevalence of positive CAC scores at baseline for each plaque category and each ICA IMT quartile. Similar figures were generated for new-onset CAC score > 0 and change in CAC over the follow-up period.

Statistical analyses were performed using Stata version 11.2 (StataCorp LP, College Station, TX). The level of statistical significance was set to be two sided at P ≤ .05.