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Impaired Coronary Vasodilation by Magnetic Resonance Angiography Is Associated With Advanced Coronary Artery Calcification

Masahiro Terashima, MD, PhD, FACC^_*, Patricia K. Nguyen, MD^_*, Geoffrey D. Rubin, MD, Carlos Iribarren, MD, MPH, PhD, Brian K. Courtney, MD^_*, Alan S. Go, MD^,||, Stephen P. Fortmann, MD, Michael V. McConnell, MD, MSEE, FACC^_*,_*

^_* Division of Cardiovascular Medicine, Stanford University School of Medicine, Stanford, California
Department of Radiology, Stanford University School of Medicine, Stanford, California
Stanford Prevention Research Center, Stanford University School of Medicine, Stanford, California
Division of Research, Kaiser Permanente of Northern California, Oakland, California
^|| Departments of Epidemiology, Biostatistics, and Medicine, University of California, San Francisco, San Francisco, California.

	Abstract

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Objectives: This study evaluated the hypothesis that impaired nitroglycerin (NTG)-induced coronary vasodilation is associated with advanced coronary atherosclerosis in asymptomatic older patients.

Background: Atherosclerosis is associated with both structural and functional abnormalities of the vessel wall. Noninvasive functional measures of subclinical coronary atherosclerosis may help characterize high-risk subjects and guide preventive therapy.

Methods: A total of 236 older patients (age 60 to 72 years, 33% female) without a history of cardiovascular disease were studied. Nitroglycerin-induced coronary vasodilation was measured by magnetic resonance angiography (MRA). Cross-sectional images of the right coronary artery were acquired before and 5 min after 0.4-mg sublingual NTG using a gated, breath-held spiral coronary MRA sequence (0.7-mm resolution). Quantitative analysis of the increase in cross-sectional area was performed in the 90% of patients (n = 212) with adequate image quality. Quantitation of coronary artery calcification (CAC) was performed by multidetector computed tomography using the Agatston method.

Results: Forty patients (19%) had advanced CAC (400). Coronary vasodilation to NTG was significantly impaired (p = 0.02) in patients with advanced CAC (median [interquartile range] = 15.9% [4.2% to 28.0%] vs. 21.5% [9.6% to 36.6%] for CAC <400). Importantly, NTG-induced coronary vasodilation remained independently associated with advanced CAC after multivariate analysis incorporating risk factors (p = 0.02) and other potential confounders (p = 0.04). There was no significant difference in coronary vasodilation between men and women, but few women (n = 3) had advanced CAC.

Conclusions: Impaired NTG-induced coronary vasodilation by MRA is associated with advanced coronary atherosclerosis in a community-based cohort of older asymptomatic subjects. Coronary MRA may provide a noninvasive functional assessment of subclinical coronary atherosclerosis.

Abbreviations and Acronyms   CAC = coronary artery calcium/calcification   CT = computed tomography   ECG = electrocardiography   HDL = high-density lipoprotein   MDCT = multidetector computed tomography   MRA = magnetic resonance angiography   MRI = magnetic resonance imaging   NTG = nitroglycerin   RCA = right coronary artery   RCAC = coronary artery calcification score for the right coronary artery   SNR = signal-to-noise ratio

For functional assessment, the standard has been the measurement of epicardial coronary artery vasodilation by invasive X-ray angiography using intra-arterial stimuli. The impairment of endothelium-dependent vasodilation is recognized as an early step in the development of atherosclerosis (2). Nitroglycerin (NTG) is an exogenous source of nitric oxide and causes endothelium-independent, smooth muscle cell–dependent vasodilation, which may also be affected by atherosclerotic changes in the vessel wall. Previous studies have demonstrated that impaired coronary vasodilation to NTG, as assessed by invasive X-ray angiography, is present in patients who have coronary artery disease and diabetes (3), and has been associated with an increased risk of future cardiac events (4,5).

We and others have developed a noninvasive method to measure NTG-induced coronary vasodilation with magnetic resonance angiography (MRA) (6,7). In this study, we hypothesized that coronary vasodilation by MRA may be impaired in older patients with advanced subclinical coronary atherosclerosis.

	Methods

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Patient characteristics.   Two hundred fifty-three patients (age 60 to 72 years, 34% women) without a history of cardiovascular disease, other major comorbidities (including prior systemic malignancy, dementia, cirrhosis, human immunodeficiency virus/acquired immunodeficiency syndrome, and organ transplantation), or contraindication to magnetic resonance imaging (MRI) were randomly selected from the membership of Kaiser Permanente of Northern California as a substudy of the ADVANCE (Atherosclerotic Disease, Vascular Function, and Genetic Epidemiology) study (8). The recruitment of these patients was performed by Kaiser Permanente of Northern California Division of Research, and all subjects underwent evaluation through the Stanford Prevention Research Center, including CT for CAC. Patients were evaluated without any alteration of their medical regimen per their health care provider, including continuation of vasoactive medications. Magnetic resonance imaging was not completed in 17 subjects due to claustrophobia, back pain, or technical problems, leaving an MRI cohort of 236 patients (mean age 66.0 ± 2.7 years, 33% women, and 37% non-Caucasian). Magnetic resonance imaging was performed within 1 week of CT in all subjects. Written informed consent was obtained from all participants. The study protocol was approved by the Institutional Review Boards at Stanford University and the Kaiser Foundation Research Institute.

NTG-induced coronary vasodilation with MRA.   Nitroglycerin-induced coronary vasodilation by MRI was performed using methods described in more detail previously (7). A 1.5-T Signa MRI scanner (GE Healthcare, Waukesha, Wisconsin) equipped with high-performance gradients (40 mT/m, 150 mT/m/ms) was used. A commercial 4-channel cardiac phased-array surface coil provided signal reception (GE Healthcare). A real-time interactive MRI system (iDrive, GE Healthcare) was used for coronary localization. High-resolution coronary MRA was performed using a multislice spiral sequence, as previously reported (9), with cardiac gating, breath-holding, and acquisition during diastole (field of view = 22 cm, in-plane spatial resolution = 0.7 mm, slice thickness = 5 mm, 3 slices, repetition time = 1 heart beat, echo time = 2.5 ms, 18 interleaves, flip angle = 60°, acquisition window = 35 ms). A subject-specific trigger delay time during diastole was determined from reviewing a 4-chamber cardiac cine magnetic resonance acquisition (10).

After real-time interactive MRI was used to prescribe in-plane views of the right coronary artery (RCA), a cross-sectional view of a linear portion of the proximal- to mid-RCA was then prescribed. Cross-sectional high-resolution coronary MRA images were then acquired both before and 5 min after 0.4-mg sublingual NTG was administered to the subject while in the magnet (Fig. 1). The time point after NTG (5 min) was chosen based on our previous data (7) on the time course of NTG-induced coronary vasodilation, demonstrating the most consistent coronary vasodilation at 3 to 7 min after NTG. Matching pre- and post-NTG images were identified, as described previously (7), and then all images were pooled and randomized, with neither patient nor NTG information provided on the images. Prior to analysis, images were judged to be good, fair, or poor based on signal-to-noise ratio (SNR), vessel border sharpness, and artifacts. The cross-sectional area of the RCA was traced by a single investigator, with a similar low intraobserver variability as found previously (7) (mean difference in cross-sectional area, expressed as a percentage of the overall mean, was 6.3 ± 5.0% with correlation coefficient = 0.97).

View larger version (57K): [in this window] [in a new window] [Download PPT slide]   Figure 1 Coronary MRA of NTG-Induced Coronary Vasodilation Coronary magnetic resonance angiography (MRA) images are shown demonstrating the analysis of nitroglycerin (NTG)-induced vasodilation. Coronary MRA of the right coronary artery (RCA) was performed pre- and post-NTG using a 0.7-mm gated, breath-held spiral MRA sequence. Cross-sectional area of the RCA was traced manually (red circles). The MRA images (A) from the patient with a coronary artery calcium (CAC) score = 0 show a greater degree of NTG coronary vasodilation (28.6%) compared with MRA images (B) from the patient with a CAC score = 443 (coronary vasodilation = 17.0%).

Statistical analysis.   Continuous variables are expressed as means with standard deviations or medians with interquartile ranges. The differences between CAC groups were compared using the unpaired t test if data were normally distributed; if otherwise distributed, the nonparametric Mann-Whitney U test was used. Differences among the 4 CAC groups, 0 CAC < 10, 10 CAC < 100, 100 CAC < 400, and CAC 400, were analyzed by the Kruskal-Wallis test. Differences in distribution of CAC scores <400 and 400 were assessed by the chi-square test. Regression analysis was performed comparing NTG-induced coronary vasodilation to log₁₀ (CAC score + 1). Multivariable logistic regression analysis was performed to identify independent predictors of advanced CAC, incorporating conventional coronary risk factors, such as age, gender, body mass index, diabetes mellitus (history of treatment or fasting glucose 126 mg/dl), systolic blood pressure, total cholesterol/high-density lipoprotein (HDL) cholesterol ratio, and smoking into the model. For multivariate analysis, tertiles of the percentages of coronary vasodilation were also used. A 2-tailed p value <0.05 was considered statistically significant.

	Results

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Patient characteristics.   Of the 236 patients, 10% were excluded prior to analysis because of poor image quality on either the pre- or post-NTG MRA, leaving 212 subjects for quantitative analysis. Median CAC was 37 (interquartile range 1 to 269). Forty patients (19%) had advanced CAC, defined as CAC 400. Clinical characteristics of the 2 groups are shown in Table 1. In subjects with CAC 400, there was a significantly higher proportion of men (p < 0.001) and of subjects with diabetes mellitus (p = 0.04).

View larger version (10K): [in this window] [in a new window] [Download PPT slide]   Figure 2 Comparison of NTG-Induced Coronary Vasodilation and CAC The degree of coronary vasodilation is shown for patients with and without advanced CAC. Coronary MRA images were analyzed in 212 patients and coronary vasodilation to NTG was quantified. Coronary artery calcium scoring was performed by computed tomography. Box and whisker plots show the median, interquartile range, and full range of percent coronary vasodilation between groups with and without advanced CAC (400). Patients with CAC 400 had significantly impaired NTG-induced coronary vasodilation compared to patients with CAC <400 by univariate and multivariate analysis (p = 0.02). Abbreviations as in Figure 1.

View this table: [in this window] [in a new window]   Table 2 Multivariate Analysis of NTG-Induced Coronary Vasodilation and Advanced CAC (400)

NTG-induced coronary vasodilation by gender.   Overall there was not a significant difference in NTG-induced coronary vasodilation between men and women (men: 20.8% [8.0% to 36.3%] vs. women: 21.1% [6.9% to 34.0%], p = 0.9). In men only (n = 143), the impaired NTG-induced coronary vasodilation in patients with CAC 400 remained significant by both univariate (p = 0.02) and multivariate analysis (p = 0.01). The sample size for women (n = 3 for CAC 400) was too small for meaningful statistical evaluation.

NT-induced coronary vasodilation and RCA coronary artery calcium.   As the MRI measurement was done on the RCA, we also compared the MRI vasodilation measures to the coronary artery calcification score for the right coronary artery (RCAC). Median RCAC was 0 (interquartile range 0 to 47). There was +RCAC (RCAC score >0) in 86 patients (41%, 71 male). Only 2 subjects with CAC 400 did not have +RCAC. The degree of NTG-induced coronary vasodilation was significantly impaired in patients with +RCAC compared to patients with –RCAC by univariate analysis (17.4% [5.0% to 33.7%] vs. 22.6% [11.4% to 36.5%], p = 0.049) and multivariate analysis (p = 0.03).

	Discussion

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Coronary MRA was successfully applied to assess NTG vasodilation in a large cohort of asymptomatic, older community-based patients. We found that NTG coronary vasodilation was impaired in patients with advanced CAC, which were primarily men in this cohort. In addition, this association was independent of other major coronary risk factors, suggesting that NTG coronary vasodilation by MRA may be a functional measure of subclinical coronary atherosclerosis.

Coronary vasodilation and CAC.   Coronary vasomotor function plays an important role in the onset and progression of coronary atherosclerosis. Endothelial dysfunction, as measured by the impairment of endothelium-dependent vasodilation, is considered to be an early marker of atherosclerosis, which can be found without overt angiographic or ultrasound evidence of coronary atherosclerosis (14,15). The assessment of epicardial coronary vasodilation is distinct from the assessment of coronary flow or perfusion reserve, which are primarily indicators of microvascular dysfunction or flow-limiting stenoses. Several studies have found a predictive value of endothelium-dependent epicardial coronary vasodilation in response to intracoronary infusion of acetylcholine in patients with mild coronary atherosclerosis or normal coronary angiograms by invasive coronary angiography (4,15,16). Although NTG-induced (endothelium-independent) vasodilation has typically been used as a "positive control" in these patients, impairment has been found (17). Schachinger et al. (4) investigated both endothelium-dependent and -independent coronary vasodilation in 147 patients with coronary artery disease and reported that NTG-induced coronary vasodilation was a significant predictor of prognosis. Similarly, von Mering et al. (5) studied 163 women with suspected myocardial ischemia and reported that women with a confirmed acute ischemic event had significantly impaired NTG-induced coronary vasodilation. These data suggest that atherosclerotic changes to the vessel wall can impair vasodilation beyond the effects on the endothelium, although the precise mechanisms underlying the impairment remain unclear. Our data suggest that there may be a threshold effect, with NTG vasodilation remaining intact until atherosclerosis is advanced.

Interestingly, smooth muscle cells are not only the primary mediators of NTG-induced vasodilation (18), but are also thought to be responsible for the formation of vascular calcification (19,20). Kullo et al. (21) recently reported that a decreased brachial artery vasodilator response to NTG, but not flow-mediated endothelium-dependent vasodilation, is associated with subclinical coronary atherosclerosis as measured by CAC in 441 asymptomatic patients. A recent study of subclinical atherosclerosis in 292 participants from the Framingham Heart Study included aortic MRI, coronary and thoracic aortic calcification by CT, and carotid intima-media thickness by ultrasound (22). The results indicated that a combination of imaging tests may be needed to fully identify higher-risk asymptomatic patients. Further studies of the prognostic significance of NTG-induced coronary vasodilation by coronary MRA and its value relative to CAC and other cardiovascular tests and risk factors are clearly needed.

Risk factors.   According to previous reports, age and gender are strong risk factors for advanced burden of CAC (23–26). In our study, the age range of enrolled subjects was narrow (60 to 72 years), and therefore we did not observe a significant association between age and advanced CAC. However, male gender was a strong predictor of advanced CAC in our study, which is consistent with prior studies (26). In other studies of asymptomatic patients, many conventional coronary risk factors, such as obesity, hypertension, hypercholesterolemia, diabetes, and smoking, were strongly associated with CAC (23,27). Interestingly, in our study, only male gender and diabetes were significantly associated with CAC 400. Besides the sample size and the relatively advanced age range, this may have been attributed to, in part, the relatively low prevalence of the other coronary risk factors in our sample compared with samples from previous studies (Table 1) (23,28). For example, the mean total cholesterol/HDL cholesterol ratio and systolic blood pressure were 3.9 and 133 mm Hg in our study compared with 4.0 to 5.3 and 139 to 144 mm Hg in other studies (23,28), and current smokers were 8.5% compared with 15% to 19% (23,28).

Study limitations.   The main limitation of the current study is that we assessed only endothelium-independent coronary vasodilation using NTG. Prior studies indicate that an endothelium-dependent method may detect abnormalities at an earlier stage of disease (2,29) and produce more divergent responses (vasoconstriction vs. vasodilation) to improve discrimination. Nonpharmacologic endothelium-dependent stimuli, such as the cold-pressor test and mental stress, are challenging in the MRI environment, and pharmacological agents, such as acetylcholine, cannot safely be given systemically. A preliminary MRI report using the cold-pressor test shows promise (30). The other main limitation is the interpatient variability of NTG vasodilation in this study. Although the RCA was used for higher SNR and spatial resolution, and has been shown previously to be very reproducible (7,31), further improvements (32,33) in SNR, resolution, and breath-hold reproducibility may improve image quality and reduce variability.

Multidetector CT angiography has emerged as a high-resolution noninvasive approach for coronary artery imaging. A recent study did look retrospectively at patients who had more than 1 coronary CT scan, where NTG was used in 1 and not in another, and did show significantly larger coronary diameter with NTG (34). However, the radiation and contrast involved make it suboptimal for repeat imaging to measure coronary vasodilation, particularly in asymptomatic subjects.

	Conclusions

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In an asymptomatic older community patient cohort, impaired NTG-induced coronary vasodilation by MRA was significantly and independently associated with advanced CAC. Noninvasive assessment of coronary vasodilation may provide an additional functional measure of subclinical coronary atherosclerosis.

	Acknowledgments

 
The authors thank the staff of the Kaiser Permanente of Northern California Division of Research and the Stanford Prevention Research Center for their assistance in patient recruitment and evaluation.

	Footnotes

 
This study was supported by grants from the Donald W. Reynolds Foundation (Las Vegas, Nevada); National Heart, Lung, and Blood Institute (grant R01HL39297, Bethesda, Maryland); and General Electric Healthcare (Waukesha, Wisconsin). Dr. Rubin is a consultant/advisory board member for TeraRecon, Inc., and Siemens Medical Solutions. Dr. McConnell receives research support from GE Healthcare, is on a scientific advisory board for Kowa, Inc., and has been an advisor for Philips Medical Systems.

^_* Reprint requests and correspondence: Dr. Michael V. McConnell, Division of Cardiovascular Medicine, Stanford University School of Medicine, 300 Pasteur Drive, Room H-2157, Stanford, California 94305-5233. (Email: mcconnell{at}stanford.edu).

Manuscript received October 18, 2007; revised manuscript received December 5, 2007, accepted December 8, 2007.

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This Article Abstract Figures Only Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Related articles in JACC Imaging Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Terashima, M. Articles by McConnell, M. V. Search for Related Content PubMed Articles by Terashima, M. Articles by McConnell, M. V.