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Clinical Investigations |
1 Department of Medicine, Kuopio University Hospital, Kuopio, Finland
2 Department of Clinical Physiology and Nuclear Medicine, Kuopio University Hospital, Kuopio, Finland
3 Department of Medicine, Division of Cardiology, Helsinki University Central Hospital, Helsinki, Finland
| ABSTRACT |
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Key Words: 123I-metaiodobenzylguanidine cardiac adrenergic function coronary artery disease
| INTRODUCTION |
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Myocardial infarction results in cardiac adrenergic denervation (811). In addition, dysfunction of adrenergic nerve endings has been reported in patients with severe coronary artery disease without myocardial infarction (12,13). During myocardial ischemia, noradrenaline spillover increases, and it has been suggested that this reflects increased adrenergic tone (14,15). However, the net transportation direction of catecholamines is reversed; that is, as the first step in acute ischemia, washout decreases because of lactate production (16). This reversal has also been shown in pacing-induced ischemia, in which net cardiac noradrenaline release, present before ischemia, reverts to net uptake during ischemia (17). Thus, a challenging question is whether the function of adrenergic nerve endings is modified similarly to endothelial function during the early phase of coronary atherosclerosis before hemodynamically significant stenoses and adrenergic denervation occur.
The function of cardiac adrenergic innervation can be assessed noninvasively with 123I-metaiodobenzylguanidine (MIBG), a noradrenaline analog taken up into the nerve endings by a specific, energy-dependent uptake-1 mechanism (18). Recently, new techniques, such as measurement of MIBG washout, have enabled more detailed assessment of the function and integrity of adrenergic innervation.
In this study, we performed quantitative coronary angiography and dual-isotope scintigraphy imaging with MIBG and 99mTc-sestamibi (MIBI) to study cardiac adrenergic innervation and myocardial perfusion, respectively, in subjects free of any anginal symptoms but with a high familial risk for coronary artery disease. Specifically, we evaluated whether myocardial MIBG kinetics, reflecting cardiac adrenergic function, are affected during the early phase of coronary artery disease.
| MATERIALS AND METHODS |
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For at least 12 h before MIBG imaging, the subjects were not allowed to drink coffee, tea, or cola beverages or to smoke. In addition, all medical therapy known to influence MIBG uptake was discontinued before the study (19).
Protocol
The study protocol consisted of quantitative coronary angiography, radionuclide studies at rest, and radionuclide studies during maximal bicycle exercise testing. The subjects gave written informed consent after the nature and possible risks associated with investigations were explained to them in detail. The study protocol was approved by the local ethics committee.
Angiographic Analyses
Coronary angiography was performed by the percutaneous femoral approach using standard angiographic techniques. Four views of the left and right coronary arteries were recorded on cine film. Intracoronary nitroglycerin (250 µg) was administered before the angiographic imaging was performed.
Quantitative analysis of coronary angiography data was performed with a previously validated method (Cardiovascular Measurement System; Medis, Neunen, The Netherlands) (20,21). The optimal framewith full opacification of the vessel, minimal motion blurring, and minimal overlapping with other brancheswas selected and was usually at end-diastole or during diastasis. The frames were selected by a cardiologist experienced in quantitative analysis of coronary angiograms. Vessel edges were measured with a computerized edge-detection algorithm. Luminal diameters were measured using the dye-filled guiding catheter as a reference, and severity of stenoses (percentage of lumen diameter) was calculated. All stages of the angiographic analyses were performed without knowledge of the patients clinical characteristics.
Exercise Test
A maximal symptom-limited exercise test was performed with a bicycle ergometer during temporary withdrawal of ß-blocking and calcium channelblocking agents. A protocol of 20 W initial workload and increments of 20 W/min was used. The test was terminated if any of the following criteria were met: severe angina pectoris, exhaustion, or decrease in systolic pressure
10 mm Hg during exercise. Myocardial ischemia was defined as horizontal or downsloping ST depression
0.1 mV measured at J point + 60 ms. In addition, maximal oxygen uptake was calculated.
Radionuclide Studies
Cardiac SPECT was performed using MIBG (MAP Medical Technologies Oy, Helsinki, Finland) to assess cardiac adrenergic innervation. Myocardial perfusion was evaluated using MIBI (DuPont Pharmaceuticals, Hertfordshire, U.K.) at rest and during exercise on 2 consecutive days. MIBG imaging and MIBI imaging at rest were performed during the first day. Myocardial perfusion during exercise was assessed on the following day. The specific activity of MIBG was 26,000 MBq/mmol. During the first day, 400 MBq MIBI tracer, followed by 200 MBq MIBG tracer 15 min later, were injected into the left antecubital vein. The first SPECT scan was started 30 min after MIBI injection to study myocardial perfusion and initial MIBG uptake. The second SPECT scan was obtained 4 h and 5 min after MIBG injection using the same imaging settings to study late MIBG uptake. On the next day, to assess perfusion during exercise, a dose of 400 MBq MIBI tracer was injected during the last minute of maximal stress and imaging was started 30 min after the injection. In addition, to assess global myocardial MIBG and MIBI uptake, the first planar scan was started 25 min after the MIBI injection and the second was obtained 4 h after injection.
A Multispect 3 gamma camera (Siemens Gammasonics, Des Plaines, IL) equipped with high-resolution collimators was used to assess myocardial distribution of MIBG and MIBI tracers. The imaging resolution was 1314 mm. Three detectors (3 x 120°) acquired 30 views with 4° steps of 45 s each. The matrix size was 128 x 128 without zooming, and the voxel size was 3 x 3 x 3 mm. The energy window of 99mTc was centered at 140 keV ± 6%, and that of 123I was shifted to 158 keV ± 5.5%. Overlapping of the energies was corrected using a matrix inversion technique with the standard doses of 123I and 99mTc. Overlapping of 123I activity onto the 99mTc window was, on average, 18%; vice versa, the overlap was 4%.
Data Analysis
The raw data of MIBG and MIBI SPECT studies were reconstructed using a Butterworth-filtered (order, 8; cutoff frequency, 0.65 cm-1) backprojection technique. For semiquantitative analysis of regional uptake on both initial and delayed images, interpolative background subtraction was used to reduce the effect of background activity. Transaxial slices were reconstructed and reoriented to represent coronal slices. Using a semiautomatic quantification program (Quantitative Heart Application; Siemens Gammasonics), mean counts were recorded from anterior, lateral, inferior, and septal regions.
Mean myocardial MIBG and MIBI counts obtained from initial and delayed planar imaging were normalized by the mean upper-mediastinal counts of the respective images and expressed as heart-to-mediastinum ratio (H/M). For assessment of regional uptake of the MIBG and MIBI tracers, the myocardium was divided into 8 segments (Fig. 1). MIBG and MIBI uptake was measured from the following coronary artery territories as previously described (22). The left anterior descending coronary artery (LAD) region included the anterior and septal areas (segments 14), the left circumflex coronary artery (LCX) region included the lateral area (segments 56), and the right coronary artery (RCA) region included the inferior area (segments 78) of the myocardium. Regional MIBG and MIBI uptake was normalized to the maximal myocardial uptake of the tracer and expressed as normalized units. The apex and the most basal layer were excluded from the analysis. The rater was unaware of the clinical data and angiographic findings.
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Statistical Methods
To estimate the significance of differences between groups, 1-way ANOVA was performed. When differences were found, the Tukey test was used as a posthoc analysis for continuous variables. In addition, the
2 test was used for categoric variables when appropriate. A least squares regression analysis was used to study univariate linear correlations. P < 0.05 was considered statistically significant. Results are expressed as mean ± SD.
| RESULTS |
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50% stenosis of the lumen diameter. Three subjects (10%) had mild (<30%) stenosis in the left main coronary artery.
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50% of maximal myocardial MIBI uptake) at rest and during exercise, and normal perfusion was also visually confirmed by an experienced clinician. Relative MIBI uptake at rest and during exercise did not significantly differ from each other in the anteroseptal region (0.65 ± 0.04 vs. 0.64 ± 0.03, respectively) or in the lateral region (0.58 ± 0.04 vs. 0.57 ± 0.05, respectively). However, the inferior myocardial region showed lower relative MIBI uptake during exercise than at rest (0.55 ± 0.06 vs. 0.60 ± 0.06, respectively; P < 0.01). Early MIBG uptake in the anteroseptal region (0.63 ± 0.04) was higher than uptake in the lateral (0.57 ± 0.06; P < 0.001) or inferior (0.58 ± 0.06; P < 0.001) region. In delayed scintigraphy imaging, MIBG uptake in the anteroseptal region (0.39 ± 0.06) did not differ from that in the lateral region (0.37 ± 0.07) but was significantly higher than that in the inferior region (0.34 ± 0.06; P < 0.001).
The average global MIBG washout rate as assessed by planar imaging was 38% ± 9%. MIBG washout in the anteroseptal region (38% ± 10%) did not differ from that in the lateral (36% ± 12%) or inferior (41% ± 8%) region. However, MIBG washout in the inferior region was higher than that in the lateral region (P < 0.05).
Relationship Between Angiographic Findings and Regional Radionuclide Measures
We found no correlation between the severity of coronary artery stenoses in the LAD, LCX, or RCA and the uptake of MIBI in the anteroseptal, lateral, or inferior myocardial regions, respectively, at rest or during exercise (Fig. 2).
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Relationship Between Regional MIBI and MIBG Measures
Regional MIBI uptake assessed at rest or during exercise did not correlate with MIBG uptake or MIBG washout of the anteroseptal or lateral myocardial region. On the other hand, MIBI uptake in the inferior region correlated directly with MIBG uptake in the corresponding region at rest (r = 0.50; P < 0.01) and during exercise (r = 0.39; P < 0.05).
| DISCUSSION |
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The severity of stenoses in the LAD, and to some extent also in the LCX, was inversely related to MIBG washout and directly related to delayed MIBG uptake in the anteroseptal and lateral myocardial regions, respectively. This finding agrees well with the fact that anterolateral myocardium perfused by the LAD and LCX has the most abundant adrenergic innervation (23). Furthermore, because the subjects were asymptomatic, it seems that myocardial MIBG kinetics, reflecting cardiac adrenergic function, respond sensitively to the development of coronary artery disease. The decrease in MIBG washout that occurs during the evolution of mild coronary artery stenosis before the stenosis is severe enough to result in denervation (which is associated with increased MIBG washout) suggests biphasic changes in myocardial MIBG kinetics during ischemic progression.
In this study, early MIBG uptake did not correlate with the severity of stenosis at any respective myocardial region. Bearing in mind that early MIBG uptake represents predominantly myocardial perfusion, one would expect this finding; most stenoses were mild, and none of the subjects had symptoms suggestive of hemodynamically significant coronary artery disease. Moreover, myocardial MIBG uptake or MIBG washout was not related to perfusion of the anteroseptal or lateral myocardial regions at rest or during exercise (the correlation between MIBG and MIBI uptake in the inferior region was most obviously caused by attenuation). Thus, we think that chronic ischemic damage of nerve endings does not explain our results.
Endothelial dysfunction is associated with paradoxic vasoconstriction of coronary arteries during adrenergic stimulus (24). There is also evidence that mast cells can be activated by neural stimulation (25,26) and that activated mast cells in the atherosclerotic vessel wall release a variety of vasoactive agents (27), of which histamine (28,29) and leukotrienes (30) can constrict atherosclerotic coronary segments. Accordingly, interaction between the neural and humoral systems may also contribute to abnormal vasoconstriction (3133). It is possible that decreased cardiac adrenergic activity may protect against unopposed vasoconstriction of sclerotic coronary arteries during the development of coronary artery disease, but the mechanisms behind this modulation remain unclear.
Studies on healthy volunteers have shown that MIBG uptake of the inferior region decreases with advancing age (34,35). In these studies, MIBG uptake in the inferior region and MIBG uptake in the anterior region have been compared with each other and have not been normalized against a fixed reference region. Thus, changes in MIBG uptake with aging can result from increased anterior MIBG uptake, decreased inferior MIBG uptake, or both. On the other hand, mild, asymptomatic atherosclerotic lesions have not been previously considered an explanation for changes in MIBG kinetics. Moreover, the association between age and myocardial MIBG uptake has not been confirmed in all studies (36). Accordingly, we suggest that changes in the MIBG kinetics caused by mild atherosclerotic lesions might have been previously considered a variation of normal because no picture about the extent of atherosclerotic disease had been provided. In our study, the age range of subjects was narrow; thus, aging itself was unlikely to have had a significant confounding effect.
The overall radionuclide uptake was lowest in the inferior myocardium, most plausibly because of the attenuation of gamma radiation, a well-known limitation of SPECT imaging. On the other hand, the density of adrenergic nerve terminals is lower in the inferior myocardial region than in the anterior region, and obviously, lower MIBG uptake in the inferior region should result (35). However, attenuation has no effect on the washout analysis, because it represents the relative difference between the initial and delayed acquisitions instead of the absolute values. Thus, we assume that attenuation does not compromise comparisons of regional MIBG washout.
The decreased relative MIBI uptake in the inferior myocardial region during exercise, as compared with uptake at rest, may be caused by a smaller perfusion reserve in that region than in the anteroseptal or lateral myocardial region. Another possibility is that scattered radiation from the liver or intestine enhances regional uptake during the rest study. Differences in the attenuation are not probable, because subject positioning under the gamma camera was similar at rest and during exercise.
We assumed that the anteroseptal, lateral, and inferior regions represent perfusion by the LAD, LCX, and RCA, respectively, as has previously been described (22). We admit that this is not always true, particularly when the left or right coronary artery clearly dominates myocardial perfusion or when rich collateral circulation is present. However, we found no abnormal dominance of coronary artery distribution or significant collaterals in our subjects.
| CONCLUSION |
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| FOOTNOTES |
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For correspondence or reprints contact: Juha Hartikainen, MD, PhD, Department of Medicine, Kuopio University Hospital, P.O. Box 1777, 70211 Kuopio, Finland.
E-mail: juha.hartikainen{at}kuh.fi
| REFERENCES |
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