Physiological Changes in Human Cardiac Sympathetic Innervation and Activity Assessed by 123 I-Metaiodobenzylguanidine (MIBG) Imaging

Transcription

1 ORIGINAL ARTICLE Imaging Circ J 2009; 73: Physiological Changes in Human Cardiac Sympathetic Innervation and Activity Assessed by 123 I-Metaiodobenzylguanidine (MIBG) Imaging Kazuyuki Sakata, MD; Kei Iida, MD; Nao Mochizuki, MD; Michitoshi Ito, RT*; Yoshihiro Nakaya, RT* Background Physiologic changes in the human sympathetic nervous system (SNS) may be associated with cardiovascular diseases, so the present study assessed the age and gender differences in global cardiac SNS in normal subjects. Methods and Results The 163 subjects (74 men, 89 women; age range years) whose coronary arteriogram was normal, and who had no other cardiac or neurohormonal diseases, and no medication affecting the autonomic nervous system were included. All study subjects underwent metaiodobenzylguanidine imaging. Both initial and delayed heart-to-mediastinum (H/M) ratios had a significant gender difference and showed a progressive decrease with aging. In addition, the initial H/M ratio had a significant positive correlation with the delayed H/M ratio (r=0.89, P<0.0001). Females (50 59 years) demonstrated significantly higher delayed H/M ratio than males of the same age. After the age of 60, the delayed H/M ratio in females progressively decreased with aging, similar to males. As for the washout rate, both genders had a significantly progressive increase with aging. In addition, there was a significant decrease in the delayed H/M ratio in 10 females with surgical menopause compared with 15 age-matched females without surgical menopause. Conclusion Cardiac SNS appears to be regulated by various physiological factors. (Circ J 2009; 73: ) Key Words: Aging; Nuclear medicine; Sex; Sympathetic nervous system The sympathetic nervous system (SNS) is an important control mechanisms of the body, dysfunction of which is well known to be associated with the development of various common cardiovascular diseases, such as ischemic heart disease, 1 congestive heart failure, 2 arrhythmias, 3 and sudden death. 4 It is also well known that the frequency and the onset of these diseases have age- and gender-related differences. 5,6 Likewise, the SNS shows physiologic fluctuations with age and sex, 7 15 which are considered to be related to gender differences in the frequency of cardiovascular diseases and the markedly increased risk of cardiovascular diseases with aging. Therefore, it is very important to assess physiological changes in sympathetic cardiovascular control in humans, especially in the cardiac SNS, because many cardiovascular diseases originate from the heart. However, as there are no appropriate methods of measuring the cardiac SNS in vivo, the precise physiological changes remain unclear. 123 I-metaiodobenzylguanidine (MIBG) imaging has become widely used to evaluate human cardiac sympathetic nervous function because it gives information about cardiac sympathetic nerve endings. Using MIBG imaging, (Received June 26, 2008; revised manuscript received September 1, 2008; accepted September 4, 2008; released online December 5, 2008) Divisions of Cardiology, *Diagnostic Radiology, Shizuoka Cancer Center Hospital, Shizuoka, Japan Mailing address: Kazuyuki Sakata, MD, Division of Cardiology, Shizuoka Cancer Center Hospital, Nagaizumi, Shizuoka , Japan. k.sakata@scchr.jp All rights are reserved to the Japanese Circulation Society. For permissions, please cj@j-circ.or.jp the aim of this study was to assess the age- and gender-related differences in the global cardiac SNS in normal subjects. Methods Patients We investigated 153 patients whose ages ranged from 40 to 89 years were referred for cardiac catheterization because of chest pain or electrocardiogram (ECG) abnormality, and which revealed a nearly normal coronary artery without coronary spasm induced by acetylcholine and with normal cardiac function and hemodynamics. After cardiac catheterization, they underwent MIBG imaging. The subjects were 74 men and 79 women who did not have obvious heart disease, hypertension ( 140 mmhg systolic blood pressure, or 90 mmhg diastolic blood pressure), obesity (body mass index (BMI) >25 kg/m 2 ), diabetes mellitus or any other diseases affecting the autonomic nervous system. In addition, they did not take any drugs known to affect MIBG kinetics. We included 10 women who fulfilled the inclusion criteria and had undergone surgical menopause without oral hormone replacement therapy before physiological menopause because we wanted to investigate the effect of menopause on the cardiac SNS. Written informed consent was given by the each patient and the study was approved by the hospital s ethics committee. Cardiac Catheterization After right-heart catheterization with a Swan-Ganz catheter, all patients underwent coronary angiography (CAG) performed by the Judkins technique. Prior to CAG, a tem-

2 Changes in Cardiac SNS on MIBG 311 Table 1 Clinical Characteristics of Each 10-Year Age Group and MIBG Parameters Total Male Female Male Female Male Female Male Female Male Female Male Female Age group (years) No Age (years) 44±4 45±3 54±3 55±3 66±2 65±6 75±4 72±3 84±3 84±3 68±12 71±12 Height (m) 1.72± ±0.06* 1.71± ±0.04* 1.66± ±0.04 # 1.65± ±0.04 # 1.62± ±0.06 # 1.68±0.06* 1.58±0.05 Weight (kg) 64.9± ±4.5* 63.6± ±3.1 # 61.8± ±6.6 # 57.8± ±6.7 $ 55.3± ± ±8.5* 51.5±6.1 BMI (kg/m 2 ) 21.9± ± ± ± ± ± ± ± ± ± ±2.3 # 20.5±2.3 Washout rate (%) 7.53± ± ± ± ± ± ± ± ± ± ± ±11.09 Initial H/M ratio 1.98± ± ± ± ± ± ± ± ± ± ± ±0.26 Delayed H/M ratio 2.16± ± ± ± ± ± ± ± ± ± ± ±0.32 *P<0.0001, # P<0.001, $ P<0.02 as compared with males of the same age. MIBG, 123 I-metaiodobenzylguanidine; BMI, body mass index; H/M, heart-to-mediastinum. Fig 1. Comparison of the initial and delayed heart-to-mediastinum (H/M) ratios in males and females by each decade. Both genders had significant decreases in the initial H/M ratio (P<0.0001, for both) and the delayed H/M ratio (P<0.0001, for both). In addition, there was a significant difference in the initial and the delayed H/M ratio between males and females (P<0.01 and P<0.03, respectively). $$$ P<0.01, $$ P<0.001, $ P< compared with females aged 40 years; *P<0.01, **P<0.001, ***P< compared with females aged 50 years; # P<0.005, ## P<0.001, ### P< compared with males aged 40 years; + P<0.005, ++ P< compared with males aged 50 years. porary pacing catheter was inserted into the right ventricle, and the pacing rate was set at 40 beats/min. Standard 12- lead ECGs were recorded continuously with a 6-channel recorder. After control, nearly normal, arteriograms were obtained, acetylcholine dissolved in saline solution, was injected in incremental doses of 20 μg and 50 μg directly into the right coronary artery and, subsequently, in incremental doses of 20, 50 and 100 μg directly into the left coronary artery through the catheter. CAG images were obtained 1 min after acetylcholine administration. After the acetylcholine provocation test, CAG and left ventriculography (LVG) were performed after intracoronary injection of 0.3 mg nitroglycerin, and binary stenosis on CAG and wall motion abnormalities on LVG were classified according to the reporting system of the American Heart Association. 21 MIBG Imaging After overnight fasting, each subject was administered a 111MBq IV dose of commercially available MIBG

3 312 SAKATA K et al. Table 2 Characteristics of Women With and Without Surgical Menopause Surgical menopause Menstruation P value No Age (years) 46±2 45± Height (m) 1.60± ± Weight (kg) 51.6± ± BMI (kg/cm 2 ) 20.1± ± Abbreviation see in Table 1. Fig 2. Comparisons of the washout rate between males and females by each decade. Both genders had significant increases in the washout rate (P<0.0001, for both). $$$ P<0.01, $ P< compared with females aged 40 years; **P<0.001, ***P< compared with females aged 50 years;! P<0.005 compared with females aged 60 years; # P<0.005, ## P<0.001, ### P< compared with males aged 40 years; ++ P< compared with males aged 50 years; & P<0.005, && P<0.001 compared with males aged 60 years. (FUJIFILM RI Pharma Co, Ltd, Tokyo, Japan) and a 5-min static acquisition was made in the anterior view at 15 min and 3 h later. Cardiac images were acquired after each static acquisition with a 3-head gamma camera (Toshiba GCA 9300A/HG) equipped with parallel-hole, high-resolution collimators. Energy discrimination was provided by a 15% window centered at 159KeV. Data processing was performed on a Toshiba GMS 5500A system. Left ventricular MIBG activity and washout rate were measured by placing a square region-of-interest over the left ventricle and then the upper mediastinum. The heart-to-mediastinum (H/M) ratios on both the initial and delayed images were calculated to quantify cardiac MIBG uptake as a fraction of the mean counts per pixel in the heart divided by that in the upper mediastinum. 17 The myocardial washout rate was defined as the percentage changes in activity from the initial to the delayed images within the left ventricle and was calculated as follows: washout rate (%)=[(A B/A)] 100, where A is the average count per pixel in the left ventricle on the initial image and B is the average decay-corrected count per pixel in the same region on the delayed image. Decay correction was performed with the assumption that the half-life of the radionuclide ( 123 I) was 13 h. 17 This technique has been confirmed as having a high reproducibility (n=12, r=0.91, P<0.001) for quantifying cardiac MIBG uptake. 17 Statistical Analysis Data are expressed as mean ± SD. Comparisons of the 10-year age groups and between males and females were performed by 1- or 2-way ANOVA followed by Bonferroni multiple comparison test. A simple linear regression analysis used the values of the initial and delayed H/M ratios. Probability values <0.05 were considered significant. Results H/M Ratios and Washout Rate Subject characteristics are presented in Table 1. The females were not as tall as the males, and their body weights were less than those of the males. However, there was no significant difference in BMI between males and females. Fig 1 and Table 1 show the decade-by-decade changes in the initial and delayed H/M ratios in both genders. Both men and women had a significantly decrease in the initial and delayed H/M ratios with advancing age (P<0.0001, for both). In addition, the initial H/M ratio had a significant positive correlation with the delayed H/M ratio (n=153, r=0.89, P<0.0001). Men in the 4 th decade had a significantly higher initial H/M ratio than those in the 7 th and 8 th decades (P<0.001 and P<0.005, respectively). In addition, men in the 5 th decade had a significantly higher initial H/M ratio than those in the 7 th and 8 th decades (P<0.005, for both). Likewise, men in the 4 th decade had a significantly higher delayed H/M ratio than those in the 6 th, 7 th and 8 th decades (P<0.005, P<0.0001, and P<0.0001, respectively). In addition, men in the 5 th decade had a significantly higher delayed H/M ratio than those in the 7 th and 8 th decades (P<0.0001, for both). In females, women in the 4 th decade had a significantly higher initial H/M ratio than those in the 7 th and 8 th decades (P< and P<0.001, respectively). In addition, women in the 5 th decade had a significantly higher initial H/M ratio than those in the 7 th and 8 th decades (P<0.001 and P<0.01, respectively). Likewise, women in the 4 th decade had a significantly higher delayed H/M ratio than those in the 6 th, 7 th and 8 th decades (P<0.01, P<0.001, and P<0.0001, respectively). Likewise, women in the 5 th decade had a significantly higher delayed H/M ratio than those in the 6 th, 7 th and 8 th decades (P<0.01, P<0.01, and P<0.0001, respectively). However, the changes in the initial and the delayed H/M ratio in females were significantly different from those in males (P<0.01 and P<0.03, respectively). The delayed H/M ratio of females (50 59 years) was significantly higher than that of males of the same age (P<0.001). After the age of 60 years, however, the delayed H/M ratio in females progressively decreased with aging. Fig 2 and Table 1 show the decade-by-decade changes in the MIBG washout rate in both genders. Both genders had a significantly progressive increase in the washout rate with advancing age (P<0.0001, for both). In males, men in the 4 th decade had a significant lower washout rate than those in the 6 th, 7 th and 8 th decades (P<0.005, P<0.001, and P<0.0001, respectively). Furthermore, men in the 5 th decade had a significant lower washout rate than those in the 7 th and 8 th decades (P<0.0001, for both) and men in the 6 th decade had a significant lower washout rate than those in the 7 th and 8 th decades (P<0.005 and P<0.001, respectively). In contrast, women in the 4 th decade had a significantly lower washout rate than those in the 6 th, 7 th and 8 th decades (P<0.01, P<0.0001, and P<0.0001, respectively). Likewise, women in the 5 th decade had a significantly lower washout rate than those in the 7 th and 8 th decades (P<0.001, P<0.0001, and P<0.0001, respectively) and women in the 6 th decade

4 Changes in Cardiac SNS on MIBG 313 Fig 3. Comparison of the metaiodobenzylguanidine parameters of females with and without surgical menopause. There was a significant decrease in the delayed heart-to-mediastinum (H/M) ratio between the 2 groups. had a significantly lower washout rate than those in the 8 th decade (P<0.005). Effect of Surgical Menopause Table 2 shows the group characteristics of females with and without surgical menopause. The females without surgical menopause were the same group as the females in their 4 th decade. No significant differences were observed in age, height, body weight or BMI. Fig 3 shows the initial and delayed H/M ratios and the washout rates of the 2 groups. There was a significant decrease in the initial H/M ratio (2.24±0.12 in females with surgical menopause vs 2.42±0.25 in menstruating females; P=0.05) and the delayed H/M ratio (2.28±0.17 in females with surgical menopause vs 2.43±0.22 in menstruating females; P=0.03) and there was a tendency toward an increase in the MIBG washout rate in females with surgical menopause than those without surgical menopause (9.7±6.1% in females with surgical menopause vs 4.7±7.5% in menstruating females; P<0.06). Discussion The present study showed that the initial and delayed H/M ratios of MIBG imaging had significant age- and genderrelated decreases and that the MIBG washout rate had a significant age-related increase in normal subjects. In addition, menstruation could influence these MIBG parameters. Comparison With Previous Studies This study examined the physiological changes in MIBG uptake and washout rate of the whole human heart. In comparison with other studies, this study had several different points. First, we paid special attention to the study population, so we could examine the age- and gender-related changes in MIBG parameters in very carefully selected subjects with a wide range of ages (40 89 years). Second, we analyzed both the initial and the delayed H/M ratio. Although the clinical value of the initial H/M ratio is controversial, 18 it is closely associated with the MIBG washout rate, which is a well-known important factor. The clinical value of the initial H/M ratio was very similar to that of the delayed H/M ratio in the present study, so this relationship is preserved in healthy normal subjects. Third, we are the first to examine the physiological effect of menstruation on MIBG parameters in the human heart. Finally, this study showed that there are gender differences in the physiological changes in the MIBG uptake and washout rate. Implications of MIBG Imaging Several studies have shown that the delayed H/M ratio and washout rate were key factors in the assessment of cardiac SNS function. The delayed H/M ratio reflects global cardiac sympathetic innervation, although MIBG imaging is able to disclose the regional changes in left ventricular innervation. 14,15 The MIBG washout rate, which indicates MIBG spillover from the presynaptic site, is enhanced in parallel with enhanced SNS activity in various cardiovascular diseases, particularly heart failure and hypertension. 16 In addition, Gao et al showed that a dexmedetomidine-induced decrease in SNS activity significantly lowered the MIBG washout, 22 suggesting that MIBG imaging is a non-invasive method of studying myocardial SNS activity. Thus, it appears that the MIBG washout rate is related to cardiac SNS activity. In the present study, changes in the delayed H/M ratio indicated that aging induces a gradual decrease in cardiac SNS innervation (norepinephrine content) in both genders, although there was a significant gender difference in cardiac

5 314 SAKATA K et al. sympathetic innervation in the 5 th decade. In contrast, changes in the MIBG washout rate suggested an age-related progressive increase in cardiac SNS activity in both genders, although in females it was less than in males at all ages. Thus, age-related changes in the cardiac SNS continued for more than 80 years in both genders. In contrast to males, there was a marked change in the cardiac SNS in females between the 5 th and 6 th decades, a sympathetic surge that seemed to be associated with menopause. Comparison With Other Techniques Various new techniques, such as muscle sympathetic nerve activity (MSNA), heart rate variability (HRV), 8,9 and norepinephrine spillover rate measurement 10 have been used to measure the SNS. However, each method has both merits and demerits, although MSNA studies appear to be generally accepted. Ng et al first described the age- and gender-specific influences on MSNA. 11 After that, more precise studies within the decades of life have been shown. Matsukawa et al showed age-related increase in MSNA that were significantly higher in males than in females under the age of 50, 12 and recently Narkiewicz et al did a similar investigation and reported that the age-related increase in MSNA was more prominent in females than in males and that there was gender difference in the younger generations. 13 Those MSNA studies showed that the sympathetic outflow can increase with aging in both genders, whereas females under the age of 40 had significant low activity and the increase in outflow in females overcame that of males after the age of years. The results are quite similar to ours, although there are several differences such as the age range, race, and subject characteristics. A most distinctly different result between the MSNA studies and the present study is that in studies of MSNA the cardiac SNS activity in females is always higher than that in the males during their lifetime. The reason why the present study did not find that phenomenon in any decade remains uncertain. It may result from MIBG imaging, or study selection, study condition, and so on. Mechanisms Contributing to Physiological Changes Possible mechanisms that contribute to the unique features of the physiologic changes in the cardiac SNS have been proposed. There are significant differences in the MIBG uptake and washout rate in the females under the age of 50 compared with males of the same age, which may result from higher parasympathetic nerve activity in younger females than in males, because HRV studies 8,9 have shown that younger females have a higher HF than males and older females. With regard to the reduced MIBG uptake and increased MIBG washout rate with aging, several mechanisms have been postulated. Animal studies report a diminished number of neurons in the heart with aging, 23,24 and increased central sympathetic nervous activity with aging has been reported by the MSNA studies, as well as attenuation of parasympathetic nerve activity with advanced aging, 9,25,26 all pof which may contribute to the age-related changes in MIBG parameters. Furthermore, over the age of 50 years, Esler et al found that extraction of tritiated norepinephrine across the heart is reduced in healthy human subjects, suggesting an age-related reduced neuronal uptake of norepinephrine at presynaptic sites. 10 A similar result was also shown by Li et al, 27 using 18F-fluorodopamine positron emission tomography. Thus, the decrease in MIBG uptake over the age of 50 appears in part to result from reduced neuronal reuptake of norepinephrine with aging. Finally, there was the cardiac sympathetic surge observed in females around the 5 th decade, a time of life that is usually consistent with the onset of menopause. Menopause enhances sympathetic nervous activity 12 and estrogen influences the autonomic nervous system. 28,29 In addition, surgical menopause could be a cardiovascular risk. 31 It is likely that the duration of menopause after surgery can affect the SNS, but it remains unclear and further studies are needed to resolve the problem. Although it seems that menstruation has a protective effect on SNS, Narkiewicz et al reported that menopause per se did not explain the age-related increase in central SNS activity 13 and hormone replacement therapy did not confer a clinical benefit on postmenopausal women. 31 The present study showed a significant difference in the delayed H/M ratio, but not in the washout rate, between females with and without surgical menopause, suggesting that menopause caused cardiac sympathetic denervation but did not increase cardiac sympathetic activity. Considering the controversy surrounding this issue, our results suggest that menopause, together with aging, dramatically changes the cardiac SNS in climacteric women, although menopause alone has more or less a deleterious effect on the cardiac SNS. Clinical Implications Advancing age brings about various physiological changes in the human body. The present study demonstrated the age- and gender-related changes in the global cardiac SNS, similar to the age- and gender-related changes in coronary heart disease mortality and morbidity in comprehensive epidemiologic studies, 5,6 suggesting that gender differences may explain the higher event rate in cardiovascular diseases in males. The concept that aging is the most powerful risk factor for cardiovascular diseases may stem from age differences in the event rate in cardiovascular disease in both genders. 32 However, it is unlikely that these changes directly lead to the development of cardiovascular diseases, because the present study showed normal physiological changes of the cardiac SNS. Therefore, the present study suggests that the healthy older person has protective mechanisms against enhanced cardiac sympathetic activity. Moreover, it is quite natural to pose the following questions: What factors influencing the cardiac SNS can lead to the cardiovascular diseases? Will modification of the cardiac SNS delay or prevent the onset of cardiovascular diseases? Where in the course of aging should we start the modification? and so on. Based on the present results, various interventional studies are needed to answer to these questions. Conclusion Cardiac sympathetic innervation (norepinephrine content) has gender- and age-related changes, whereas cardiac sympathetic activity probably had age-related changes alone. Such information might help us unravel the contribution of cardiac SNS to age- and gender- related disease interactions and, furthermore, its modification may be useful in preventing cardiovascular diseases. References 1. Remme WJ. The sympathetic nervous system and ischaemic heart disease. Eur Heart J 1998; 19: F62 F Kaye D, Esler M. Sympathetic neuronal regulation of the heart in aging and heart failure. Cardiovasc Res 2005; 66:

6 Changes in Cardiac SNS on MIBG 3. Anderson KP. Sympathetic nervous system activity and ventricular tachyarrhythmias: Recent advances. Ann Noninvasive Electrocardiol 2003; 8: Schwartz PJ, La Rovere MT, Vanoli E. Autonomic nervous system and sudden cardiac death: Experimental basis and clinical observations for post-myocardial infarction risk stratification. Circulation 1992; 85(Suppl): I-77 I Lerner DJ, Kannel WB. Pattern of coronary heart disease morbidity and mortality in the sexes: A 26-year follow-up of the Framingham population. Am Heart J 1986; 111: Stokes J 3rd, Kannel WB, Wolf PA, Cupples LA, D Agostino RB. The relative importance of selected risk factors for various manifestations of cardiovascular disease among women and men from years: 30 years of follow-up in the Framingham Study. Circulation 1987; 75: V-65 V Ziegler MG, Lake CR, Kopin IJ. Plasma noradrenaline increases with age. Nature 1976; 261: Kuo TB, Lin T, Yang CC, Li CL, Chen CF, Chou P. Effect of aging on gender differences in neural control of heart rate. Am J Physiol 1999; 277: H2233 H Ramaekers D, Ector H, Aubert AE, Rubens A, Van de Werf F. Heart rate variability and heart rate in healthy volunteers: Is the female autonomic nervous system cardioprotective? Eur Heart J 1998; 19: Esler MD, Thompson JM, Kaye DM, Turner AG, Jennings GL, Cox HS, et al. Effects of aging on the responsiveness of the human cardiac sympathetic nerves to stressors. Circulation 1995; 91: Ng AV, Callister R, Johnson DG, Seals DR. Age and gender influence muscle sympathetic nerve activity at rest in healthy humans. Hypertension 1993; 21: Matsukawa T, Sugiyama Y, Watanabe T, Kobayashi F, Mano T. Gender difference in age-related changes in muscle sympathetic nerve activity in healthy subjects. Am J Physiol 1998; 275: R1600 R Narkiewicz K, Philips BG, Kato M, Hering D, Bieniaszewski L, Somers VK. Gender-selective interaction between aging, blood pressure, and sympathetic nerve activity. Hypertension 2005; 45: Sakata K, Shirotani M, Yoshida H, Kurata C. Physiological fluctuation of the human left ventricle sympathetic nervous system assessed by Idine-123-MIBG. J Nucl Med 1998; 39: Tsuchimochi S, Tamaki N, Tadamura E, Kawamoto M, Fujita T, Yonekura Y, et al. Age and gender differences in normal myocardial adrenergic neuronal function evaluated by iodine-123-mibg imaging. J Nucl Med 1995; 36: Estorch M, Carrio I, Berna L, Lopez-Pousa J, Torres G. Myocardail iodine-labeled metaiodobenzylguanidine 123 uptake relates to age. J Nucl Cardiol 1995; 2: Sakata K, Shirotani M, Yoshida H, Kurata C. Comparison of effects of enalapril and nitrendipine on cardiac sympathetic nervous system in essential cardiology. J Am Coll Cardiol 1998; 32: Yamazaki J, Muto H, Kabano T, Yamashita S, Nanjo S, Inoue A. 315 Evaluation of beta-blocker therapy in patients with dilated cardiomyopathy: Clinical meaning of iodine 123-metaiodobenzylguanidine myocardial single-photon emission computed tomography. Am Heart J 2001; 141: Tsutamoto T, Tanaka T, Sakai H, Nishiyama K, Fujii M, Yamamoto T, et al. Beneficial effect of perindopril on cardiac sympathetic nerve activity and brain natriuretic peptide in patients with chronic heart failure. Circ J 2008; 72: Arimoto T, Sukekawa H, Harada M, Takayama S, Ikeno E, Nishigi K, et al. Short cardiac iodine-123-metaiodobenzylguanidine imaging protocol in heart failure. Circ J 2008; 72: Austin WG, Edwards JE, Frye RL, Gensini GG, Gott VL, Griffith LS, et al. A reporting system on patients evaluated for coronary artery disease: Report of the Ad Hoc Committee for Grading of Coronary Artery Disease, Council on Cardiovascular Surgery, American Heart Association. Circulation 1975; 51: Gao DW, Talke PO, Dae MW. Effects of reduced sympathetic activity on myocardial metaiodobenzylguanidine (MIBG) washout. J Auton Pharmacol 2001; 21: Akamatsu FE, De-Souza RR, Liberti EA. Fall in the number of intracardiac neurons in aging rats. Mech Ageing Dev 1999; 109: Goldberg PB, Kreider MS, McLean MR, Roberts J. Effects of aging at the adrenergic cardiac neuroeffector junction. Fed Proc 1986; 45: Poller U, Nedelka G, Radke J, Ponicke K, Brodde OE. Age-dependent changes in cardiac muscarinic receptor function in healthy volunteers. J Am Coll Cardiol 1997; 29: Du XJ, Dart AM, Riemersma RA. Sex differences in the parasympathetic nerve control of rat heart. Clin Exp Pharmacol Physiol 1994; 21: Li ST, Holmes C, Kopin IJ, Goldstein DS. Aging-related changes in cardiac sympathetic function in humans, assessed by 6 18F-fluorodopamine PET scanning. J Nucl Med 2003; 44: Vongpatanasin WV, Tuncel M, Mansour Y, Arbique D, Victor RG. Transdermal estrogen replacement therapy decreases sympathetic activity in postmenopausal women. Circulation 2001; 103: Liu CC, Kuo TB, Yang CC. Effects of estrogen on gender-related autonomic differences in humans. Am J Physiol Heart Circ Physiol 2003; 285: H2188 H Lobo RA. Surgical menopause and cardiovascular risks. Menopause 2007; 14: Rossouw JE, Anderson GL, Prentice RL, LaCroix AZ, Kooperberg C, Stefanick ML, et al, Writing Group for the Women s Health Initiative Investigators. Risks and benefits of estrogen plus progestin in healthy postmenopausal women: Principal results From the Women s Health Initiative randomized controlled trial. JAMA 2002; 288: Lakatta EG, Levy D. Arterial and cardiac aging: Major shareholders in cardiovascular disease enterprises. Part I: Aging arteries: A Set up for vascular disease. Circulation 2003; 107: