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J Am Coll Cardiol, 2001; 37:169-174 © 2001 by the American College of Cardiology Foundation |
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* State University of New York Downstate Medical Center, Brooklyn, New York, USA
Veterans Affairs Medical Center, Long Beach, California, USA
Manuscript received May 18, 2000; revised manuscript received August 3, 2000, accepted September 20, 2000.
Reprint requests and correspondence: Dr. Michael A. Weber, 350 Fifth Avenue, Suite 4002, New York, New York 10118
michaelwebermd{at}cs.com
| Abstract |
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We sought to test whether the differences in activity of the renin-angiotensin and sympathetic nervous systems at rest or during exercise can explain the differing cardiovascular properties and outcomes of lean and obese hypertensive patients.
BACKGROUND
Although lean hypertensive patients have fewer metabolic abnormalities than obese hypertensive patients, paradoxically they appear to have a poorer cardiovascular prognosis.
METHODS
To evaluate the heightened risks in lean hypertensive patients, this study compared metabolic, neuroendocrine and cardiovascular characteristics at rest and during a standardized treadmill protocol in obese (body mass index [BMI] = 32.5 ± 0.3 kg/m2, n = 55) and lean (BMI = 24.3 ± 0.2 kg/m2, n = 66) hypertensive patients. Normotensive obese (n = 21) and lean (n = 55) volunteers served as control subjects.
RESULTS
Compared with the lean normotensive subjects, the lean and obese hypertensive patients had greater left ventricular mass index (LVMI) values, but on multivariate analysis, LVMI correlated with plasma renin activity (p < 0.001) and plasma norepinephrine (PNE) (p < 0.01) in the lean but not the obese hypertensive patients. Arterial compliance (stroke volume/pulse pressure ratio) was reduced in the lean hypertensive patients, in whom it correlated (p = 0.033) with PNE. The PNE rose less (22%) in the obese than in the lean (55%) hypertensive patients in response to standing (p < 0.05). Likewise, during treadmill exercise, there were lesser increases in renin (65% vs. 145%, p < 0.01) and epinephrine (200% vs. 500%, p < 0.05) in the obese hypertensive patients. These changes were also less in obese patients than in lean control subjects, indicating attenuated neurohormonal responses to stress in obesity.
CONCLUSIONS
Compared with obese hypertensive patients, cardiovascular properties in lean hypertensive patients are more dependent on catecholamines and the renin system. The different neuroendocrine responses to dynamic stimuli in lean and obese patients also might help to explain the disparity in their cardiovascular outcomes.
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The main goal of the present study was to test the hypothesis that differences in catecholamine and renin values might discriminate between obese hypertension and lean hypertension. In particular, we have examined whether responses to standardized treadmill testing might reveal properties of these two conditions that might further explain the differences in their clinical outcomes. In addition, we have performed the same observations in obese and lean normotensive control subjects to determine which characteristics can be attributed to hypertension and which to obesity.
| Methods |
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All subjects were carefully instructed in the collection of a 24-h urine sample. Collections were regarded as adequate if 24-h urinary creatinine measurements exceeded 10 mg/kg body weight in women and 15 mg/kg in men. Blood samples for insulin were drawn after an overnight fast. Blood samples for measurement of plasma renin activity were drawn from an indwelling venous port after at least 1 h in the upright position. Blood samples for catecholamine measurements were also drawn from an indwelling venous port after the patient had been quietly recumbent for 30 min. In addition, samples for renin and catecholamines were drawn from an indwelling venous port immediately before treadmill exercise (with the patient standing ready on the treadmill) and immediately on completion of the exercise protocol. Urinary protein values were measured by nephelometry. Plasma values of renin activity (9) and insulin (10) were measured by radioimmunoassay techniques, and plasma catecholamine concentrations were measured by a radioenzyme method (11).
M-mode and two-dimensional echocardiographic studies were performed from parasternal and apical windows. Standard views were obtained by using a commercially available, phased-array, Doppler echocardiographic instrument (General Electric, Pass II or RT500). Standard M-mode echocardiograms were recorded from the parasternal window on strip chart paper at a speed of 50 mm/s. Left ventricular mass was calculated in grams with the equation: left ventricular mass = 1.05 ([LVDD + PWT + VST]3 [LVDD]3), where VST = ventricular septal thickness; LVDD = left ventricular diastolic dimension; and PWT = (left ventricular) posterior wall thickness (12). The ratio of echocardiographically derived stroke volume to pulse pressure (SV/PP ratiothe difference between systolic and diastolic blood pressures measured in close proximity to the echocardiographic procedure) was used as an index of total body arterial compliance.
Each participant underwent a full treadmill test on two separate occasions, according to a modified Balke-Ware treadmill protocol (13). The data from the initial study were used to individualize the protocol for each subject during the definitive study treadmill session, which was performed between 7 and 14 days after the initial session (14). After a 1-min warm-up at 2.0 mph/0% grade, the changes in speed and grade were then computer-adjusted (based on each subjects exercise capacity during the baseline test) to yield a test duration of 10 min. The walking speed was increased in ramp fashion to a level between 2.7 and 4.2 mph, where it remained constant; after that, the treadmill grade began increasing at a rate ranging from 1.0% to 2.5%/min. For a given subject, the overall ramp rate was constant during changes in both speed and grade (14). Exercise was continued until volitional fatigue. A standard 12-lead electrocardiogram was obtained throughout the exercise test for safety purposes and to exclude from analysis any patient who revealed evidence of cardiac disease (there were no such instances).
The principal analytic technique was to compare measurements or responses to exercise among the four separate groups (obese and lean hypertensive patients and obese and lean normotensive subjects) using two-way analysis of variance. One factor was lean/obese, and the other was normotension/hypertension. The interaction term between these two factors was also tested (although, in fact, it did not reach significance for any of the values measured in this study). Regression analysis was done using the Pearson method. Data are shown as the mean value ± SEM. All participants in the study signed an informed consent approved by the Institutional Review Board of the Long Beach Veterans Affairs Medical Center.
| Results |
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| Discussion |
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Responses to exercise. The primary goal of this investigation was to determine whether obese and lean hypertensive patients could be differentiated by their responses to a standardized treadmill protocol. In fact, there was no difference between the two hypertensive groups, or their respective normotensive control groups, in their maximal systolic or diastolic blood pressure changes during the procedure. However, there were significantly greater increases in both plasma epinephrine concentrations and plasma renin activity in the lean hypertensive patients as compared with the obese ones. The corresponding changes in the normotensive groups were virtually identical to those in the hypertensive patients, indicating that obesityregardless of whether or not hypertension is presentmay have an inhibitory effect on the neuroendocrine response to physical stress. We also observed that the increase in PNE concentrations on standing, possibly a reflection of acute sympathetic activation, was greater in the lean hypertensive patients than in the obese ones.
Cardiovascular and renal findings. The role of sympathetic factors in hypertension in the obese individual is not clear. In a large number of studies reporting measurements of plasma or urinary norepinephrine levels at rest or during unspecified activity, approximately equal numbers of studies showed that catecholamine levels were higher in obese hypertensive patients or were higher in lean hypertensive patients, or were similar in each (15). This is consistent with the findings in the present study, where rest plasma catecholamine levels were similar in obese and lean hypertensive patients. It has been suggested, though, by such techniques as norepinephrine spillover (16) or microneurography (17), that there might be increased sympathetic activity in obesity; but, again, these findings may not be consistent (15). Indeed, cardiac left ventricular muscle mass in the present study correlated significantly with plasma concentrations of norepinephrine and epinephrine, as well as with plasma renin activity, in the lean hypertensive patients but not in the obese ones. However, other investigators have previously shown that left ventricular mass in obese individuals appears to be primarily related to increased body weight (4), perhaps reflecting the known myocardial trophic effects of the peptide hormone leptin, which is produced by adipose tissue (18).
The SV/PP ratio, which is used as an index of total arterial compliance (19), differed between the two hypertensive groups. It was significantly lower, suggesting increased arterial stiffness, in the lean hypertensive patients as compared with the obese hypertensive patients or normotensive control subjects. This finding may be of clinical relevance, for SV/PP has been shown to be predictive of physiologic changes in the vasculature (20) and recently was found to have predictive value, even when adjusted for other risk factors, for subsequent clinical cardiovascular events (21). In the present study, this measure of arterial compliance correlated inversely with plasma concentrations of norepinephrine in the lean hypertensive group, but not in any of the other groups. It is possible, therefore, that arterial stiffening may be a characteristic of lean hypertension that distinguishes it from obese hypertension.
There are differences in renal function between obese and lean hypertensive patients. Obesity appears to increase renal sodium reabsorption (22,23), very likely because of heightened renal sympathetic activity (24). This, in turn, could help explain the increased plasma volume and cardiac output found in obese individuals (25). This study has shown that the glomerular filtration rate, as measured by creatinine clearance, is significantly higher in both hypertensive and normotensive obese individuals as compared with lean normotensive control subjects. The lean hypertensive patients also exhibited an increase in glomerular filtration. Similar findings have been reported previously (5). Likewise, we found an increase in the albumin excretion rate in the obese hypertensive patients, probably reflecting their high glomerular filtration rate. Another finding, also previously well demonstrated (3), was the clearly abnormal lipid profile in the obese hypertensive patients. Obesity, per se, may not fully explain these abnormalities, because as compared with the obese normotensive group, the obese hypertensive group had higher low density lipoprotein and lower high density lipoprotein cholesterol concentrations. Thus, the combination of obesity and hypertension in this particular group seems to have additive adverse effects on the lipid profile.
Therapeutic implications.
The hemodynamic data of obese hypertension are characterized by increased stroke volume but normal peripheral resistance, whereas in lean hypertension, stroke volume is normal but peripheral resistance in increased (2628). A recently published report based on a re-analysis of the Systolic Hypertension in the Elderly Program (SHEP) has highlighted the prognostic importance of these differing hemodynamic profiles (29). Treatment with the diuretic chlorthalidone was most effective at reducing mortality in overweight hypertensive patients. Indeed, in hypertensive patients with BMI
24 kg/m2, normally considered a desirable weight, the risk of adverse events rose sharply (29). The findings of the present study that cardiovascular changes in lean hypertensive patients are at least partly mediated by activity of the renin-angiotensin and sympathetic systems may help explain why diuretic therapy, which stimulates these systems, is not fully effective in preventing clinical end points in lean patients. It would also be most interesting to test whether such agents as angiotensin-converting enzyme inhibitors might be preferentially effective in thin hypertensive patients.
Overall, these findings emphasize that obese hypertension and lean hypertension are two distinct conditions. As established previously (26), obese patients with hypertension are characterized by abnormalities of insulin and lipid metabolism and have evidence for left ventricular hypertrophy, renal hyperfiltration and albuminuria. Lean hypertensive patients have similar findings, but they are more dependent on the sympathetic and renin systems. Moreover, in this study, there was an attenuated neuroendocrine response to stress in the obese patients, which might also help to explain why they appear to have a cardiovascular prognostic advantage over lean patients (68,29).
| Footnotes |
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| References |
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