© 1998 American Heart Association, Inc.
Original Contributions |
Hormone Replacement Therapy Improves Cardiovascular Risk by Lowering Plasma Viscosity in Postmenopausal Women
From the Preventive Cardiology Center, Lipoprotein and Hemorheology Research Facility, and the Departments of Medicine, Pathology (R.S.R.), and Clinical Nutrition (C.C.T.), Rush Medical College, Chicago, Ill, and the Preventive Cardiology Research and Education Program (L.J.M.), The University of Michigan, Ann Arbor.
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Abstract |
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Abstract—Hormone replacement therapy may protect against cardiovascular disease through several mechanisms that have variable actions on the major determinants of plasma viscosity. Plasma viscosity is an important predictor of incident and recurrent cardiovascular events and mortality in coronary heart disease patients. The effect of estrogen alone or in combination with progestin on plasma viscosity is not known. Using a randomized, double-blind design, we examined the impact of the following daily hormone regimens on plasma viscosity in 23 women: (1) 1 mg estradiol and 2.5 mg medroxyprogesterone (n=7); (2) 1 mg estradiol alone (n=8); and (3) placebo (n=8). Plasma viscosity, fibrinogen, and standard lipoprotein levels were determined at baseline and after 12 weeks of intervention. Plasma viscosity was measured at 37°C with a coaxial microviscometer. Fibrinogen was measured by the Clauss method. Significant changes in plasma viscosity (mPa · s) levels occurred among treatment groups (P<0.01) after the intervention. Plasma viscosity was significantly reduced with estrogen replacement therapy (P<0.01). These data demonstrate that estrogen replacement therapy lowers plasma viscosity. This study suggests an additional mechanism for the cardiovascular protection conferred to postmenopausal women on estrogen replacement therapy.
Key Words: hormone replacement therapy • estrogen • plasma viscosity • coronary heart disease risk
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Introduction |
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Hormone replacement therapy has been associated with lower rates of cardiovascular diseases in postmenopausal women.1 2 The underlying metabolic basis for the reduced vascular risk has been explained, in part, by favorable changes in LDL, HDL, and thrombotic markers such as plasminogen activator inhibitor, fibrinogen, and D-dimer.3 4 5 In contrast, most postmenopausal estrogen formulations increase plasma triglycerides by 24% to 67%.6 7 Elevations in triglycerides are associated with a dose-dependent increase in plasma viscosity8 that may mitigate the cardiovascular benefit of hormone replacement therapy.
Elevated plasma viscosity is an established predictor of initial and recurrent cardiovascular events and mortality in coronary heart disease patients.9 10 11 Plasma viscosity is influenced by the concentration of lipoproteins and other plasma proteins, with the major contribution resulting from fibrinogen.12 Because hormone replacement therapy is accompanied by lipid and fibrinogen changes that may both increase and decrease plasma viscosity, we evaluated whether a 3-month period of daily hormone replacement therapy with either estrogen alone or estrogen and progesterone changed plasma viscosity levels.
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Methods |
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Subjects
Twenty-three women of postmenopausal status as determined by the absence of menses for at least 12 months or a hysterectomy with bilateral oophorectomy were recruited from outpatient clinics at the University of Michigan Medical Center between 1995 and 1996. Subjects were eligible for participation if they had not been on hormone replacement therapy within the previous 6 months and had a nonsuspicious Pap smear and mammography within the past year. Exclusion criteria were current smoking, history of diabetes mellitus, abnormal hepatic or renal function, an acute medical condition within the previous 3 months, fasting triglycerides
3.95 mmol/L
(350 mg/dL) on the initial screening laboratory test, or a
contraindication to estrogen replacement therapy. All subjects were
required to give informed consent, and the protocol was reviewed by the
University of Michigan Institutional Review Board.
Design
The study was a 12-week, double-blind, placebo-controlled
clinical trial. Before treatment, 2 baseline visits 1 week apart were
scheduled to obtain fasting blood levels and other clinical measures.
At the end of the intervention period, 2 follow-up laboratory
evaluations were performed at 11 and 12 weeks. After the baseline
evaluation, participants were randomly assigned to 1 of 3 possible
groups for which daily oral treatment involved the following: (1) 1 mg
estradiol plus 2.5 mg medroxyprogesterone acetate
daily (n=7); (2) 1 mg estradiol (n=8); and (3) placebo control (n=8).
Estradiol (Estrace) and the corresponding placebo were provided by Mead
Johnson Laboratories (Princeton, NJ).
Medroxyprogesterone acetate (Provera) and the
corresponding placebo were provided by The Upjohn Company (Kalamazoo,
Mich). One subject experienced vaginal bleeding on combined hormone
treatment, and therapy was discontinued at 5 weeks. Follow-up
laboratory studies were obtained, and these are included in the
analyses. There were 2 subjects who reported adverse reactions
but continued therapy. The reported symptoms included breast
tenderness, vaginal bleeding, cramps, and swelling in the hands and
feet. The 22 subjects who completed the trial were completely adherent
with the intervention, as based on pill counts performed at 6 and
12 weeks.
Blood Handling
All subjects fasted for a minimum of 12 hours. Blood was drawn
from subjects who were seated for a minimum of 5 minutes. A tourniquet
was applied lightly and for <1 minute, as described
previously.13 Blood was drawn into vacuum tubes
containing a serum separation gel for serum viscosity and total protein
quantification; 15% K3 EDTA for plasma lipid
analysis and plasma viscosity measurements; and 0.129 mol/L
(3.8%) sodium citrate solution for fibrinogen analysis. Serum
and plasma were separated at 4°C for 10 minutes at 3000 rpm. Aliquots
of plasma and serum were transferred with disposable plastic pipettes
into stoppered plastic tubes. All samples were refrigerated at 4°C
until the time of analysis. Plasma and serum aliquots for
viscosity determinations were shipped on wet ice to the Lipoprotein and
Hemorheology Research Facility, Chicago, Ill. Viscosity measurements
were made within 24 hours after venipuncture.
Laboratory Analyses
Viscosity measurements were made on plasma by using a coaxial
cylinder microviscometer and DIN 412 measuring cup (Mettler-Toledo AG),
as described previously.14 The batch coefficient
of variation for plasma viscosity is 2.8%. Plasma concentrations of
total cholesterol, HDL cholesterol, and
triglycerides were assayed by standard chemical techniques.
The LDL cholesterol concentration was estimated by the
Friedewald formula.15 Plasma fibrinogen
measurements were made by the Clauss method (Data-Fi fibrinogen
determination kit)16 and represent an
average of 3 individually acquired measurements that were selected to
minimize previously recognized methodological
variability.17 The batch coefficient of variation
for fibrinogen replicates is 5.8%,17 and the
3-month coefficient of variation using an average of 3 replicate
samples is 8.1%.18 Because the intraindividual
variability in plasma fibrinogen17 and
triglycerides19 is large, 2
independent blood samples were obtained 1 week apart both at baseline
and after 3 months.
Statistics
Statistical analyses were performed with SPSS Windows
version 6.01. All subject data were analyzed on the basis of
intention to treat. Histograms of variables were examined for
normality and transformed, if necessary. Data are expressed as means
and SDs for continuous variables and as proportions for the
categorical variables, such as history of angina and prior history
of myocardial infarction. Because 2 blood samples were obtained 1 week
apart both at baseline and after 3 months, the reported analytes
reflect the average of these 2 independent samples at baseline and at 3
months. Pearson correlation tests were used to examine the associations
between select variables that included age, body mass index, and
waist-to-hip ratio with plasma viscosity. Percent differences in each
variable were calculated as follows: (3-month value-baseline
value)/baseline valuex100. Differences across time and across
treatment groups were compared by using a repeated-measures ANOVA
followed by paired t tests with an adjusted Bonferroni
correction (
of 0.01). Stepwise multivariate linear
regression analyses were also performed to evaluate the
potential influence of group assignment, age, body mass index,
waist-to-hip ratio, and changes in plasma lipids and fibrinogen on
plasma viscosity changes.
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Results |
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There were no significant differences in age (mean±SD, 65.1±7.7 years), body mass index (26.2±3.8 kg/m2), waist-to-hip ratio (0.78±0.06), heart rate (72.4±11.2 bpm), systolic (132.3±19.5 mm Hg) or diastolic (80.7±7.8 mm Hg) blood pressure, or previous history of angina or myocardial infarction among the 3 treatment groups. No significant differences in baseline plasma lipids, fibrinogen, or viscosity levels among treatment groups were observed (the Table
). Plasma viscosity levels were
positively correlated with fibrinogen (r=0.46,
P=0.03) but not with age (r=-0.06,
P=0.78).
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Three-month use of either hormone replacement formulation against
placebo did not result in significant changes in plasma lipids or
fibrinogen. In contrast, changes in plasma viscosity levels were
significant (P<0.01). The estradiol plus
medroxyprogesterone group had a nonsignificant
(P=0.03) lowering of plasma viscosity from (mean±SD)
1.449±0.085 to 1.392±0.051 mPa · s (
4%), the
estradiol-assigned group had a significant (P<0.01)
reduction in plasma viscosity from 1.420±0.039 to 1.365±0.041
mPa · s (4%), and the placebo-assigned women experienced a
nonsignificant (P=0.03) increase in plasma viscosity
(3%) from 1.417±0.082 to 1.464±0.091 mPa · s (the Table
).
No other significant changes among groups were found. However,
significant reductions in fibrinogen (P=0.003) and LDL
cholesterol (P<0.01) levels were observed after
3 months, regardless of treatment assignment.
Three-month differences in plasma viscosity were best explained by hormonal treatment group when body mass index; age; waist-to-hip ratio; systolic and diastolic blood pressures; treatment group; and changes in fibrinogen, LDL cholesterol, HDL cholesterol, total cholesterol, total triglycerides, or the cross product of LDL cholesterol and fibrinogen were available for selection (adjusted r2=0.44, P=0.0004). Treatment status and age were selected by backward selection analyses with an adjusted r2=0.50, P=0.0004.
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Discussion |
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One well-known consequence of most common hormone replacement formulations is an increase in plasma triglycerides (24% to 67%) that occurs through estrogen-induced stimulation of hepatic VLDL secretion and inhibition of hepatic triglyceride lipase.5 6 Elevated triglyceride levels in women are independent predictors of coronary heart disease mortality and angiographic progression of coronary atherosclerosis.20 21 Thus, any resulting increase in plasma triglycerides may negate the beneficial influence of hormone replacement therapy in some women, because elevated triglycerides are associated with elevations in plasma viscosity.8
In contrast to potential hypertriglyceridemia-induced hyperviscosity, hormone replacement therapy lowers fibrinogen,5 which is a major determinant of plasma viscosity.12 14 The Postmenopausal Estrogen Progestin Interventions trial showed that placebo-treated women experienced significant increases in plasma fibrinogen levels that were blunted by estrogen alone or combined hormone replacement treatment.5 Findings from the Atherosclerosis Risk in Communities study22 and the FINRISK Hemostasis study23 showed that women using estrogen alone or combined hormone formulations had lower fibrinogen concentrations than those observed in nonusers.
In this study of postmenopausal women with fasting
triglycerides <3.2 mmol/L (283 mg/dL), short-term
estradiol therapy lowered plasma viscosity by 0.055±0.022 mPa
· s (P<0.01). Combined hormone replacement therapy
lowered plasma viscosity by a similar magnitude (0.057±0.052 mPa
· s); however, these changes were not statistically significant at an
of 0.01 (P=0.03). This magnitude of plasma viscosity
change approximates a 1-SD change in women as determined by our
group.14 The Caerphilly and Speedwell studies
reported that a 2% plasma viscosity difference, or 0.010 mPa ·
s, in men was associated with a 4% change in coronary heart
disease risk.24 An age-standardized difference of
0.032 to 0.047 mPa · s discriminated between men with and
without incident ischemic heart
disease.24 In this study, hormone replacement
therapies resulted in much larger changes in plasma viscosity, but we
are not ascertain whether the magnitude of such changes would occur
among women with higher plasma triglyceride levels or
whether these changes are maintained with a longer treatment
duration.
Estradiol treatment was accompanied by a nonsignificant increase in plasma triglycerides of +19% that did not offset the favorable changes on plasma viscosity. The lipid changes with estradiol are consistent with 2 other studies that used this estrogen formulation.7 25 In contrast to estradiol treatment alone, women on combined hormone therapy had a nonsignificant reduction in triglycerides of -21%, and women assigned to placebo therapy had a nonsignificant increase in triglycerides of 10%. Plasma fibrinogen levels did not change as a function of treatment assignment, although there was a significant fall in fibrinogen levels with time (P=0.003). The reasons for the observed reductions in fibrinogen and LDL cholesterol are not apparent. This outcome would be unlikely in studies of longer duration and may reflect a Hawthorne effect. In addition, the variability in plasma fibrinogen exceeds that of plasma viscosity and may have precluded detection of a groupxtime interaction for fibrinogen measurement (P=0.22).
The literature on hormonal therapy and plasma viscosity is sparse. In 1 report, oral contraceptive users had no significant changes in plasma viscosity, but these conclusions were limited by small sample size.26 Another small study of women treated with triphasic oral contraceptives showed an increase in plasma viscosity after 3 to 6 months of treatment.27 This difference may be due to the higher concentrations of estrogen and progestins in oral contraceptive preparations compared with those for hormone replacement.
To our knowledge, this is the first report that evaluates the influence of hormone replacement therapy on plasma viscosity in postmenopausal women. Plasma viscosity is a more sensitive indicator of change as a consequence of hormonal replacement therapy than are triglycerides17 or fibrinogen,19 owing to smaller measurement variability.18 28 Our work supports a new mechanism through which estrogen replacement therapy reduces cardiovascular risk in postmenopausal women. This beneficial property was reduced by concomitant progestin administration. Further studies are needed to evaluate the safety of hormone replacement therapy in postmenopausal women with moderate to severe elevations in triglycerides and the relationship between fasting versus postprandial triglyceride levels on plasma viscosity. In conclusion, estrogen replacement therapy lowers plasma viscosity in postmenopausal women with fasting triglycerides <3.2 mmol/L.
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Acknowledgments |
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L.J.M.'s work was supported by Mead Johnson Laboratories, Princeton, NJ.
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Footnotes |
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Reprint requests to Robert S. Rosenson, MD, FACC, Preventive Cardiology Center, Rush-Presbyterian–St Luke's Medical Center, 1725 W Harrison St, Chicago, IL 60612.
Received October 22, 1997; accepted May 26, 1998.
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References |
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