Natural menopause induces impairment of effort tolerance and peak oxygen consumption in sedentary women free from cardiovascular diseases and risk factors1,2 in comparison with premenopausal women of the same age. It has been hypothesized that these changes may be related to the endothelium-dependent dysfunction in peripheral vasomotility. This develops in healthy postmenopausal women3 as a consequence of ovarian hormone deficiency, which does not guarantee an adequate blood supply to the exercising skeletal muscles.2,4 These findings seem to suggest an early introduction of postmenopausal hormone (estrogen-progestin) therapy (HT), but the few studies that observed the effects of HT on exercise performance showed negative or unconvincing results.5–7 However, it should be taken into consideration that the majority of HT studies were performed on female populations whose mean age was approximately 60 years. Thus, these women had generally been postmenopausal for about 10 years at the time of enrollment.
In this context, the present study evaluated vasodilator reserve, exercise tolerance, and aerobic capacity in a population of healthy, sedentary postmenopausal women free from cardiovascular risk factors, in whom menopause occurred within a time span of 2–3 years before their enrollment. The objective was to compare postmenopausal women with matched premenopausal controls, as well as postmenopausal women before and after 3 months of HT.
MATERIALS AND METHODS
The study was a case-control study that included a comparison group; it was approved by the Institutional Ethics Committee (Policlinico Universitario, University of Cagliari). The enrolled women were informed about the purpose and methodology of the study and gave their written consent for inclusion. On the basis of previous studies, which found a 25% difference in peak oxygen consumption between premenopausal and postmenopausal women, we hypothesized an improvement of 20% with HT. Therefore, the number of patients needed was 20.
The study population included 53 healthy, untrained postmenopausal women, aged younger than 55 years, who were consecutively seen at our outpatient clinic over a 6-month period. Thirty of these women (age, mean±standard deviation, 50.6±1.1 years, range 48–52 years) were enrolled in the study and completed it. Reasons for patient exclusion or dropout were the following: nine women were deemed ineligible for inclusion after initial assessments; three refused to give their consent; four were initially included but refused one or more examinations related to the end point; four were found unable to comply with the protocol; and three decided to interrupt HT (see below). In all of the recruited women, menopause occurred between 2 and 3 years before inclusion in the study. Menopause was defined as more than 12 months of amenorrhea confirmed by a follicle-stimulating hormone concentration of more than 20 International Units/mL. Exclusion criteria were clinical history of cardiovascular diseases, family history of breast cancer, recent (within 1 year) deep vein thrombosis, impaired hepatic function (twofold increase in liver serum enzymes), and any condition likely to interfere with exercise testing. Moreover, a baseline gynecologic evaluation, with transvaginal ultrasonogram and mammogram, excluded suspicion of malignancy, uterine myomata, endometrial pathology, and breast cancer. Thirty premenopausal women (mean age 49.2±1.1 years, range 47–51 years), with comparable body mass index and degree of fitness reporting regular menses, randomly selected from subjects evaluated in our outpatient cardiology service, were used as a control group. They were enrolled among women who had been contacted during a project of primary prevention of cardiovascular disease in our department. These subjects were tested in the early follicular phase. None of the enrolled premenopausal and postmenopausal women were characterized by specific risk factors for cardiovascular disease such as being overweight, high total cholesterol, low-density lipoprotein and triglycerides, diabetes, hypertension, or habitual smoking. Women underwent a full cardiovascular assessment (including physical examination, electrocardiogram [ECG], transthoracic echocardiography, and ultrasonography of the neck vessels), spirometry, and laboratory tests (hematology and chemistry and thyroid hormones and antibodies), which revealed all subjects as being free from cardiovascular or pulmonary diseases, euthyroid, and in a state of mental wellbeing. None of the women had used HT before the study was performed.
After baseline evaluation, the postmenopausal group received conjugated equine estrogens 0.625 mg and continuous combined medroxyprogesterone 2.5 mg for 3 months, in accordance with a prospective open-label protocol. During the study, the subjects were asked to maintain their regular diet and degree of physical activity. In addition, in the 2 days immediately preceding each experimental session, premenopausal and postmenopausal subjects were instructed to refrain from consuming alcohol, coffee, or aspirin and to abstain from severe physical exercise. Women were familiarized with the medical environment and instrumentation before testing. On the morning of the testing day, they reported to our laboratory at approximately 8:00 am.
Studies were performed between 9:00 and 10:00 am (after a 12-hour overnight fast), in a quiet room at a controlled temperature (22±1°C) and relative humidity (65±10%). The left brachial artery diameter was assessed by means of high-resolution ultrasonography in basal condition and after forearm ischemia, a procedure that allows measurement of reactive hyperemia of the vessel. In detail, after 15 minutes of supine rest, the artery was imaged by a linear-array transducer (Sonos 5500 equipped with a 7.5–12.5 MHz linear-array transducer, Hewlett Packard, Palo Alto, CA) and scanned over a longitudinal section, 3–5 cm above the elbow, where the clearest image was obtained. The ECG was recorded throughout the study. Blood pressure was monitored by a digital detector (Finapres 2300, Ohmeda, Louisville, CO). The focus zone was set to the depth of the anterior vessel wall. Depth and gain settings were optimized to identify the lumen-vessel wall interface. A pneumatic tourniquet was placed around the forearm, distal to the target artery, and inflated to a pressure of 50 mm Hg above the patient's systolic blood pressure, for 5 minutes. Reactive hyperemia was induced by sudden cuff deflation. The diameter of the right brachial artery was measured three times: 1) at basal rest; 2) during ischemia and the subsequent reactive hyperemia, with the brachial artery continuously imaged for 30 seconds before and 180 seconds after cuff release; and 3) after sublingual glyceryl trinitrate (0.4 mg), to assess endothelium-independent vasodilation. The ultrasound images were recorded directly into the ultrasound machine and analyzed offline. The diameter of the brachial artery was measured from the anterior to the posterior interface. At each recording, the mean arterial diameter was calculated from four cardiac cycles synchronized with the R-wave peak on the electrocardiogram. All measurements were made at end diastole. The flow-mediated dilation was then calculated as the percentage change in diameter compared with basal measurements. The same experienced investigator, blinded to the premenopausal or postmenopausal status of women, performed all studies of each patient to avoid interobserver variability.
Approximately 1 hour after vascular examination and after a light, carbohydrate-rich meal, all women underwent an integrated cardiopulmonary exercise test according to a previously described protocol.2 In brief, after a 3-minute warm-up at a low workload, they cycled at a constant pedal frequency of 60 rpm on a Case 15 bicycle ergometer (Marquette, Milwaukee, WI) with an initial external work of 10 watts, increased by 10 watts/min. Women were strongly encouraged to continue as long as possible and reach muscular exhaustion. Heart rate and rhythm were continuously monitored from a 12-lead ECG; arterial blood pressure was ascertained by the standard sphygmomanometric technique every 3 minutes during exercise and recovery. Breathing rate (f), minute ventilation (Ve), oxygen consumption (Vo2), and carbon dioxide production (Vco2) were determined on a breath-by-breath basis with a Medical Graphics System 2000 (Medical Graphics Corporation, St. Paul, MN). Peak oxygen consumption was defined as the highest value of oxygen consumption achieved at the end of exercise (Vo2max). Anaerobic threshold was determined from the Vco2/Vo2 plot using the V-slope method.8 Finally, the amount of oxygen used during incremental exercise in relation to the quantity of external work performed was estimated by means of the ratio between the increase in oxygen consumption and that in work rate (ΔVo2/ΔWR).
Postmenopausal women were studied at inclusion (T0) and after 3 months of HT use (T1); also premenopausal controls repeated the ergometric test after 3 months to nullify any possible adaptation or training effect. At each visit, a blood sample was also obtained for evaluation of biochemical and hematologic parameters.
Values are reported as mean±1 standard deviation. Differences in mean values between groups were assessed by using the repeated measures analysis of variance. The correlations between the flow-mediated dilation variations and those of ergometabolic records were evaluated by computing the Pearson correlation coefficient. All calculated P values are two-tailed and considered as significant when P<.05.
Baseline clinical and biophysical characteristics were comparable in postmenopausal and control women (Table 1).
No significant differences were found between premenopausal and postmenopausal women with regard to resting heart rate, blood pressure, and double product (heart rate×systolic blood pressure), as well as brachial artery diameter (Table 2). However, endothelial function was considerably impaired in postmenopausal women, as shown by significantly reduced flow-mediated dilation in response to forearm ischemia (P<.001; Table 2). No significant differences were observed between the two groups in regard to endothelium-independent response to sublingual glyceryl trinitrate. The ischemic stress-induced increase in blood pressure was greater in postmenopausal women than in the premenopausal population (6.6±2.2 compared with 3.2±2.0 mm Hg, P<.01), whereas the heart rate response was comparable in the two groups. Finally, ventilation parameters (breathing rate, minute ventilation, and oxygen consumption), measured at rest in seated position on the bicycle ergometer, did not show differences in the comparison between the two groups of women (Table 2).
At peak exercise, heart rate, systolic blood pressure, and double product values resulted in the predictable maximal range9 and were comparable in the two groups of women (Fig. 1). Both premenopausal and postmenopausal subjects exceeded 90% of predicted maximal heart rate (94±5% and 93±3%, respectively) and interrupted the exercise test due to fatigue. No effort-induced ECG changes suggestive of ischemia were found in any woman, but a nonsignificant (less than 0.1 mV) ST depression occurred in nine subjects. Moreover, diastolic blood pressure was significantly higher in postmenopausal than in premenopausal women (92±5 mm Hg compared with 87±5 mm Hg, P<.01; Fig. 1).
Various functional parameters were significantly reduced in postmenopausal women in comparison with the control group (Fig. 2): maximal workload (91±7 watts compared with 96±5 watts, P<.01), Vo2max (1.2±0.3 L/min compared with 1.6±0.1 L/min, P<.001), ΔVo2/ΔWR (11±2 mL/min/watts compared with 15±3 mL/min/watts, P<.001), and anaerobic threshold, expressed in percentage of Vo2max (43±7% compared with 50±6 %, P<.001). Finally, the mean minute ventilation at peak of oxygen consumption was higher in postmenopausal than in premenopausal women (57±9 L/min compared with 52±10 L/min), but the difference was not statistically significant.
No significant differences were observed in hemodynamic parameters at rest in postmenopausal women after 3 months of HT in comparison with the time of enrollment (data not shown). Conversely, in comparison with baseline, HT produced a significant improvement in flow-mediated dilation (P<.001), which became comparable with the respective value of the control group of women. After 3 months of therapy, all ergometabolic measurements that had been found changed at baseline showed significant improvement and did not differ from those of premenopausal women (Fig. 2). In particular, a significant increase of maximal workload (from 91±7 watts to 96±4 watts, P<.001), Vo2max (from 1.2±0.3 L/min to 1.7±0.2 L/min, P<.001), ΔVo2/ΔWR (from 11±2 mL/min/watts to 14±2 mL/min/watts, P<.001), and anaerobic threshold (from 43±7% to 49±5%, P<.01) was found. Flow-mediated dilation variations induced by 3-month HT were positively correlated with those of Vo2max (r=0.77, P<.001) and ΔVo2/ΔWR (r=0.73, P<.001) (Fig. 3).
Metabolic features of postmenopausal women were unaffected by therapy, if we exclude a reduction of body mass index and increase in high-density lipoprotein cholesterol, which did not attain statistical significance.
The present data confirm the recent evidence suggesting that natural menopause induces an impairment of exercise tolerance and maximal oxygen uptake in women free from cardiovascular diseases and risk factors.2 Furthermore, our data show that a compromised peripheral vasodilator reserve is associated with reduced physical performance and that 3 months of HT is able to restore both exercise capacity and endothelium-dependent vasodilation in postmenopausal women.
After natural menopause, ovarian failure produces a direct detrimental effect on vessel-wall physiology. This is mainly due to the reduced levels of nitric oxide, with consequent impairment of arterial vasomotion.10,11 Accordingly, a worsened arterial vasodilating response was found in healthy postmenopausal subjects in comparison with premenopausal women of the same age, both at rest and in the course of physical exercise.3,4 Moreover, accumulating evidence suggests that, after menopause, even in the absence of cardiovascular diseases or profiles of elevated coronary risk, women exhibit a reduced exercise capacity.2,4 Specifically, a decrease of maximal workload, associated with a worsening of aerobic capacity that is the highest oxygen uptake obtainable in course of strenuous exercise, has been recently found in healthy sedentary postmenopausal women2 and confirmed in the present study.
There is much unanimity among authors in ascribing deteriorated physical performance of postmenopausal women to their impaired vasodilator response, with consequent critical reduction in the blood supply to exercising skeletal muscles.3,4,10,11 Our data clearly show such association between peripheral blood flow and exercise capacity. Indeed, we found a statistical correlation between flow-mediated dilation variation and impaired maximal aerobic capacity (Vo2max) and lower values of ΔVo2/ΔWR, an expression of the increase in oxygen uptake in response to a simultaneous increase in work rate. This large set of ergometabolic abnormalities is quite suitable to identify the peripheral circulation as the limiting system in individuals experiencing exercise intolerance.12
After an average time interval of 10 years from the onset of menopause, further alterations follow the aforementioned worsening of vascular function, taking place from deterioration of the metabolic risk profile,13,14 until initiation and progression of atherosclerosis.15 For this reason, we investigated women who had entered menopause a maximum of 3 years before the study, in whom ovarian exhaustion had not produced significant negative consequences, including abdominal fat accumulation, insulin resistance, hypertriglyceridemia, and increased concentration of low-density lipoprotein.
Another unfavorable consequence of ovarian hormone exhaustion is a shift of autonomic cardiovascular control toward sympathetic hyperactivity. A series of enhanced cardiovascular and neuroendocrine responses to different stressors has been demonstrated in postmenopausal compared with premenopausal women of the same age.16–18 Furthermore, women who had undergone bilateral ovariectomy showed higher levels of atherogenic lipids, stress-induced systolic and diastolic blood pressure, and catecholamines than did controls who had undergone hysterectomy alone.19 The increased pressure response found in our postmenopausal women, both at peak of postischemic vascular reactivity and at peak exercise, may be interpreted in accordance with the aforesaid sympathetic prevalence in the postmenopausal cardiovascular system.
The available evidence about the effects of postmenopausal HT on cardiovascular function during exercise in women without known cardiac disease is poor and contradictory. The very few studies that found an improvement of effort tolerance and maximal oxygen consumption were limited by the absence of a control group20 or the use of estrogen alone as replacement therapy.21 On the other hand, the lack of modifications observed by the remaining studies was determined by confounding factors. In a previous report, a cyclical format of HT (1 mg 17β-estradiol daily with cyclic micronized progesterone, 200 mg for 10 days/month)7 was used. Because 17β-estradiol was found to induce an up-regulation of progestin receptors at the level of endothelium and smooth muscle cells of the arterial wall, whereas progestins activate a negative feedback on functional activity of such receptors,22,23 it could be hypothesized that unfavorable vascular effects of progestins are more pronounced in a cyclical rather than in a continuous, combined HT. In another study,6 the advanced age (62.1 years) of the examined subjects hindered the differentiation of hemodynamic consequences of menopause from those caused by aging. For instance, reduced insulin sensitivity usually appears when women are in their 60s or older and have accumulated levels of visceral fat that approximate those of men of the same age.24 Thus, we can infer that, when HT is administered to women who have been postmenopausal for at least 10 years, the acquired cardiovascular modifications scarcely, if at all, benefit from treatment. In contrast with previous studies, we suggest that exercise tolerance and aerobic capacity are favorably influenced by HT in a continuous, combined scheme and administered to 50.6-year-old women who have been postmenopausal for no longer than 30 months.
We believe we can exclude other possible reasons for the significantly improved exercise tolerance in the postmenopausal women than the use of HT. Firstly, the only other possible causes for an improvement in maximal oxygen consumption, such as that observed in our study, are drugs acting on the cardiovascular system or those increasing muscle mass or the regular aerobic exercise; but there was no difference in the level of daily activities and exercise in women taking HT and controls. Secondly, the majority of measured ergometabolic parameters, such as anaerobic threshold, ΔVo2/ΔWR, and flow-mediated dilation, are intrinsic properties of the cardiovascular system. They are not suited to being voluntarily modified, nor are they sensitive to the psychological-emotional status.
Our data give evidence of a close association between the enhancement of vasodilation capacity and the improvement of aerobic muscular work, both induced by HT. These findings are in line with several studies showing the positive hormonal activity of 17β-estradiol on the endothelial function,25,26 as well as the ability of estrogen therapy and HT, both in acute and in chronic administrations, to increase the endothelium-dependent vasodilation, both at rest and in response to isotonic exercise.27,7 A decrease in plasma norepinephrine after administration of 17β-estradiol has been reported in healthy postmenopausal women,28 but this was not investigated here. Thus, in this study, the correction produced by HT on impaired vasodilation and physical performance could only hypothetically be ascribed to a modulation of ovarian hormones on the postmenopausal adrenergic hyperactivity.
One limitation of the present study is the lack of a placebo arm. In placebo-controlled studies of HT, especially in those conducted in early postmenopause, blinding is difficult because of the effect of the hormones on symptoms, breast tenderness, and bleeding. In Women's Health Initiative,29 which studied asymptomatic women, almost 70% of the women taking hormones had either vaginal bleeding or breast tenderness.
Our findings confirm that, even in women without cardiovascular risk factors, menopause causes per se an impairment of exercise capacity within 3 years. The abnormal peripheral vasoreactivity observed in postmenopausal women seems to prevent oxygen flow from matching the oxygen requirement during strenuous isotonic effort. The notion that this effort intolerance is a consequence of estrogen deficiency on peripheral circulation suggested the use of HT. In this study, the use of HT during early menopause proved effective in improving peripheral vascular function and in restoring exercise tolerance, thus anticipating the occurrence of vascular and metabolic changes.
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