CLINICAL STUDY: HEART FAILURE
Deconditioning fails to explain peripheral skeletal muscle alterations in men with chronic heart failure
Brian D. Duscha, MS*,*,
Brian H. Annex, MD*, ,
Howard J. Green, PhD ,
Anne M. Pippen, MS* and
William E. Kraus, MD
* Department of Medicine, Division of Cardiology, Duke University Medical Center, Durham, North Carolina, USA
Department of Cell Biology, Duke University Medical Center, Durham, North Carolina, USA
Department of Medicine, Division of Cardiology, Durham Veterans Administration Medical Center, Durham, North Carolina, USA
Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada
Manuscript received August 13, 2001;
revised manuscript received November 14, 2001,
accepted January 9, 2002.
* Reprint requests and correspondence: Brian D. Duscha, MS, Duke University Medical Center, Box 3022, Duke Center for Living, Durham, North Carolina 27710, USA.
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Abstract
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OBJECTIVES: It remains controversial whether the skeletal muscle alterations in chronic heart failure (CHF) are due to disease pathophysiology or result from chronic deconditioning. The purpose of this study was to compare the skeletal muscle of CHF patients to peak oxygen consumption (peak VO2) matched sedentary controls.
BACKGROUND: It has been established that skeletal muscle abnormalities are related to the exercise intolerance observed in patients with CHF.
METHODS: We studied the skeletal muscle of sedentary controls and patients with CHF matched for age, gender and peak VO2.
RESULTS: Hypothesis testing for the effects of group (CHF vs. normal), gender, and the interaction group x gender were performed. For capillary density only gender (p = 0.002) and the interaction of group x gender (p = 0.007) were significantly different. For 3-hydroxyl coenzyme A (CoA) dehydrogenase only group effect (p = 0.004) was significantly different. Mean values for capillary density were 1.46 ± 0.28 for CHF men versus 1.87 ± 0.32 for sedentary control men, 1.40 ± 0.32 for CHF women versus 1.15 ± 0.35 for sedentary control women. The activities for 3-hydroxyl CoA dehydrogenase were 3.09 ± 0.88 for CHF men versus 4.05 ± 0.42 for sedentary control men, 2.93 ± 0.72 for CHF women versus 3.51 ± 0.78 for sedentary control women.
CONCLUSIONS: This study suggests that women and men adapt to CHF differently: men develop peripheral skeletal muscle abnormalities that are not attributable to deconditioning; women do not develop the same pathologic responses in skeletal muscle when compared with normal women matched for aerobic capacity.
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Abbreviations and Acronyms
| | CHF | | chronic heart failure | | CoA | | coenzyme A | | LVEF | | left ventricular ejection fraction | | VO2 | | oxygen consumption |
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Studies in exercise physiology have revealed a strong and direct relationship between skeletal muscle phenotype and maximal peak oxygen consumption (VO2) in healthy individuals (1–4). This relationship is true for both cross-sectional studies and longitudinal exercise training studies in healthy subjects. It has also been established that alterations in the characteristics that define a skeletal muscle’s oxidative capacity are related to the exercise intolerance observed in patients with chronic heart failure (CHF). Numerous studies have linked abnormalities in fiber typing, oxidative enzymes, mitochondria, contractile proteins and capillary density to increased glycolytic metabolism and early fatigue in this population (5–9). Although accepted as a partial explanation of decreased exercise capacity in patients with CHF, these skeletal muscle changes have long been the subject of debate: are they due to an abnormal pathophysiology unique to CHF, or are they simply a consequence of decreased physical activity beyond normal deconditioning of sedentary, but healthy, normals? If the latter hypothesis were true, the skeletal muscle of extremely deconditioned normal humans would be expected to take on the characteristics of New York Heart Association class II to III ambulatory patients with mild CHF matched for peak VO2. The present investigation was designed to study the skeletal muscle of normal volunteers and patients with CHF matched for age, gender and peak VO2. The purpose of this study was to compare the skeletal muscle histology and biochemistry of CHF patients with those of aerobically matched normal subjects to address the hypothesis that the alterations observed in CHF skeletal muscle are due to detraining effects independent of disease pathology.
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Methods
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Patient population.
Twenty-six patients (16 men and 10 women) with class II to IV CHF due to left ventricular systolic dysfunction (left ventricular ejection fraction, or LVEF, <35%) and 15 age- and aerobically matched normal sedentary controls (6 men and 9 women) participated in the study. All CHF subjects were on a stable medical regimen for a minimum of three months prior to study. Medications were as follows: 15/16 CHF men and 9/10 CHF women were on digoxin, 15/16 CHF men and 9/10 CHF women were taking diuretics, 16/16 CHF men, 9/10 CHF women were taking angiotensin-converting enzyme inhibitors and 4/16 CHF men and 6/10 CHF women were receiving beta-blocker therapy. All CHF subjects were symptom limited by dyspnea and/or leg fatigue. Seven CHF men and eight CHF women had idiopathic cardiomyopathy; nine CHF men and two CHF women had ischemic cardiomyopathy. All subjects were free of claudication, rales and peripheral bruits. Exclusion criteria included insulin dependent diabetes mellitus, clinically significant chronic obstructive pulmonary disease and peripheral vascular disease. All CHF patients were sedentary and not involved in any form of regular physical activity. There was no indication of cardiopulmonary dysfunction in normal subjects by either history or physical examination, and none exhibited symptoms of ischemic heart disease. Normal subjects were on no medications and none were engaged in any type of regular aerobic exercise.
Study protocol.
All studies were performed under research protocols approved by the Institutional Review Boards of Duke University and the Durham Veterans Administration Medical Center. Each subject was informed of testing protocols and the potential risks and benefits of participation. All subjects provided written consent before participation.
Exercise testing.
All patients with CHF underwent graded upright bicycle exercise to a symptom-limited maximum on a cycle ergometer (Fitron, Lumex, Inc., Ronkonkoma, New York, or Monarck, Varberg, Sweden) with a 12 lead EKG, as previously described in our laboratory (6). The workload began at 150 kpm/min and advanced in 3-min stages of 150 kpm/min. Equilibrium radionuclide angiograms were obtained for subjects at rest using a low energy mobile gamma camera. Expired gases were analyzed continuously using a Sensormedics 4400 unit (Yorba Linda, California).
Muscle biopsies.
Biopsy samples were obtained from the vastus lateralis using a modified Bergstrom needle technique (10). Biopsy sites were anesthetized with a 2% lidocaine solution, and 0.5-cm incisions were made through the skin and fascia lata. The needle was consistently inserted to a depth of 40 to 60 mm. Samples were then mounted in cross-section, in optimal cutting temperature compound (Miles Pharmaceutical, West Haven, Connecticut) beds and snap frozen at –80°C.
Immunohistochemical analysis of vascular density.
Vascular density, expressed as endothelial cells/muscle fiber, was determined by examining the total number of endothelial cells relative to the total number of muscle fibers via light microscopy. Endothelial cells were identified in histologic sections using immunohistological techniques with an endothelial cell specific monoclonal antibody in methods previously described (11,12).
Enzymology.
Maximal activity of the oxidative enzyme 3-hydoxyl-coenzyme A (CoA) dehydrogenase was measured. A frozen tissue sample was homogenized in a phosphate buffer (pH 7.4) containing 0.02% bovine serum albumin (BSA), 5 mM B-mercaptoethanol and 0.05 mM EDTA and diluted (1:100) in 20 mM imidazole buffer with 0.02% BSA (13). Activities were performed on frozen homogenates stored (–80°C) until the time of analysis. Enzyme assays were performed fluorometrically using an end point assay at a temperature of 23°C as outlined previously (13,14).
Statistical analysis.
All data were screened for homogeneity and outlying data points. No outliers were identified. Descriptive statistics were used to calculate means and standard errors for demographic data. An unpaired Student t test was used to determine differences between groups for demographic data. An analysis of variance (ANOVA) was used to detect differences between the four groups (CHF men, CHF women, normal men and normal women). Fixed factors included gender and group. The dependent factors (continuous variables) were capillary density and 3-hydroxyl CoA dehydrogenase. If the corrected ANOVA model revealed significant differences, a post hoc analysis was performed using a Bonferroni correction model. A p value <0.05 was considered statistically significant for all tests.
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Results
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Table 1 contains demographic data for the CHF patients and normal controls. There were no significant differences in age, body mass index or peak VO2 between CHF patients and normal controls. The average respiratory exchange ratio values at peak exercise were 1.32 for normal men and 1.21 for normal women, therefore ensuring a maximal effort was achieved. There were differences in LVEF between both CHF men and normal men (21.3 ± 10.0% vs. 52.5 ± 6.7%, p < 0.001) and CHF women and normal women (29.4 ± 9.0% vs. 61.2 ± 9.0%, p < 0.001). Left ventricular ejection fraction was also higher in CHF women than in CHF men (29.4 ± 9.0% vs. 21.3 ± 10.0%, p < 0.05).
Hypothesis testing for the effects of group, gender and the interaction between group and gender were as follows for capillary density: group effect p = NS, gender effect p = 0.002, and the interaction of group and gender p = 0.007. Hypothesis testing for the effects of group, gender and the interaction between group and gender were as follows for 3-hydroxyl CoA dehydrogenase: group effect p = 0.004, gender effect p = NS, and the interaction of group and gender p = NS.
Group differences for vascular density measurements are based on noncorrected post hoc testing and are represented by Figure 1. The number of endothelial cells/muscle fiber was 1.46 ± 0.28 for CHF men versus 1.87 ± 0.32 for normal men (p < 0.02), 1.40 ± 0.32 for CHF women versus 1.15 ± 0.35 for normal women (p = NS). There were no significant differences in fiber area or fiber diameter between the groups (data not shown).
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Figure 1 Comparison of capillary density (endothelial cells/muscle fiber; mean ± SE) between men and women with chronic heart failure (CHF) and normal subjects (*p < 0.02).
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Figure 2 illustrates the differences between groups for the oxidative enzyme 3-hydroxyl CoA dehydrogenase. The enzyme activities (mol/kg protein/h) were 3.09 ± 0.88 for CHF men versus 4.05 ± 0.42 for normal men (p < 0.02) and 2.93 ± 0.72 for CHF women versus 3.51 ± 0.78 for normal women (p = NS).
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Figure 2 Comparison of the oxidative enzyme 3-hydroxyl coenzyme A (CoA) dehydrogenase (3-HAD) activity (mol/kg/protein; mean ± SE) between men and women with chronic heart failure (CHF) and normal subjects (*p = 0.02).
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In addition, analyses were performed on the oxidative enzyme citrate synthetase and myosin heavy chain isoforms I, IIa and IIx. As in previous investigations (15), these did not yield significant differences between the CHF and normal groups for either gender (data not shown).
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Discussion
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Skeletal muscle abnormalities play an extremely important role in the exercise intolerance observed in CHF. However, the mechanisms underlying the skeletal muscle metabolic abnormalities in CHF are currently poorly understood. Moreover, there is considerable debate regarding the role of deconditioning in this process. Therefore, studies that address this mechanistic component of pathophysiology are of extreme importance because they point toward potential countermeasures for preventing this abnormality. We hypothesized that CHF patients and normal controls matched for age, gender and peak VO2 would have similar skeletal muscle characteristics.
Effect of gender and CHF on skeletal muscle.
The most important finding in this study was that men with CHF and sedentary normal men had different skeletal muscle characteristics, despite having similar peak VO2. Interestingly, this was not found to be true when comparing CHF women and normal women. We found no significant differences in any of the skeletal muscle characteristics examined in this study when comparing normal women to CHF women. In previous work (15), we compared normal and CHF men and women in skeletal muscle characteristics and found significant differences between men and women in the apparent adaptation of skeletal muscle to heart failure. The present study expands and confirms the previous findings by making comparisons with men and women matched for peak VO2 across groups. Power calculations reveal that the analyses for women and men were equally powered to detect differences between normal and CHF subjects in both enzymology and capillary measures. This analysis implies that the observations of significant differences in men, but not women, are indicative of truly distinct biological responses to the heart failure state in skeletal muscle between the genders.
A major, but unexplained, finding in this study was that CHF women had slightly increased (p = NS) capillary density in the presence of reduced 3-hydroxyl CoA (p = NS) compared to normal women. In men, the changes track together (both decreased). These data suggest an adaptive process in women with CHF that is different than that in men. It is possible that alterations in the two domains (metabolic enzymes and vascular remodeling) are controlled by different signaling pathways and molecular responses, thus preserving the oxidative capacity in the skeletal muscle of women CHF patients. Furthermore, cardiopulmonary exercise testing is widely used as a marker of the severity of heart failure and an indicator for the need of cardiac transplantation. If the peripheral adaptations to heart failure in women differ from those in men, and if the underlying pathophysiology is different, then the criteria used diagnostically, therapeutically and prognostically in men may not be appropriate for women. These hypotheses will require further study in larger populations for confirmation.
Deconditioning and skeletal muscle.
A stimulus for conducting this study was the hypothesis that an important noncardiac etiology contributing to leg fatigue in both normals and CHF patients may be an ultra-deconditioned state of the peripheral skeletal muscle. By our definition, ultra-deconditioning is a state of muscle beyond that which would ordinarily exist in a sedentary normal individual. Longitudinal studies of space flight, prolonged bed rest, and hind limb suspension all argue that disuse over and above that observed with activities of daily living can be a major contributor to decreased oxidative capacity in skeletal muscle. Studies in other chronic disease populations, such as cerebral vascular disease and chronic obstructive pulmonary disease, have also observed some skeletal muscle characteristics consistent with severe muscle deconditioning (16,17). Similarly, some have hypothesized, based on previous literature linking skeletal muscle alterations to decreased exercise tolerance in CHF, that this may have represented a tier of deconditioning that has gone unexplored through investigations of normally active subjects.
Limitations of previous studies.
The likely reason that the ultra-deconditioning theory has not been previously studied is that all investigations to date have compared normal subjects not engaged in regular physical activity to class II to IV CHF patients. Despite best efforts in these studies, this has usually resulted in a normal group with a substantially higher peak VO2 than the CHF group. Due to the large differences in peak VO2 between normal controls and CHF patients in most of these studies, it is difficult to discern if the abnormal skeletal muscle alterations found in CHF are a result of disease pathophysiology or ultra-deconditioning. One might expect to find skeletal muscle differences between groups differing in peak VO2 of 15 to 20 ml/kg/min, as they do in these earlier studies. The primary barrier limiting this type of comparison is the difficulty in finding age-matched normal volunteers with peak VO2 below 20 ml/kg/min. For example, a normally active 55-year old male with a height of 180 cm and weight of 90 kg has a predicted peak VO2 of approximately 35 ml/kg/min (18). Therefore, in order to effectively answer the question of whether changes in skeletal muscle of CHF patients are due to ultra-deconditioning or if the pathophysiology of disease is responsible for additional skeletal alterations, a normal control group with matched peak VO2 must be used for comparison. A strength of this investigation, and what makes it especially unique, was that we studied 15 normal controls matched for age, gender and aerobic capacity to the CHF subjects. When matching for aerobic capacity, we found that, at least in men, the skeletal muscle abnormality existing in CHF subjects is well beyond the level that can be attributed to deconditioning alone.
Other possible contributors.
Besides deconditioning, there are other potential mechanisms whereby the pathophysiologic state of CHF may lead to peripheral maladaptive changes. It is possible that chronic hypoperfusion and skeletal muscle hypoxia leads to irreversible alterations. Green (19) has shown chronic hypoxia in normals to be a stimulus for decreased aerobic enzymes. This may be one explanation as to why the men with CHF in this study have decreased 3-hydroxyl CoA dehydrogenase and capillary density compared to age- and peak VO2-matched normal men (Figs. 1 and 2). Other contributors may include abnormal gene regulation, increased sympathetic activity leading to catabolic metabolites, receptor abnormalities or responses to abnormal circulating cytokines. None of the latter potential contributors would be present in normal subjects, despite low aerobic capacities, and therefore may explain the differences observed between CHF men and normal men in this study.
Conclusions.
In conclusion, this study suggests that women and men adapt to CHF differently: men develop peripheral skeletal muscle abnormalities that are not attributable to deconditioning, whereas women with CHF either do not develop the same pathologic responses in skeletal muscle as do men with CHF, or their responses are not as profound as in men. These findings must be confirmed in larger studies and may have important implications for both the diagnosis and treatment of heart failure in women.
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Footnotes
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Supported by Grant HLI7670 (to Dr. Kraus) from the National Heart, Lung, and Blood Institute, Bethesda, Maryland, and by a Merit Review Grant from the Office of Research and Development, Medical Research Service, Department of Veterans Affairs (to Dr. Annex). Dr. Kraus and Dr. Annex received support from an Established Investigator Award from the American Heart Association.
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Abstract
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