Beth A. Parker

Assessment of flow-mediated dilation in humans: a methodological and physiological guideline

Endothelial dysfunction is now considered an important early event in the development of atherosclerosis, which precedes gross morphological signs and clinical symptoms. The assessment of flow-mediated dilation (FMD) was introduced almost 20 years ago as a noninvasive approach to examine vasodilator function in vivo. FMD is widely believed to reflect endothelium-dependent and largely nitric oxide-mediated arterial function and has been used as a surrogate marker of vascular health. This noninvasive technique has been used to compare groups of subjects and to evaluate the impact of interventions within individuals. Despite its widespread adoption, there is considerable variability between studies with respect to the protocols applied, methods of analysis, and interpretation of results. Moreover, differences in methodological approaches have important impacts on the response magnitude, can result in spurious data interpretation, and limit the comparability of outcomes between studies. This review results from a collegial discussion between physiologists with the purpose of developing considered guidelines. The contributors represent several distinct research groups that have independently worked to advance the evidence base for improvement of the technical approaches to FMD measurement and analysis. The outcome is a series of recommendations on the basis of review and critical appraisal of recent physiological studies, pertaining to the most appropriate methods to assess FMD in humans.

Effect of Statins on Skeletal Muscle Function

Background— Many clinicians believe that statins cause muscle pain, but this has not been observed in clinical trials, and the effect of statins on muscle performance has not been carefully studied. Methods and Results— The Effect of Statins on Skeletal Muscle Function and Performance (STOMP) study assessed symptoms and measured creatine kinase, exercise capacity, and muscle strength before and after atorvastatin 80 mg or placebo was administered for 6 months to 420 healthy, statin-naive subjects. No individual creatine kinase value exceeded 10 times normal, but average creatine kinase increased 20.8±141.1 U/L (P<0.0001) with atorvastatin. There were no significant changes in several measures of muscle strength or exercise capacity with atorvastatin, but more atorvastatin than placebo subjects developed myalgia (19 versus 10; P=0.05). Myalgic subjects on atorvastatin or placebo had decreased muscle strength in 5 of 14 and 4 of 14 variables, respectively (P=0.69). Conclusions— These results indicate that high-dose atorvastatin for 6 months does not decrease average muscle strength or exercise performance in healthy, previously untreated subjects. Nevertheless, this blinded, controlled trial confirms the undocumented impression that statins increase muscle complaints. Atorvastatin also increased average creatine kinase, suggesting that statins produce mild muscle injury even among asymptomatic subjects. This increase in creatine kinase should prompt studies examining the effects of more prolonged, high-dose statin treatment on muscular performance. Clinical Trial Registration— URL: http://www.clinicaltrials.gov. Unique identifier: NCT00609063.

An assessment by the Statin Muscle Safety Task Force: 2014 update

The National Lipid Associations Muscle Safety Expert Panel was charged with the duty of examining the definitions for statin-associated muscle adverse events, development of a clinical index to assess myalgia, and the use of diagnostic neuromuscular studies to investigate muscle adverse events. We provide guidance as to when a patient should be considered for referral to neuromuscular specialists and indications for the performance of a skeletal muscle biopsy. Based on this review of evidence, we developed an algorithm for the evaluation and treatment of patients who may be intolerant to statins as the result of adverse muscle events. The panel was composed of clinical cardiologists, clinical lipidologists, an exercise physiologist, and a neuromuscular specialist.

Effect of statins on skeletal muscle: exercise, myopathy, and muscle outcomes.

Statins are effective in reducing low-density lipoprotein cholesterol and cardiac events but can produce muscle side effects. We have hypothesized that statin-related muscle complaints are exacerbated by exercise and influenced by factors including mitochondrial dysfunction, membrane disruption, and/or calcium handling. The interaction between statins, exercise, and muscle symptoms may be more effectively diagnosed and treated as rigorous scientific studies accumulate.

Effect of Statins on Creatine Kinase Levels Before and After a Marathon Run

We measured the serum levels of myoglobin, total creatine kinase (CK), and the CK myocardial (CK-MB), muscle (CK-MM), and brain (CK-BB) isoenzymes in 37 subjects treated with statins and 43 nonstatin-treated controls running the 2011 Boston Marathon. Venous blood samples were obtained the day before (PRE) and within 1 hour (FINISH) and 24 hours after (POST) the race. The hematocrit and hemoglobin values were used to adjust for changes in the plasma volume. The CK distribution was normalized using log transformation before analysis. The exercise-related increase in CK 24 hours after exercise, adjusted for changes in plasma volume, was greater in the statin users (PRE to POST 133 ± 15 to 1,104 ± 150 U/L) than in the controls (PRE to POST 125 ± 12 to 813 ± 137 U/L; p = 0.03 for comparison). The increase in CK-MB 24 hours after exercise was also greater in the statin users (PRE to POST 1.1 ± 3.9 to 8.9 ± 7.0 U/L) than in the controls (PRE to POST 0.0 ± 0.0 to 4.2 ± 5.0 U/L; p <0.05 for comparison). However, the increases in muscle myoglobin did not differ at any point between the 2 groups. Increases in CK at both FINISH and POST race measurements were directly related to age in the statin users (r(2) = 0.13 and r(2) = 0.14, respectively; p <0.05) but not in the controls (r(2) = 0.02 and r(2) = 0.00, respectively; p >0.42), suggesting that susceptibility to exercise-induced muscle injury with statins increases with age. In conclusion, our results show that statins increase exercise-related muscle injury.

Evidence for sex differences in cardiovascular aging and adaptive responses to physical activity

There are considerable data addressing sex-related differences in cardiovascular system aging and disease risk/progression. Sex differences in cardiovascular aging are evident during resting conditions, exercise, and other acute physiological challenges (e.g., orthostasis). In conjunction with these sex-related differences—or perhaps even as an underlying cause—the impact of cardiorespiratory fitness and/or physical activity on the aging cardiovascular system also appears to be sex-specific. Potential mechanisms contributing to sex-related differences in cardiovascular aging and adaptability include changes in sex hormones with age as well as sex differences in baseline fitness and the dose of activity needed to elicit cardiovascular adaptations. The purpose of the present paper is thus to review the primary research regarding sex-specific plasticity of the cardiovascular system to fitness and physical activity in older adults. Specifically, the paper will (1) briefly review known sex differences in cardiovascular aging, (2) detail emerging evidence regarding observed cardiovascular outcomes in investigations of exercise and physical activity in older men versus women, (3) explore mechanisms underlying the differing adaptations to exercise and habitual activity in men versus women, and (4) discuss implications of these findings with respect to chronic disease risk and exercise prescription.

25(OH) Vitamin D is Associated with Greater Muscle Strength in Healthy Men and Women

PURPOSE The purpose of the study was to examine the relation between serum 25-hydroxy vitamin D (25(OH)D) levels and muscle strength in 419 healthy men and women over a broad age range (20-76 yr). METHODS Isometric and isokinetic strength of the arms and legs was measured using computerized dynamometry, and its relation to vitamin D was tested in multivariate models controlling for age, gender, resting HR, systolic blood pressure, diastolic blood pressure, body mass index, maximal oxygen uptake (VO(2max)), physical activity counts, and season of vitamin D measurement. RESULTS Vitamin D was significantly associated with arm and leg muscle strength when controlling for age and gender. When controlling for other covariates listed previously, vitamin D remained directly related to both isometric and isokinetic arm strength but only to isometric leg strength. CONCLUSION These data suggest that there may be a differential effect of vitamin D on upper and lower body strength. The mechanism for this difference remains unclear but could be related to differences in androgenic effects or to differences in vitamin D receptor expression. Our study supports a direct relation between vitamin D and muscle strength and suggests that vitamin D supplementation be evaluated to determine whether it is an effective therapy to preserve muscle strength in adults.

Heterogenous vasodilator pathways underlie flow-mediated dilation in men and women.

This study investigated the sex differences in the contribution of nitric oxide (NO) and prostaglandins (PGs) to flow-mediated dilation (FMD). Radial artery (RA) FMD, assessed as the dilatory response to 5-min distal cuff occlusion, was repeated after three separate brachial artery infusions of saline (SAL), N(G)-monomethyl-L-arginine (L-NMMA), and ketorolac (KETO) + L-NMMA in healthy younger men (M; n = 8) and women (W; n = 8). In eight subjects (4 M, 4W) RA FMD was reassessed on a separate day with drug order reversed (SAL, KETO, and L-NMMA + KETO). RA FMD was calculated as the peak dilatory response observed relative to baseline (%FMD) and expressed relative to the corresponding area under the curve shear stress (%FMD/AUC SS). L-NMMA reduced %FMD similarly and modestly (P = 0.68 for sex * trial interaction) in M and W (all subjects: 10.0 ± 3.8 to 7.6 ± 4.7%; P = 0.03) with no further effect of KETO (P = 0.68). However, all sex * trial and trial effects on %FMD/AUC SS for l-NMMA and KETO + l-NMMA were insignificant (all P > 0.20). There was also substantial heterogeneity of the magnitude and direction of dilator responses to blockade. After l-NMMA infusion, subjects exhibited both reduced (n = 14; range: 11 to 78% decrease) and augmented (n = 2; range: 1 to 96% increase) %FMD. Following KETO + l-NMMA, seven subjects exhibited reduced dilation (range: 10 to 115% decrease) and nine subjects exhibited augmented dilation (range: 1 to 212% increase). Reversing drug order did not change the nature of the findings. These findings suggest that RA FMD is not fully or uniformly NO dependent in either men or women, and that there is heterogeneity in the pathways underlying the conduit dilatory response to ischemia.

Relation of Vitamin D Level to Maximal Oxygen Uptake in Adults

Low cardiorespiratory fitness and low serum 25-hydroxy vitamin D (25[OH]D) levels are associated with increased cardiovascular and all-cause mortality, but whether low 25(OH)D is independently associated with cardiorespiratory fitness in healthy adults is not known. We examined 25(OH)D levels and fitness in 200 healthy adults participating in a double-blind clinical trial investigating statins and muscle performance (STOMP study). Maximal aerobic exercise capacity (Vo₂(max)) was measured using metabolic gas analysis during graded treadmill exercise to exhaustion. 25(OH)D was measured using an enzyme-linked immunosorbent assay. Daily physical activity was assessed using the Paffenbarger Physical Activity Questionnaire. Serum 25(OH)D concentration was positively related to Vo₂(max) (r = 0.29, p = 0.0001), even after adjusting for relevant predictors (e.g., age, gender, and body mass index). There was also a significant interaction between 25(OH)D level and self-reported hours of moderate to vigorous physical activity (MVPA; p < 0.02). With each SD increase in 25(OH)D, Vo₂(max) increased by 2.6 ml/kg/min (p = 0.0001) when MVPA was low (16 hours/week) and 1.6 ml/kg/min (p <0.0004) when MVPA was moderate (35 hours/week) but only 0.01 ml/kg/min (p = 0.9) when MVPA was high (64 hours/week). In conclusion, serum 25(OH)D levels predict Vo₂(max) in adults; the effect is greatest in those with low levels of physical activity.

A Randomized Clinical Trial to Assess the Effect of Statins on Skeletal Muscle Function and Performance: Rationale and Study Design

Hydroxymethylglutaryl-coenzyme A reductase inhibitors or statins are the most effective medications for reducing elevated concentrations of low-density lipoprotein cholesterol (LDL-C). Statins reduce cardiac events in patients with coronary artery disease and previously healthy persons. Current recommendations for LDL-C treatment goals indicate that more patients will be treated with higher doses of these medications. Statins have been extremely well-tolerated in controlled clinical trials but are increasingly recognized to produce skeletal muscle myalgia, cramps, and weakness. The reported frequency of such mild symptoms is not clear, and muscle performance has not been examined with these medications. Accordingly, the present investigation, the Effect of Statins on Skeletal Muscle Function and Performance (STOMP) study, will recruit approximately 440 healthy persons. Participants will be randomly assigned to treatment with atorvastatin 80 mg/d or placebo. Handgrip, elbow and knee isometric and isokinetic strength, knee extensor endurance, and maximal aerobic exercise performance will be determined at baseline. Participants will undergo repeat testing after 6 months of treatment or after meeting the study definition of statin myalgia. This study will determine the effect of statins on skeletal muscle strength, endurance, and aerobic exercise performance and may ultimately help clinicians better evaluate statin-related muscle and exercise complaints.