Age and flow-mediated dilation: a comparison of dilatory responsiveness in the brachial and popliteal arteries

Redundant vasodilator pathways underlying radial artery flow-mediated dilation are preserved in healthy aging

Background. Blocking nitric oxide (NO) and vasodilator prostanoids (PN) does not consistently reduce flow-mediated dilation (FMD) in young adults. The impact of aging on the contribution of NO and PG to FMD is unknown. Methods. FMD was measured in older adults (n = 10, 65 ± 3 y) after arterial infusion of saline, N(G)-monomethyl-L-arginine (L-NMMA), and ketorolac + L-NMMA. Data were compared to published data in young adults. Results. L-NMMA reduced FMD in older adults (8.9 ± 3.6 to 5.9 ± 3.7%) although this was not statistically significant (P = 0.08) and did not differ (P = 0.74) from the reduction observed in young adults (10.0 ± 3.8 to 7.6 ± 4.7%; P = 0.03). Blocking PN did not affect FMD in young or older adults. In older adults, L-NMMA reduced (n = 6; range = 36-123% decrease), augmented (n = 3; 10-122% increase), or did not change FMD (n = 1; 0.4% increase). After PN blockade, FMD responses were reduced (n = 2), augmented (n = 6), or unaffected (n = 1). Conclusions. NO or PN blockade did not consistently reduce FMD in healthy older adults, suggesting the existence of redundant vasodilator phenotypes as observed previously in young adults.

Endothelial dysfunction in metabolic and vascular disorders

Vascular endothelium has important regulatory functions in the cardiovascular system and a pivotal role in the maintenance of vascular health and metabolic homeostasis. It has long been recognized that endothelial dysfunction participates in the pathogenesis of atherosclerosis from early, preclinical lesions to advanced, thrombotic complications. In addition, endothelial dysfunction has been recently implicated in the development of insulin resistance and type 2 diabetes mellitus (T2DM). Considering that states of insulin resistance (eg, metabolic syndrome, impaired fasting glucose, impaired glucose tolerance, and T2DM) represent the most prevalent metabolic disorders and risk factors for atherosclerosis, it is of considerable scientific and clinical interest that both metabolic and vascular disorders have endothelial dysfunction as a common background. Importantly, endothelial dysfunction has been associated with adverse outcomes in patients with established cardiovascular disease, and a growing body of evidence indicates that endothelial dysfunction also imparts adverse prognosis in states of insulin resistance. In this review, we discuss the association of insulin resistance and T2DM with endothelial dysfunction and vascular disease, with a focus on the underlying mechanisms and prognostic implications of the endothelial dysfunction in metabolic and vascular disorders. We also address current therapeutic strategies for the improvement of endothelial dysfunction.

Acute impact of retrograde shear rate on brachial and superficial femoral artery flow-mediated dilation in humans

Retrograde shear rate (SR) in the brachial artery (BA) is associated with endothelial dysfunction; a precursor to atherosclerosis. The BA does not typically manifest clinical atherosclerosis, whereas the superficial femoral artery (SFA) is more prone to developing plaque. Examine whether the impact of incremental levels of retrograde SR differs between atherosclerosis-prone (i.e., SFA) and -resistant vessels (i.e., BA) in healthy men. Thirteen healthy young men reported three times to the laboratory. We examined BA flow-mediated dilation (FMD) before and after 30-min exposure to cuff inflation around the forearm at 0, 30, and 60 mmHg, to manipulate retrograde SR. Subsequently, the 30-min intervention was repeated in the SFA, using the same cuff pressure as in the forearm. Order of testing (vessel and intervention) was randomized among subjects. We found a dose-dependent increase in retrograde SR with 30 and 60 mmHg cuff inflation, which was present in both the BA and SFA (all P < 0.05). BA and SFA FMD decreased after the 30-min intervention ("time": P = 0.012), and this was dependent on cuff pressure ("cuff × time": P = 0.024). A significant decrease in FMD was observed after 60 mmHg only and this change was similarly present in both arteries ("time × artery": P = 0.227). Moreover, the BA and SFA demonstrate a similar relationship between changes in retrograde SR and FMD (r = 0.498 and 0.475, respectively). Our study demonstrates that acute exposure to an increase in retrograde shear leads to comparable decreases in FMD in atherosclerotic-prone and -resistant conduit arteries in humans.

Change in the lower limb deep venous flow in peripheral atherosclerotic arterial disease

Aim

This prospective study was undertaken to determine how peripheral atherosclerotic disease influences the flow in the deep veins of the leg.

Material and method

Thirty one subjects with peripheral atherosclerotic disease and 23 age matched control subjects were studied. The popliteal vein flow velocity was measured at rest and during reactive hyperemia by means of color duplex ultrasound scanning. Patient age, ankle-brachial index (ABI) and the presence of risk factors for venous thrombosis were also recorded.

Results

There was a negative correlation between the ankle-brachial index and venous flow velocity among subjects with peripheral arterial disease (p=0.001). There was a negative correlation between dyslipidemia and resting venous flow velocity (p=0.049). During reactive hyperemia, venous flow velocity increased less in subjects with peripheral arterial disease than it did in control subjects (p=0.007). The subjects with dyslipidemia showed no changes in venous flow velocity in reactive hyperemia measurements (p=0.908).

Conclusion

Increasing the venous flow velocity in peripheral arterial disease, may confer some protection against the deep venous thrombosis.

Effect of head-out water immersion on vascular function in healthy subjects

Immersion in thermoneutral water increases cardiac output and peripheral blood flow and reduces systemic vascular resistance. This study examined the effects of head-out water immersion on vascular function. Twelve healthy middle-aged males were immersed during 60 min in the seated position, with water at the level of xiphoid. Local and central vascular tone regulating systems were studied during that time. Brachial artery diameter and blood flow were recorded using ultrasonography and Doppler. Endothelial function was assessed with flow-mediated dilation. Results were compared with the same investigations performed under reference conditions in ambient air. During water immersion, brachial artery diameter increased (3.7 ± 0.2 mm in ambient air vs. 4 ± 0.2 mm in water immersion; p < 0.05). Endothelium-mediated dilation was significantly lower in water immersion than in ambient air (10% vs. 15%; p = 0.01). Nevertheless, the difference disappeared when the percentage vasodilatation of the brachial artery was normalized to the shear stimulus. Smooth muscle-mediated dilation was similar in the 2 conditions. Spectral analysis of systolic blood pressure variability indicated a decrease in sympathetic vascular activity. Plasma levels of nitric oxide metabolites remained unchanged, whereas levels of natriuretic peptides were significantly elevated. An increase in brachial blood flow, a decrease in sympathetic activity, a warming of the skin, and an increase in natriuretic peptides might be involved in the increase in reference diameter observed during water immersion. Endothelial cell reactivity and smooth muscle function did not appear to be altered.

Errors in the estimation of wall shear stress by maximum Doppler velocity

OBJECTIVE

Wall shear stress (WSS) is an important parameter with links to vascular (dys)function. Difficult to measure directly, WSS is often inferred from maximum spectral Doppler velocity (Vmax) by assuming fully-developed flow, which is valid only if the vessel is long and straight. Motivated by evidence that even slight/local curvatures in the nominally straight common carotid artery (CCA) prevent flow from fully developing, we investigated the effects of velocity profile skewing on Vmax-derived WSS.

METHODS

Velocity profiles, representing different degrees of skewing, were extracted from the CCA of image-based computational fluid dynamics (CFD) simulations carried out as part of the VALIDATE study. Maximum velocities were calculated from idealised sample volumes and used to estimate WSS via fully-developed (Poiseuille or Womersley) velocity profiles, for comparison with the actual (i.e. CFD-derived) WSS.

RESULTS

For cycle-averaged WSS, mild velocity profile skewing caused ±25% errors by assuming Poiseuille or Womersley profiles, while severe skewing caused a median error of 30% (maximum 55%). Peak systolic WSS was underestimated by ~50% irrespective of skewing with Poiseuille; using a Womersley profile removed this bias, but ±30% errors remained. Errors were greatest in late systole, when skewing was most pronounced. Skewing also introduced large circumferential WSS variations: ±60%, and up to ±100%, of the circumferentially averaged value.

CONCLUSION

Vmax-derived WSS may be prone to substantial variable errors related to velocity profile skewing, and cannot detect possibly large circumferential WSS variations. Caution should be exercised when making assumptions about velocity profile shape to calculate WSS, even in vessels usually considered long and straight.

Peak and time-integrated shear rates independently predict flow-mediated dilation

BACKGROUND

To determine whether peak and time-integrated shear rates independently predict flow-mediated dilation (FMD).

METHODS

Eleven physically active (25 ± 5 years old) male subjects were tested. FMD was defined as the shear rate-diameter relationship. Hierarchical linear modeling was used to estimate brachial artery diameter change with repeated measures of shear rate nested within each subject. Two models were tested: 1) FMD was induced using ischemia-induced hyperemia (2, 4, 6, and 10 minutes); and 2) FMD was induced following transient (ischemia) and steady-state (forearm heating and handgrip exercise) increases in shear rate. For both models we determined whether peak, in addition to time-integrated shear rates, explained a significant portion of variation for diameter change.

RESULTS

Model 1: Time integrated shear rates explained most of the variation for diameter change. However, peak shear rate explained an additional significant portion of variation. Model 2: The transient condition resulted in significantly (p = 0.012) smaller diameter change per shear rate change than the steady-state condition. However, when specifying peak shear rate as a covariate, the difference between conditions became nonsignificant (p = 0.138).

CONCLUSIONS

Peak and time-integrated shear rates independently predict FMD. Future studies using the FMD test should consider both parameters.

Relationship of flow-mediated arterial dilation and exercise capacity in older patients with heart failure and preserved ejection fraction

BACKGROUND

Older heart failure patients with preserved ejection fraction (HFpEF) have severely reduced exercise capacity and quality of life. Both brachial artery flow-mediated dilation (FMD) and peak exercise oxygen uptake (peak VO(2)) decline with normal aging. However, uncertainty remains regarding whether FMD is reduced beyond the degree associated with normal aging and if this contributes to reduced peak VO(2) in elderly HFpEF patients.

METHODS

Sixty-six older (70 ± 7 years) HFpEF patients and 47 healthy participants (16 young, 25 ± 3 years, and 31 older, 70 ± 6 years) were studied. Brachial artery diameter was measured before and after cuff occlusion using high-resolution ultrasound. Peak VO(2) was measured using expired gas analysis during upright cycle exercise.

RESULTS

Peak VO(2) was severely reduced in older HFpEF patients compared with age-matched healthy participants (15.2 ± 0.5 vs 19.6 ± 0.6 mL/kg/min, p < .0001), and in both groups, peak VO(2) was reduced compared with young healthy controls (28.5 ± 0.8 mL/kg/min; both p < .0001). Compared with healthy young participants, brachial artery FMD (healthy young, 6.13% ± 0.53%) was significantly reduced in healthy older participants (4.0 ± 0.38; p < .0002) and in HFpEF patients (3.64% ± 0.28%; p < .0001). However, FMD was not different in HFpEF patients compared with healthy older participants (p = .86). Although brachial artery FMD was modestly related to peak VO(2) in univariate analyses (r = .19; p = .048), it was not related in multivariate analyses that accounted for age, gender, and body size.

CONCLUSION

These results suggest that endothelial dysfunction may not be a significant independent contributor to the severely reduced exercise capacity in elderly HFpEF patients.

Velocity acceleration as a determinant of flow-mediated dilation

Shear stress is the established stimulus for flow-mediated dilation (FMD). In vivo, shear stress is typically estimated using mean blood velocity. However, mean blood velocity may not adequately characterize the shear stimulus. Pulsatile flow results in large shear gradients (velocity acceleration) at the onset of flow. The purpose of this study was to determine the importance of velocity acceleration to FMD. We define FMD as the brachial artery shear rate-diameter slope. Fourteen physically active, young (26 ± 5 years), male subjects were tested. Progressive forearm heating and handgrip exercise elicited steady-state increases in shear rate. FMD was measured prior to and following induced increases in velocity acceleration. Velocity acceleration was increased by inflating a tourniquet around the forearm to 40 mm Hg. Hierarchical linear modeling was used to estimate change in diameter with repeated measures of shear stress nested within each subject. Averaged across conditions, the 40 mm Hg cuff resulted in a 14% increase in velocity acceleration (p = 0.001). FMD was attenuated by 11.0% (p = 0.015) for the acceleration vs. control condition. However, after specifying velocity acceleration as a covariate, FMD was no longer significantly (p = 0.619) different between acceleration and control conditions. This finding suggests that mean blood velocity alone may not adequately characterize the shear stimulus.

Brachial artery adaptation to lower limb exercise training: role of shear stress

Lower limb exercise increases upper limb conduit artery blood flow and shear stress, and leg exercise training can enhance upper limb vascular function. We therefore examined the contribution of shear stress to changes in vascular function in the nonexercising upper limbs in response to lower limb cycling exercise training. Initially, five male subjects underwent bilateral brachial artery duplex ultrasound to measure blood flow and shear responses to 30-min cycling exercise at 80% of maximal heart rate. Responses in one forearm were significantly (P < 0.05) attenuated via cuff inflation throughout the exercise bout. An additional 11 subjects participated in an 8-wk cycle training study undertaken at a similar intensity, with unilateral cuff inflation around the forearm during each exercise bout. Bilateral brachial artery flow-mediated dilation responses to a 5-min ischemic stimulus (FMD%), an ischemic handgrip exercise stimulus (iEX), and endothelium-independent NO donor administration [glyceryl trinitrate (GTN)] were measured at 2, 4, and 8 wk. Cycle training increased FMD% in the noncuffed limb at week 2, after which time responses returned toward baseline levels (5.8 ± 4.1, 8.6 ± 3.8, 7.4 ± 3.5, 6.0 ± 2.3 at 0, 2, 4 and 8 wk, respectively; ANOVA: P = 0.04). No changes in FMD% were observed in the cuffed arm. No changes were evident in response to iEX or GTN in either the cuffed or noncuffed arms (P > 0.05) across the 8-wk intervention period. Our data suggest that lower limb cycle training induces a transient increase in upper limb vascular function in healthy young humans, which is, at least partly, mediated via shear stress.

Use of ultrasound for non-invasive assessment of flow-mediated dilation

The pathological complications of atherosclerosis, namely heart attacks and strokes, remain the leading cause of mortality in the Western world. Preceding atherosclerosis is endothelial dysfunction. There is therefore interest in the application of non-invasive clinical tools to assess endothelial function. The flow-mediated dilation (FMD) test is the standard tool used to assess endothelial function. Reduced FMD is an early marker of atherosclerosis and has been noted for its capacity to predict future cardiovascular disease events. This review discusses the measurement of endothelial function using ultrasound, with a focus on the FMD technique.

The importance of velocity acceleration to flow-mediated dilation

The validity of the flow-mediated dilation test has been questioned due to the lack of normalization to the primary stimulus, shear stress. Shear stress can be calculated using Poiseuille's law. However, little attention has been given to the most appropriate blood velocity parameter(s) for calculating shear stress. The pulsatile nature of blood flow exposes the endothelial cells to two distinct shear stimuli during the cardiac cycle: a large rate of change in shear at the onset of flow (velocity acceleration), followed by a steady component. The parameter typically entered into the Poiseuille's law equation to determine shear stress is time-averaged blood velocity, with no regard for flow pulsatility. This paper will discuss (1) the limitations of using Posieuille's law to estimate shear stress and (2) the importance of the velocity profile-with emphasis on velocity acceleration-to endothelial function and vascular tone.

The impact of baseline artery diameter on flow-mediated vasodilation: a comparison of brachial and radial artery responses to matched levels of shear stress

An inverse relationship between baseline artery diameter (BAD) and flow-mediated vasodilation (FMD) has been identified using reactive hyperemia (RH) to create a shear stress (SS) stimulus in human conduit arteries. However, RH creates a SS stimulus that is inversely related to BAD. The purpose of this study was to compare FMD in response to matched levels of SS in two differently sized upper limb arteries [brachial (BA) and radial (RA) artery]. With the use of exercise, three distinct, shear rate (SR) stimuli were created (SR = blood velocity/vessel diameter; estimate of SS) in the RA and BA. Artery diameter and mean blood velocity were assessed with echo and Doppler ultrasound in 15 healthy male subjects (19–25 yr). Data are means ± SE. Subjects performed 6 min of adductor pollicis and handgrip exercise to increase SR in the RA and BA, respectively. Exercise intensity was modulated to achieve uniformity in SR between arteries. The three distinct SR levels were as follows: steady-state exercise 39.8 ± 0.6, 57.3 ± 0.7, and 72.4 ± 1.2 s−1 ( P < 0.001). %FMD and AbsFMD (mm) at the end of exercise were greater in the RA vs. the BA at each shear level [at the highest level: RA = 15.7 ± 1.5%, BA = 5.4 ± 0.8% ( P < 0.001)]. The mean slope of the within-subject SR-%FMD regression line was greater in the RA (RA = 0.33 ± 0.04, BA = 0.13 ± 0.02, P < 0.001), and a strong within-subjects relationship between %FMD and SR was observed in both arteries (RA: r2 = 0.92 ± 0.02; BA: r2 = 0.90 ± 0.03). Within the RA, there was a significant relationship between baseline diameter and %FMD; however, this relationship was not present in the BA (RA: r2 = 0.76, P < 0.001; BA: r2 = 0.03, P = 0.541). These findings suggest that the response to SS is not uniform across differently sized vessels, which is in agreement with previous studies.

Plasma aldosterone levels and aldosterone-to-renin ratios are associated with endothelial dysfunction in young to middle-aged subjects

OBJECTIVE

Small clinical studies suggested a role for aldosterone in the development of endothelial dysfunction. We investigated whether the plasma aldosterone concentration (PAC) or the aldosterone-to-renin ratio (ARR) were associated with decreased endothelial function as measured by flow-mediated dilation (FMD) of the brachial artery in the general population.

METHODS

Our study population comprised 972 participants from the Study of Health in Pomerania, who were not treated with antihypertensive medication. We performed age-stratified (<50 and ≥ 50 years) ordinal logistic regression analyses. FMD was categorised as decreased (1st quintile), moderate (2nd-4th quintile), or increased (5th quintile). PAC and ARR were divided into low, moderate, and high values according to age- and sex-specific tertiles. All models were re-calculated for 871 subjects with PAC and ARR within the study-specific reference ranges. Odds ratios (OR) and 95% confidence intervals (CI) are presented.

RESULTS

Subjects <50 years with high PAC (OR 1.60; 95% CI 1.07-2.38) or ARR (OR 1.81; 95% CI 1.21-2.73) had higher odds for decreased FMD than subjects with low PAC or ARR, respectively. Similar results were obtained in analyses restricted to subjects with PAC and ARR within the reference range. High-normal PAC (OR 1.62; 95% CI 1.07-2.47) or ARR (OR 1.62; 95% CI 1.05-2.50) was associated with higher odds for decreased FMD when compared with low-normal PAC or ARR, respectively. These associations were not observed in subjects ≥ 50 years.

CONCLUSIONS

High and high-normal PAC or ARR contribute to an impaired FMD and subsequently the progression of subclinical atherosclerosis in young to middle-aged subjects.

The association of arterial shear and flow-mediated dilation in diabetes

While adjusting flow-mediated dilation (FMD), a measure of vascular function, for shear rate may be important when evaluating endothelial-dependent vasodilation, the relationship of FMD with shear rate in study populations with cardiovascular risk factors is unclear. We aimed to investigate the association of four measures of shear rate (peak shear rate (SR(peak)) and shear rate area under the curve through 30 seconds (SR(AUC 0-30)), 60 seconds (SR(AUC 0-60)), and time to peak dilation (SR(AUC 0-ttp))) with FMD in 50 study subjects with type 2 diabetes and mild hypertension undergoing baseline FMD testing for an exercise intervention trial. Associations among measures of shear rate and FMD were evaluated using Pearson's correlations and R(2). The four measures of shear rate were highly correlated within subjects, with Pearson's correlations ranging from 0.783 (p < 0.001) to 0.972 (p < 0.001). FMD was associated with each measure of shear rate, having a correlation of 0.576 (p < 0.001) with SR(AUC 0-30), 0.529 (p < 0.001) with SR(AUC 0-60), and 0.512 (p < 0.001) with SR(peak). Nine of 50 subjects (18%) did not dilate following the shear stimulus. Among the 41 responders, FMD had a correlation of 0.517 (p < 0.001) with SR(AUC 0-ttp) and similar correlations to those found in the full sample for SR(AUC 0-30), SR(AUC 0-60), and SR(peak). In conclusion, shear rate appears to explain up to a third of between-person variability in FMD response and our results support the reporting of shear rate and FMD with and without adjustment for shear rate in similar clinical populations with CVD risk factors.

Low flow-mediated constriction: prevalence, impact and physiological determinant

Flow-mediated dilation (FMD) is a surrogate marker for endothelial function. In the FMD procedure, arterial response during cuff inflation is not taken into consideration yet studies have demonstrated vasoconstriction, vasodilation and no change in the brachial artery during cuff inflation. The term low flow-mediated constriction (L-FMC) has been introduced to describe the vasoconstriction that occurs in some individuals during inflation of the cuff. The aims of this study were to examine (i) whether brachial artery response during cuff inflation differed in a population with varied coronary artery disease (CAD) risk factor profiles, (ii) the impact of this response on the subsequent calculation of FMD and (iii) the role of arterial stiffness in this variable response. L-FMC, 'traditional' FMD and 'modified' FMD, which accounts for brachial artery response during cuff inflation, were studied in a total of 46 subjects varying in risk factor profiles for coronary artery disease. During cuff inflation, brachial artery responses varied widely from -5·6% (vasoconstriction) to 5·0% (vasodilation). When subjects were divided into healthy versus multiple risk factors (n = 34), L-FMC and FMD were not different between the groups but modified FMD was significantly different (P = 0·02). L-FMC was modestly but significantly associated with FMD (r = 0·41) and positively correlated with brachial artery pulse wave velocity (r = 0·30). Our results indicate that brachial artery responses to inflation of the cuff are very variable and are associated with arterial stiffness and that accounting for so-called L-FMC may provide a more comprehensive assessment of endothelial vasodilatory function.

Relationship between upper and lower limb conduit artery vasodilator function in humans

Brachial artery flow-mediated dilation (FMD) is a strong predictor of future cardiovascular disease and is believed to represent a "barometer" of systemic endothelial health. Although a recent study [Padilla et al. Exp Biol Med (Maywood) 235: 1287-1291, 2010] in pigs confirmed a strong correlation between brachial and femoral artery endothelial function, it is unclear to what extent brachial artery FMD represents a systemic index of endothelial function in humans. We conducted a retrospective analysis of data from our laboratory to evaluate relationships between the upper (i.e., brachial artery) vs. lower limb (superficial femoral n = 75; popliteal artery n = 32) endothelium-dependent FMD and endothelium-independent glyceryl trinitrate (GTN)-mediated dilation in young, healthy individuals. We also examined the relationship between FMD assessed in both brachial arteries (n = 42). There was no correlation between brachial and superficial femoral artery FMD (r(2) = 0.008; P = 0.46) or between brachial and popliteal artery FMD (r(2) = 0.003; P = 0.78). However, a correlation was observed in FMD between both brachial arteries (r(2) = 0.34; P < 0.001). Brachial and superficial femoral artery GTN were modestly correlated (r(2) = 0.13; P = 0.007), but brachial and popliteal artery GTN responses were not (r(2) = 0.08; P = 0.11). Collectively, these data indicate that conduit artery vasodilator function in the upper limbs (of healthy humans) is not predictive of that in the lower limbs, whereas measurement of FMD in one arm appears to be predictive of FMD in the other. These data do not support the hypothesis that brachial artery FMD in healthy humans represents a systemic index of endothelial function.

Effects of leg blood flow restriction during walking on cardiovascular function

INTRODUCTION

Exercise with limb blood flow restriction (BFR) is a very popular exercise modality in Japan and is spreading widely to the rest of the world. The underlying principle of this training modality is that under the conditions of restricted blood flow, even low-intensity exercise can provide significant muscle strength and hypertrophy. One concern, however, is that BFR during exercise may place unnecessary burden on those with compromised cardiac function.

METHODS

We determined the impact of leg BFR during walking on cardiovascular function in 17 young (26 ± 1 yr) healthy volunteers. Each subject underwent five bouts of 2-min treadmill walking at 2 miles·h(-1) with 1-min interval either with or without tourniquet cuffs inflated on both thighs.

RESULTS

Heart rate increased more during the BFR session, whereas stroke volume decreased greater during the BFR session. Blood pressure increased significantly and substantially during the BFR session. Consequently, an increase in double product, an index of myocardial oxygen demand, was more than threefold higher in the BFR condition. Systemic arterial compliance evaluated by stroke volume/pulse pressure ratio significantly increased during the control session by 14% but reduced during the BFR condition by 19%. Popliteal artery flow-mediated vasodilation decreased significantly after the exercise with BFR but not after the control session.

CONCLUSIONS

Even at low intensity, the aerobic exercise with BFR requires a greater cardiac work and decreases endothelial function. Limb BFR during exercise may need to be more cautiously prescribed to those with compromised cardiac conditions.

A local increase in red blood cell aggregation can trigger deep vein thrombosis: evidence based on quantitative cellular ultrasound imaging

BACKGROUND

Recurrent deep vein thrombosis (DVT) risk factors include a first idiopathic DVT, strongly suggesting the existence of undiagnosed and/or unidentified prothrombotic abnormalities.

OBJECTIVES

To evaluate the impact of locally increased red blood cell (RBC) aggregation on DVT pathogenesis in a rabbit model.

METHODS

DVT presence, flow and aggregation were measured in situ with ultrasound. Greatly enhanced aggregation was achieved by covalent linkage of Pluronic F98 to the RBC surface; coating with Pluronic F68, which very mildly enhances aggregation, was used as a coating control. On day 1, endothelial damage and a partial stenosis were surgically created on the left femoral vein whereas the right femoral vein was not manipulated.

RESULTS

A thrombus was formed within 30 min in six out of seven left femoral veins of animals receiving a 30% volume blood exchange with F98-coated RBC, whereas a thrombus occurred in only one out of seven veins in F68-transfused controls. In vivo imaging using quantitative ultrasound confirmed increased aggregation in the thrombosed veins of the F98 group compared with the F68 group and the contralateral vessel. For each group, five animals were followed for 2 weeks before being killed. In F98-transfused animals, lysis of clots occurred and the presence of chronic thrombi totally occluding the vein in three out of five animals was confirmed by histology. Conversely, in the F68 group, a single disorganized blood clot was observed in one out of five animals.

CONCLUSIONS

A marked increase in RBC aggregation promotes thrombosis in rabbit femoral veins, confirming a pathophysiological role of locally altered hemorheology in the onset of DVT.

Human vascular aging: limb-specific lessons

Unlike quadrupeds, human limbs are exposed to differing homeostatic challenges and uses across the life span. This raises the question of whether vascular function and skeletal muscle blood flow regulation differs between human limbs and whether these limb-specific parameters may be altered during exercise as a consequence of aging and disease.

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.

Low flow-mediated constriction occurs in the radial but not the brachial artery in healthy pregnant and nonpregnant women

Radial artery diameter decreases when a wrist cuff is inflated to stop blood flow to distal tissue. This phenomenon, referred to as low flow-mediated vasoconstriction (L-FMC), was proposed as a vascular function test. Recommendations that L-FMC be measured concurrently with flow-mediated dilation (FMD) were based on radial artery data. However, cardiovascular disease prediction studies traditionally measure brachial artery FMD. Therefore, studies should determine whether L-FMC occurs in the brachial artery. The hypothesis that reduced shear causes L-FMC has not been tested. Brachial and radial artery L-FMC and FMD were assessed in active nonpregnant (n=17), inactive nonpregnant (n=10), active pregnant (n=15, 34.1+/-1.2 wk gestation), and inactive pregnant (n=8, 34.2+/-2.2 wk gestation) women. Radial artery diameter decreased significantly during occlusion in all groups (nonpregnant, -4.4+/-4.2%; pregnant, -6.4+/-3.2%). Brachial artery diameter did not change in active and inactive nonpregnant, and inactive pregnant women; however, the small decrease in active pregnant women was significant. Occlusion decreased shear rate in both arteries, yet L-FMC only occurred in the radial artery. Radial artery L-FMC was not correlated with the reduction in shear rate. L-FMC occurs in the radial but not the brachial artery and is not related to changes in shear rate. Positive correlations between L-FMC (negative values) and FMD (positive values) suggest that radial artery FMD may be reduced among women who experience greater L-FMC. Studies should clarify the underlying stimulus and mechanisms regulating L-FMC, and test the hypothesis that endothelial dysfunction is manifested as enhanced brachial artery L-FMC, but attenuated radial artery L-FMC.

Vascular health in children and adolescents: effects of obesity and diabetes

The foundations for cardiovascular disease in adults are laid in childhood and accelerated by the presence of comorbid conditions, such as obesity, diabetes, hypertension, and dyslipidemia. Early detection of vascular dysfunction is an important clinical objective to identify those at risk for subsequent cardiovascular morbidity and events, and to initiate behavioral and medical interventions to reduce risk. Typically, cardiovascular screening is recommended for young adults, especially in people with a family history of cardiovascular conditions. Children and adolescents were once considered to be at low risk, but with the growing health concerns related to sedentary lifestyle, poor diet and obesity, cardiovascular screening may be needed earlier so that interventions to improve cardiovascular health can be initiated. This review describes comorbid conditions that increase cardiovascular risk in youth, namely obesity and diabetes, and describes noninvasive methods to objectively detect vascular disease and quantify vascular function and structure through measurements of endothelial function, arterial compliance, and intima-media thickness. Additionally, current strategies directed toward prevention of vascular disease in these populations, including exercise, dietary interventions and pharmacological therapy are described.

Is the ratio of flow-mediated dilation and shear rate a statistically sound approach to normalization in cross-sectional studies on endothelial function?

It has been deemed important to normalize flow-mediated dilation (FMD), a marker of endothelial function, for between-subject differences in the eliciting shear rate (SR) stimulus. Conventionally, FMD is divided by the area under the curve of the SR stimulus. In the context of a cross-sectional comparison across different age cohorts, we examined whether this ratio approach adhered to established statistical assumptions necessary for reliable normalization. To quantify brachial artery FMD and area under the curve of SR, forearm cuff inflation to suprasystolic pressure was administered for 5 min to 16 boys aged 10.9 yr (SD 0.3), 48 young men aged 25.3 yr (SD 4.2), and 15 older men aged 57.5 yr (SD 4.3). Mean differences between age groups were statistically significant (P < 0.001) for nonnormalized FMD [children: 10.4% (SD 5.4), young adults: 7.5% (SD 2.9), older adults: 5.6% (SD 2.0)] but not for ratio-normalized FMD (P = 0.10). Moreover, all assumptions necessary for reliable use of ratio-normalization were violated, including regression slopes between SR and FMD that had y-intercepts greater than zero (P < 0.05), nonlinear and unstable relations between the normalized ratios and SR, skewed data distributions, and heteroscedastic variance. Logarithmic transformation of SR and FMD before ratio calculation improved adherence to these assumptions and resulted in age differences similar to the nonnormalized data (P = 0.03). In conclusion, although ratio normalization of FMD altered findings about age differences in endothelial function, this could be explained by violation of statistical assumptions. We recommend that exploration of these assumptions should be routine in future research. If the relationship between SR and FMD is generally found to be weak or nonlinear or variable between samples, then ratio normalization should not be applied.

Estrogen replacement restores flow-induced vasodilation in coronary arterioles of aged and ovariectomized rats

The risk for cardiovascular disease (CVD) increases with advancing age; however, the age at which CVD risk increases significantly is delayed by more than a decade in women compared with men. This cardioprotection, which women experience until menopause, is presumably due to the presence of ovarian hormones, in particular, estrogen. The purpose of this study was to determine how age and ovarian hormones affect flow-induced vasodilation in the coronary resistance vasculature. Coronary arterioles were isolated from young (6 mo), middle-aged (14 mo), and old (24 mo) intact, ovariectomized (OVX), and ovariectomized + estrogen replaced (OVE) female Fischer-344 rats to assess flow-induced vasodilation. Advancing age impaired flow-induced dilation of coronary arterioles (young: 50 +/- 4 vs. old: 34 +/- 6; % relaxation). Ovariectomy reduced flow-induced dilation in arterioles from young females, and estrogen replacement restored vasodilation to flow. In aged females, flow-induced vasodilation of arterioles was unaltered by OVX; however, estrogen replacement improved flow-induced dilation by approximately 160%. The contribution of nitric oxide (NO) to flow-induced dilation, assessed by nitric oxide synthase (NOS) inhibition with N(G)-nitro-l-arginine methyl ester (l-NAME), declined with age. l-NAME did not alter flow-induced vasodilation in arterioles from OVX rats, regardless of age. In contrast, l-NAME reduced flow-induced vasodilation of arterioles from estrogen-replaced rats at all ages. These findings indicate that the age-induced decline of flow-induced, NO-mediated dilation in coronary arterioles of female rats is related, in part, to a loss of ovarian estrogen, and estrogen supplementation can improve flow-induced dilation, even at an advanced age.

Impact of acute exposure to increased hydrostatic pressure and reduced shear rate on conduit artery endothelial function: a limb-specific response

Unlike quadrupeds, humans exhibit a larger hydrostatic pressure in the lower limbs compared with the upper limbs during a major part of the day. It is plausible that repeated episodes of elevated pressure in the legs may negatively impact the endothelium, hence contributing to the greater predisposition of atherosclerosis in the legs. We tested the hypothesis that an acute exposure to increased hydrostatic pressure would induce conduit artery endothelial dysfunction. In protocol 1, to mimic the hemodynamic environment of the leg, we subjected the brachial artery to a hydrostatic pressure gradient (∼15 mmHg) by vertically hanging the arm for 3 h. Brachial artery flow-mediated dilation (FMD) was assessed in both arms before and following the intervention. In protocol 2, we directly evaluated popliteal artery FMD before and after a 3-h upright sitting (pressure gradient ∼48 mmHg) and control (supine position) intervention. Our arm-hanging model effectively resembled the hemodynamic milieu (high pressure and low shear rate) present in the lower limbs during the seated position. Endothelium-dependent vasodilation at the brachial artery was attenuated following arm hanging ( P < 0.05); however, contrary to our hypothesis, upright sitting did not have an impact on popliteal artery endothelial function ( P > 0.05). These data suggest an intriguing vascular-specific response to increased hydrostatic pressure and reduced shear rate. Further efforts are needed to determine if this apparent protection of the leg vasculature against an acute hydrostatic challenge is attributable to posture-induced chronic adaptations.

Impact of age, sex, and exercise on brachial artery flow-mediated dilatation

Flow-mediated dilatation (%FMD), an index of nitric oxide (NO)-mediated vasodilator function, is regarded as a surrogate marker of cardiovascular disease. Aging is associated with endothelial dysfunction, but underlying sex-related differences may exist and the effects of fitness and exercise on endothelial dysfunction in men (M) and women (W) are poorly understood. We compared %FMD of the brachial artery in 18 young [Y, 26 +/- 1 yr; 9 M and 9 W], 12 older fit (OF, 57 +/- 2 yr; 6 M and 6 W), and 16 older sedentary (OS, 59 +/- 2 yr; 8 M and 8 W) subjects. Glyceryl trinitrate (GTN) administration was used to assess endothelium-independent vasodilatation, and the FMD-to-GTN ratio was calculated to characterize NO dilator function in the context of smooth muscle cell sensitivity. Brachial %FMD in Y (7.1 +/- 0.8%) was significantly higher compared with OS (4.8 +/- 0.7%, P < 0.05), but not OF (6.4 +/- 0.7%). Differences between Y and OS subjects were due primarily to lower FMD in the OS women (4.3 +/- 0.6%). OS women exhibited significantly lower FMD-to-GTN ratios compared with Y (P < 0.05) and OF women (P < 0.05), whereas these differences were not apparent in men. Exercise training improved brachial artery NO dilator function (FMD-to-GTN ratio) after 24 wk (P < 0.05) in OS women, but not men. These findings indicate that maintaining a high level of fitness, or undertaking exercise training, prevents the age-related decline in the brachial artery vasodilator function evident in women. In OS men, who had relatively preserved NO dilator function, no training adaptations were observed. This study has potential implications for the prevention of conduit artery endothelial dysfunction in men and women.

Endothelium-dependent vasodilatation and exercise hyperaemia in ageing humans: impact of acute ascorbic acid administration

Age-related increases in oxidative stress impair endothelium-dependent vasodilatation in humans, leading to the speculation that endothelial dysfunction contributes to impaired muscle blood flow and vascular control during exercise in older adults. We directly tested this hypothesis in 14 young (22 +/- 1 years) and 14 healthy older men and women (65 +/- 2 years). We measured forearm blood flow (FBF; Doppler ultrasound) and calculated vascular conductance (FVC) responses to single muscle contractions at 10, 20 and 40% maximum voluntary contraction (MVC) before and during ascorbic acid (AA) infusion, and we also determined the effects of AA on muscle blood flow during mild (10% MVC) continuous rhythmic handgrip exercise. For single contractions, the peak rapid hyperaemic responses to all contraction intensities were impaired approximately 45% in the older adults (all P < 0.05), and AA infusion did not impact the responses in either age group. For the rhythmic exercise trial, FBF (approximately 28%) and FVC (approximately 31%) were lower (P = 0.06 and 0.05) in older versus young adults after 5 min of steady-state exercise with saline. Subsequently, AA was infused via brachial artery catheter for 10 min during continued exercise. AA administration did not significantly influence FBF or FVC in young adults (1-3%; P = 0.24-0.59), whereas FBF increased 34 +/- 7% in older adults at end-exercise, and this was due to an increase in FVC (32 +/- 7%; both P < 0.05). This increase in FBF and FVC during exercise in older adults was associated with improvements in vasodilator responses to acetylcholine (ACh; endothelium dependent) but not sodium nitroprusside (SNP; endothelium independent). AA had no effect on ACh or SNP responses in the young. We conclude that acute AA administration does not impact the observed age-related impairment in the rapid hyperaemic response to brief muscle contractions in humans; however, it does significantly increase muscle blood flow during continuous dynamic exercise in older adults, and this is probably due (in part) to an improvement in endothelium-dependent vasodilatation.

Role of {alpha}1-adrenergic vasoconstriction in the regulation of skeletal muscle blood flow with advancing age

alpha(1)-Adrenergic vasoconstriction during dynamic leg exercise is diminished in younger individuals, although the extent of this exercise-induced "sympatholysis" in the elderly remains uncertain. Thus, in nine young (25 +/- 1 yr) and six older (72 +/- 2 yr) healthy volunteers, we evaluated changes in leg blood flow (ultrasound Doppler) during blood flow-adjusted intra-arterial infusion of phenylephrine (PE; a selective alpha(1)-adrenergic agonist) at rest and during knee-extensor leg exercise at 20, 40, and 60% of maximal work rate (WR(max)). To probe the potential contributors to exercise-induced changes in alpha(1)-adrenergic receptor sensitivity, exercising leg O(2) consumption (Vo(2)) and lactate efflux were also evaluated (n = 10). At rest, the PE-induced vasoconstriction (i.e., decrease in leg blood flow) was diminished in older (-37 +/- 3%) compared with young (-54 +/- 4%) subjects. During exercise, the magnitude of alpha(1)-adrenergic vasoconstriction in the active leg decreased in both groups. However, compared with young, older subjects maintained a greater vasoconstrictor response to PE at 40% WR(max) (-14 +/- 3%, older; -7 +/- 2%, young) and 60% WR(max) (-11 +/- 3%, older; -4 +/- 3%, young). It is possible that this observation may be attributed to lower absolute work rates in the older group, because, for a similar absolute work rate ( approximately 10 W) and leg Vo(2) ( approximately 0.36 l/min), vasoconstriction to PE was not different between groups (-14 +/- 3%; older; -17 +/- 5%, young). Together, these data challenge the concept of reduced sympatholysis in the elderly, suggesting instead that the inhibition of alpha(1)-adrenergic vasoconstriction in the exercising leg is associated with work performed and, therefore, more closely related to the rate of oxidative metabolism than to age per se.

Does arterial shear explain the magnitude of flow-mediated dilation?: a comparison between young and older humans

Flow-mediated dilatation (FMD) has become a commonly applied approach for the assessment of vascular function and health in humans. Recent studies emphasize the importance of normalizing the magnitude of FMD to its apparent eliciting stimulus, the postdeflation arterial shear. However, the relationship between shear stress and the magnitude of FMD may differ between groups. The aim of this study was to examine the relationship between the brachial FMD and four different indexes of postdeflation shear rate (SR) in healthy children (n = 51, 10 +/- 1 yr) and young (n = 57, 27 +/- 6 yr) and older (n = 27, 58 +/- 4 yr) adults. SR was calculated from deflation (time 0) until 9 s (peak), 30 s (0-30), 60 s (0-60), or until the time-to-peak diameter in each individual (0-ttp). Edge detection and wall tracking of high resolution B-mode arterial ultrasound images were used to calculate the conduit artery diameter. In young adults, the brachial artery FMD demonstrated a significant correlation with the area under the SR curve (SR(AUC)) 0-30 s (r(2) = 0.12, P = 0.009), 0-60 s (r(2) = 0.14, P = 0.005), and 0-ttp (r(2) = 0.14, P = 0.005) but not for the peak SR(AUC) 0-9 s (r(2) = 0.04, P = 0.12). In children and older adults, the magnitude of the brachial artery FMD did not correlate with any of the four SR(AUC) stimuli. These findings suggest that in young subjects, postdeflation SR(AUC) correlates moderately with the magnitude of the FMD response. However, the relationship between FMD and postdeflation shear appears to be age dependent, with less evidence for an association in younger and older subjects. Therefore, we support presenting SR(AUC) stimuli but not normalizing FMD responses for the SR(AUC) when using this technique.

Aging affects vascular structure and function in a limb-specific manner

The limb-specific effects of aging upon vessel structure and function are not well understood. Consequently, in 12 young (26 +/- 2 yr) and 12 old (72 +/- 1 yr) healthy subjects, we utilized ultrasound Doppler to evaluate intima-media thickness (IMT), ischemic reperfusion, and flow-mediated dilation (FMD) following (5 min) suprasystolic cuff occlusion in both the arm [brachial artery (BA)] and the leg [popliteal artery (PA)]. Structural measurements, whether normalized for vessel size or not, revealed a greater IMT in both the BA and PA with age (young: BA 0.028 +/- 0.001 and PA 0.046 +/- 0.003 cm, old: BA 0.039 +/- 0.002 and PA 0.073 +/- 0.005 cm; P < 0.05). Ischemic reperfusion revealed a similar pattern as IMT in terms of limb and age-related differences. There was an age-related attenuation in both BA FMD (old: 38% smaller BA FMD compared with young) and PA FMD (old: 71% smaller PA FMD compared with young). However, when this percent change was normalized for shear rate, only the PA FMD of the old group was still significantly attenuated (old: 41% smaller PA FMD/shear rate compared with young). Together, the finding of differential structural and functional parameters in the arms and legs of healthy young people, and the somewhat negative findings that are specific to the legs of otherwise healthy older people (greater IMT and attenuated FMD), support and may help to better understand the increased propensity to develop a vascular pathology in the legs with age.

The impact of baseline diameter on flow-mediated dilation differs in young and older humans

Flow-mediated dilation (FMD) has become a commonly applied approach for the assessment of vascular function and health, but methods used to calculate FMD differ between studies. For example, the baseline diameter used as a benchmark is sometimes assessed before cuff inflation, whereas others use the diameter during cuff inflation. Therefore, we compared the brachial artery diameter before and during cuff inflation and calculated the resulting FMD in healthy children (n=45; 10+/-1 yr), adults (n=31; 28+/-6 yr), and older subjects (n=22; 58+/-5 yr). Brachial artery FMD was examined after 5 min of distal ischemia. Diameter was determined from either 30 s before cuff inflation or from the last 30 s during cuff inflation. Edge detection and wall tracking of high resolution B-mode arterial ultrasound images was used to calculate conduit artery diameter. Brachial artery diameter during cuff inflation was significantly larger than before inflation in children (P=0.02) and adults (P<0.001) but not in older subjects (P=0.59). Accordingly, FMD values significantly differed in children (11.2+/-5.1% vs. 9.4+/-5.2%; P=0.02) and adults (7.3+/-3.2% vs. 4.6+/-3.3%; P<0.001) but not in older subjects (6.3+/-3.4% vs. 6.0+/-4.2%; P=0.77). When the diameter before cuff inflation was used, an age-dependent decline was evident in FMD, whereas FMD calculated using the diameter during inflation was associated with higher FMD values in older than younger adults. In summary, the inflation of the cuff significantly increases brachial artery diameter, which results in a lower FMD response. This effect was found to be age dependent, which emphasizes the importance of using appropriate methodology to calculate the FMD.

Sex and limb-specific ischemic reperfusion and vascular reactivity

With little known regarding sex and limb heterogeneity, we investigated vascular reactivity and ischemic reperfusion (IR) in the upper and lower extremities of 15 healthy men (26 +/- 2 yr) and women (23 +/- 1 yr). Doppler ultrasound was used to evaluate IR and flow-mediated dilation (FMD) after suprasystolic cuff occlusion in both the arm [brachial artery (BA)] and the leg [popliteal artery (PA)]. Cumulative IR [area under the curve (AUC)], normalized for muscle mass, revealed no sex-related differences in either limb (forearm: men 38 +/- 3 and women 44 +/- 4 ml/100 g; lower leg: men 12 +/- 2 and women 14 +/- 2 ml/100 g), while both groups revealed a greater IR per unit of arm muscle mass (AUC) compared with the lower leg (P < 0.05). The BA and PA were smaller in women (BA 0.31 +/- 0.1, PA 0.47 +/- 0.1 cm) than in men (BA 0.41 +/- 0.1, PA 0.6 +/- 0.2 cm). Absolute FMD/shear rate revealed attenuated vascular function in the PA of the women [women 3.3 +/- 0.6, men 5.0 +/- 0.8 (all x10(-6)) cm/s(-1).s] and no sex difference in the BA [women 1.2 +/- 0.2, men 1.6 +/- 0.1 (all x10(-6)) cm/s(-1).s]. In both sexes the PA demonstrated greater vascular reactivity than the BA. Thus vascular reactivity in healthy young people is greater in the legs, regardless of sex, and women have vascular function similar to men in the upper extremities but appear to have poorer vascular function normalized for shear rate in the lower extremities.

Association between brachial artery reactivity and cardiovascular disease status in an elderly cohort: the cardiovascular health study

BACKGROUND AND OBJECTIVES

The association of brachial flow-mediated dilation (FMD) and cardiovascular disease (CVD) status is unclear especially in older adults whose FMD is greatly diminished. We assessed the association of FMD and the presence or absence of subclinical and clinical CVD in a population based cohort of older adults.

METHODS AND RESULTS

FMD was measured in 2971 adults aged 72-98 years (mean age 78.6 years) who participated in the Cardiovascular Health Study. Multiple linear regression analysis was used to examine the association between FMD and CVD status (clinical, subclinical and free of CVD). Out of 2791 with complete data, 82.7% were Caucasians and 59% females. Seven hundred and forty-three were classified as having clinical CVD, 607 as subclinical CVD and 1441 as neither clinical CVD nor subclinical CVD (CVD free). FMD was higher in the CVD free group compared with either the clinical (3.13+/-0.05% vs 2.93+/-0.07%, p=0.025) or the subclinical CVD group (3.13+/-0.05% vs 2.95+/-0.08%, p=0.05) after adjusting for covariates. There was no significant difference between the FMD of subjects with clinical and subclinical CVD (2.93+/-0.07% vs 2.95+/-0.08%, p=0.84). Similar but inverted associations were observed between height adjusted brachial artery diameter (BAD) and CVD status. However, FMD and BAD had poor diagnostic accuracies for identifying older adults with subclinical CVD.

CONCLUSION

Among older adults, those with either clinical or subclinical CVD have lower FMD than CVD free subjects. BAD showed similar but inverted associations with CVD status in this cohort. FMD and BAD had poor diagnostic accuracies for identifying older adults with subclinical CVD.

Effects of posture on shear rates in human brachial and superficial femoral arteries

Shear rate is significantly lower in the superficial femoral compared with the brachial artery in the supine posture. The relative shear rates in these arteries of subjects in the upright posture (seated and/or standing) are unknown. The purpose of this investigation was to test the hypothesis that upright posture (seated and/or standing) would produce greater shear rates in the superficial femoral compared with the brachial artery. To test this hypothesis, Doppler ultrasound was used to measure mean blood velocity (MBV) and diameter in the brachial and superficial femoral arteries of 21 healthy subjects after being in the supine, seated, and standing postures for 10 min. MBV was significantly higher in the brachial compared with the superficial femoral artery during upright postures. Superficial femoral artery diameter was significantly larger than brachial artery diameter. However, posture had no significant effect on either brachial or superficial femoral artery diameter. The calculated shear rate was significantly greater in the brachial (73 +/- 5, 91 +/- 11, and 97 +/- 13 s(-1)) compared with the superficial femoral (53 +/- 4, 39 +/- 77, and 44 +/- 5 s(-1)) artery in the supine, seated, and standing postures, respectively. Contrary to our hypothesis, our current findings indicate that mean shear rate is lower in the superficial femoral compared with the brachial artery in the supine, seated, and standing postures. These findings of lower shear rates in the superficial femoral artery may be one mechanism for the higher propensity for atherosclerosis in the arteries of the leg than of the arm.

Hormone therapy is associated with preserved smooth muscle structure and dilation in the arterial vasculature of the leg in older women

Long-term hormone therapy (HT) is associated with reduced intima-medial thickness (IMT), an established risk factor for atherosclerotic disease, in the femoral artery of healthy older women relative to age-matched non-hormone users. However, the influence of continuous, long-term HT on the relation between age, IMT, and smooth muscle dilation has not been investigated in the popliteal artery, an artery prone to stiffening and calcification. In the present study, popliteal artery IMT and smooth muscle dilation (the increase in diameter to sublingual nitroglycerin, NTG) were assessed with Doppler ultrasound in young (Y: n=16; age 23+/-1 [mean+/-S.E.M.]), older non-HT (O non-HT: n=14; age 69+/-1), and older HT (O HT: n=8; age 67+/-1) healthy women. The approximately 0.5 mm increase in resting diameter observed in older non-HT women relative to young women was absent in older HT women, as was the age-related increase in IMT (Y: 0.52+/-0.02 mm; O non-HT: 0.63+/-0.02 mm; O HT: 0.56+/-0.02 mm; p<0.05 for age and hormone comparisons). NTG dilation (percent change above rest) was similarly attenuated in older non-HT women (Y: 8.6+/-1%; O non-HT: 3.0+/-0.7%; O HT: 7.4+/-1.7%; p<0.05 for age and hormone comparisons), and NTG dilation was inversely related to IMT (p<0.01). Collectively, these results suggest that long-term, continuous HT may alleviate the detrimental effects of aging on both structural changes and smooth muscle dilation of the popliteal artery in healthy women.

Sex-specific influence of aging on exercising leg blood flow

Our previous work suggests that healthy human aging is associated with sex-specific differences in leg vascular responses during large muscle mass exercise (2-legged cycling) (Proctor DN, Parker BA. Microcirculation 13: 315-327, 2006). The present study determined whether age x sex interactions in exercising leg hemodynamics persist during small muscle mass exercise that is not limited by cardiac output. Thirty-one young (20-30 yr; 15 men/16 women) and 31 older (60-79 yr; 13 men/18 women) healthy, normally active adults performed graded single-leg knee extensions to maximal exertion. Femoral artery blood velocity and diameter (Doppler ultrasound), heart rate (ECG), and beat-to-beat arterial blood pressure (mean arterial pressure, radial artery tonometry) were measured during each 3-min work rate (4.8 and 8 W/stage for women and men, respectively). The results (means +/- SE) were as follows. Despite reduced resting leg blood flow and vascular conductance, older men exhibited relatively preserved exercising leg hemodynamic responses. Older women, by contrast, exhibited attenuated hyperemic (young: 52 +/- 3 ml.min(-1).W(-1); vs. older: 40 +/- 4 ml.min(-1).W(-1); P = 0.02) and vasodilatory responses (young: 0.56 +/- 0.06 ml.min(-1).mmHg(-1).W(-1) vs. older: 0.37 +/- 0.04 ml.min(-1).mmHg(-1) W(-1); P < 0.01) to exercise compared with young women. Relative (percentage of maximal) work rate comparisons of all groups combined also revealed attenuated vasodilator responses in older women (P < 0.01 for age x sex x work rate interaction). These sex-specific age differences were not abolished by consideration of hemoglobin, quadriceps muscle, muscle recruitment, and mechanical influences on muscle perfusion. Collectively, these findings suggest that local factors contribute to the sex-specific effects of aging on exercising leg hemodynamics in healthy adults.

Flow-mediated dilatation in the superficial femoral artery is nitric oxide mediated in humans

Flow-mediated dilatation (FMD) of the brachial and radial arteries is an important research tool for assessment of endothelial function in vivo, and is nitric oxide (NO) dependent. The leg skeletal muscle vascular bed is an important territory for studies in exercise physiology. However, the role of endothelial NO in the FMD response of lower limb arteries has never been investigated. The purpose of this study was to examine the contribution of NO to FMD in the superficial femoral artery in healthy subjects. Since physical inactivity may affect endothelial function, and therefore NO availability, spinal cord-injured (SCI) individuals were included as a model of extreme deconditioning. In eight healthy men (34 +/- 13 years) and six SCI individuals (37 +/- 10 years), the 5 min FMD response in the superficial femoral artery was assessed by echo-Doppler, both during infusion of saline and during infusion of the NO synthase blocker N(G)-monomethyl-L-arginine (L-NMMA). In a subset of the controls (n = 6), the 10 min FMD response was also examined using the same procedure. The 5 min FMD response in controls (4.2 +/- 0.3%) was significantly diminished during L-NMMA infusion (1.0 +/- 0.2%, P < 0.001). In SCI, L-NMMA also significantly decreased the FMD response (from 8.2 +/- 0.4% during saline to 2.4 +/- 0.5% during L-NMMA infusion). The hyperaemic flow response during the first 45 s after cuff deflation was lower in both groups during infusion of L-NMMA, but the effect of L-NMMA on FMD persisted in both groups after correction for the shear stress stimulus. The 10 min FMD was not affected by L-NMMA (saline: 5.4 +/- 1.6%, L-NMMA: 5.6 +/- 1.5%). Superficial femoral artery FMD in response to distal arterial occlusion for a period of 5 min is predominantly mediated by NO in healthy men and in the extremely deconditioned legs of SCI individuals.

Importance of measuring the time course of flow-mediated dilatation in humans

Flow-mediated dilatation (FMD) is widely used to describe conduit artery endothelial function. The traditional approaches to FMD calculation assess diameter change at arbitrary time points after occluding cuff deflation. The aim of this study was to examine the time course of brachial artery FMD after a 5-minute period of forearm ischemia in 12 young, 12 fitness matched older and 12 older untrained subjects. Edge-detection and wall tracking of high resolution B-mode arterial ultrasound images, combined with synchronized Doppler waveform envelope analysis, were used to calculate brachial artery diameter, blood flow, and shear rate continuously across the cardiac cycle after forearm ischemia. FMD was significantly higher in young healthy subjects (7.8+/-3.2%) compared with sedentary older subjects (5.2+/-2.8%, P<0.05) but not trained older subjects (6.4+/-2.3%). Time to peak diameter differed between young (50+/-11 seconds) and both older groups (trained; 80+/-21, P<0.001; sedentary: 83+/-36 seconds, P<0.001). A large proportion (>42%) of true peak diameters fell outside the time frames typically used to assess FMD in the literature. When calculated according to the commonly used approach, ie, 60 s after cuff deflation, FMD was significantly lower compared with true peak FMD in all groups (P<0.001), and no differences were evident between the groups. The time course of FMD differs significantly between young and older subjects. Studies assuming that peak dilation occurs at an arbitrary time point, or within limited time windows, may draw misleading conclusions regarding differences between groups. More sophisticated approaches to measurement of FMD are required if it is to be considered a valid biomarker of vascular disease.