The relationship between aerobic fitness and low-flow-mediated constriction in older adults

Sitting knee-flexion angle does not influence endothelial-dependent vasodilation in laboratory or free-living conditions

INTRODUCTION

Single bouts of prolonged bent-legged sitting attenuate popliteal endothelial-dependent vasodilation (as assessed via flow-mediated dilation [FMD]), which is partially attributed to arterial 'kinking'. However, the impact of knee-flexion angle on sitting-induced popliteal FMD is unknown. The objective of this study was to perform separate laboratory and free-living studies to test the hypotheses that: (1) popliteal FMD impairments would be graded between knee flexions at 90° (bent-legged sitting) > 45° > 0° (straight-legged sitting) following a 3-hour bout of sitting; and (2) more habitual time spent bent-legged sitting (< 45°) would be associated with lower FMD.

METHODS

The laboratory study included eight young, healthy adults (24 ± 2 years; four women) who underwent two sitting bouts over 2 days with one leg positioned at a knee-flexion angle of 0° or 90° and the opposite leg at 45° knee flexion. Popliteal FMD was assessed at pre- and postsitting timepoints.

RESULTS

Sitting-induced reductions in FMD were similar between all knee-flexion angles (all, p > 0.674). The free-living study included 35 young, healthy adults (23 ± 3 years; 16 women) who wore three activPAL monitors (torso, thigh, shin) to determine detailed sedentary postures. Time spent sedentary (624 ± 127 min/day), straight-legged sitting (112 ± 98 min/day), and bent-legged sitting (442 ± 106 min/day) were not related to relative FMD (5.3 ± 1.8%; all, p > 0.240).

CONCLUSION

These findings suggest that knee-flexion angle-mediated arterial 'kinking' during sitting is not a major contributor toward sitting-induced popliteal endothelial-dependent vasodilatory dysfunction.

Nadir blood pressure responses to longer consecutive cardiac cycle sequences absent of sympathetic bursts are associated with popliteal endothelial-dependent dilation

PURPOSE

The nadir pressure responses to cardiac cycles absent of muscle sympathetic nerve activity (MSNA) bursts (or non-bursts) are typically reported in studies quantifying sympathetic transduction, but the information gained by studying non-bursts is unclear. We tested the hypothesis that longer sequences of non-bursts (≥8 cardiac cycles) would be associated with a greater nadir diastolic blood pressure (DBP) and that better popliteal artery function would be associated with an augmented reduction in DBP.

METHODS

Resting beat-by-beat DBP (via finger photoplethysmography) and common peroneal nerve MSNA (via microneurography) were recorded in 39 healthy, adults (age 23.4 ± 5.3 years; 19 females). For each cardiac cycle absent of MSNA bursts, the mean nadir DBP (ΔDBP) during the 12 cardiac cycles following were determined, and separate analyses were conducted for ≥8 or < 8 cardiac cycle sequences. Popliteal artery endothelial-dependent (via flow-mediated dilation; FMD) and endothelial-independent vasodilation (via nitroglycerin-mediated dilation; NMD) were determined.

RESULTS

The nadir DBP responses to sequences ≥8 cardiac cycles were larger (-1.40 ± 1.27 mmHg) than sequences <8 (-0.38 ± 0.46 mmHg; p < 0.001). In adjusting for sex and burst frequency (14 ± 8 bursts/min), larger absolute or relative FMD (p < 0.01), but not NMD (p > 0.53) was associated with an augmented nadir DBP. This overall DBP-FMD relationship was similar in sequences ≥8 (p = 0.04-0.05), but not <8 (p > 0.72).

CONCLUSION

The DBP responses to non-bursts, particularly longer sequences, were inversely associated with popliteal endothelial function, but not vascular smooth muscle sensitivity. This study provides insight into the information gained by quantifying the DBP responses to cardiac cycles absent of MSNA.

Mechanistic, participant, and movement-related factors that contribute to low-flow-mediated constriction

Endothelial function is commonly determined via the ultrasound-based flow-mediated dilation (FMD) technique which assesses arterial dilation in response to a hyperemia response following distal cuff occlusion. However, the low-flow-mediated constriction (L-FMC) response during cuff-induced ischemia is often overlooked. L-FMC provides unique information regarding endothelial function, but vascular researchers may be unclear on what this metric adds. Therefore, the objective of this review was to examine the mechanistic determinants and participant-level factors of L-FMC. Existing mechanistic studies have demonstrated that vasoreactivity to low flow may be mediated via non-nitric oxide vasodilators (i.e., endothelial hyperpolarizing factors and/or prostaglandins), inflammatory markers, and enhancement of vasoconstriction via endothelin-1. In general, participant-level factors such as aging and presence of cardiovascular conditions generally are associated with attenuated L-FMC responses. However, the influence of sex on L-FMC is unclear with divergent results between L-FMC in upper versus lower limb vessels. The ability of aerobic exercise to augment L-FMC (i.e., make more negative) is well supported, but there is a major gap in the literature concerning the mechanistic underpinnings of this observation. This review summarizes that while larger L-FMC responses are generally healthy, the impact of interventions to augment/attenuate L-FMC has not included mechanistic measures that would provide insight into non-nitric oxide-based endothelial function. Clarifications to terminology and areas of further inquiry as it relates to the specific pharmacological, individual-level factors, and lifestyle behaviors that impact L-FMC are highlighted. A greater integration of mechanistic work alongside applied lifestyle interventions is required to better understand endothelial cell function to reductions in local blood flow.

Cardiorespiratory Fitness and Endothelial Function in Aging Healthy Subjects and Patients With Cardiovascular Disease

Background

Individuals with a higher lifelong cardiorespiratory fitness show better vascular health with aging. Studies on fitness-related effects on endothelial function either analyzed samples with a narrow age-range or incompletely assessed endothelial responsiveness. This study aims to assess the impact of cardiorespiratory fitness on the association of brachial-arterial flow-mediated vasodilation (FMD) and low flow-mediated vasoconstriction (L-FMC) with age in healthy adults and patients with cardiovascular diseases.

Methods

FMD, L-FMC and V.O₂peak were prospectively measured in a population-based sample including 360 healthy adults and 99 patients with cardiovascular disease of European descend. Non-linear models were applied to assess V.O₂peak-associated variations in age-related differences of endothelial function independent of classical cardiovascular risk factors.

Results

FMD was negatively associated with age in healthy adults (adjusted R² = 0.27, partial R² = 0.07, p < 0.001) and in cardiovascular patients (adjusted R² = 0.29, partial R² = 0.05, p = 002). L-FMC showed no association with age. In models predicting the change of FMD with higher age, V.O₂peak accounted for 2.8% of variation in FMD (χ²(5) = 5.37, p = 0.372, s = 1.43). Thereby, V.O₂peak-stratified changes of FMD started to fan out at around 30 years of age in women and 50 years of age in men, with 7–12% lower values at old age with V.O₂peak ≤3rd percentile compared to V.O₂peak ≥97th percentile) in both, the healthy sample and in cardiovascular patients.

Conclusion

The statistical effect of cardiorespiratory fitness on the association of FMD with age independent of classical cardiovascular risk factors was small in both, healthy aging adults as well as patients with cardiovascular diseases. Its clinical significance should be assessed further.

Impact of habitual sedentary patterns on popliteal artery endothelial-dependent vasodilation in healthy adults

Introduction:

Acute, laboratory-based bouts of prolonged sitting attenuate lower-limb arterial endothelial-dependent vasodilation. However, the impact of habitual sedentary patterns on popliteal artery endothelial health is unclear. We tested the hypothesis that greater habitual total sedentary time, more time spent in prolonged sedentary bouts, and fewer sedentary breaks would be associated with worse popliteal flow-mediated dilation (FMD) responses.

Methods:

This cross-sectional study used 98 healthy participants (19–77 years, 53 females) that wore an activPAL monitor on the thigh for 6.4 ± 0.8 days to objectively measure sedentary activity and completed a popliteal ultrasound assessment to determine FMD. Both relative (%baseline diameter) and absolute (mm) FMD were calculated. Using bivariate correlation and multiple regression analyses, we examined if there were relationships between sedentary outcomes and FMD while statistically controlling for any potential confounders.

Results:

In the multiple regression model, age (p = 0.006, β = −0.030, 95% CI = −0.051, −0.009) and total time in sedentary bouts > 1 hour (p = 0.031, β = −0.005, 95% CI = −0.009, −0.001) were independent predictors of relative FMD. Age (β = −0.002, 95% CI = −0.003, −0.001), mean blood flow (β = 0.013, 95% CI = 0.002, 0.024), moderate-intensity physical activity (β = 155.9E⁻⁵, 95% CI = 22.4E⁻⁵, 289.4E⁻⁵), sedentary breaks (β = 0.036, 95% CI = 0.007, 0.066), and total time spent in sedentary bouts > 1 hour (β = −25.02E⁻⁵, 95% CI = −47.67E⁻⁵, −2.378E⁻⁵) were predictors of absolute FMD (all, p < 0.047). All independent outcomes remained significant after partially controlling for all other predictor variables (all, p < 0.031).

Conclusions:

Habitual prolonged sedentary bouts and sedentary breaks, but not total sedentary time, were predictors of popliteal endothelial-dependent vasodilatory function. The patterns by which sedentary time is accumulated may be more important than the total sedentary time on lower-limb arterial health.

What is the impact of aerobic fitness and movement interventions on low-flow-mediated vasoconstriction? A systematic review of observational and intervention studies

The cardiovascular benefits of physical exercise are well established. The vasoreactivity that occurs during reductions in local arterial blood flow, termed low-flow-mediated constriction (L-FMC), is a measure of endothelial-dependent vasoconstrictor function. It is unclear whether aerobic fitness and movement (or lack thereof) influences L-FMC. We systematically reviewed studies examining the impact of physical behaviours on L-FMC. To be included, cross-sectional and interventional studies had to examine the impact of a physical behaviour on L-FMC in adults. There were no language or date of publication restrictions. Sources were searched in May, 2021 and included Scopus, Embase, MEDLINE, CINAHL, and Academic Search Premier. National Institutes of Health quality assessment tools were used. Fourteen studies (15 arms; 313 participants; 398 total observations from four arteries) met the inclusion criteria. The study quality varied from four out of 14 (controlled intervention scoring) to nine out of 12 (longitudinal intervention with no control group scoring) with the total points dependent upon the study design. Conflicting results were reported for acute prolonged sitting studies (attenuated L-FMC: n = 1; no change: n = 1) and resistance exercise (increased L-FMC: n = 2; no change: n = 2). Most observational studies examining aerobic fitness (3/4 studies) and aerobic exercise interventions (4/5 studies) observed a favourable effect on L-FMC. Overall, the included studies support that higher aerobic fitness and engaging in aerobic exercise training may augment L-FMC responses. Our systematic review highlights the heterogeneity between studies and identifies current gaps and future directions to better our understanding of (in)activity, exercise, and posture on endothelial vasoconstrictor function. PROSPERO Registration No.: CRD42021248241.

Endothelial function of healthy adults from 20 to 91 years of age: prediction of cardiovascular risk by vasoactive range

OBJECTIVES

Brachial arterial low flow-mediated constriction (L-FMC) and flow-mediated dilation (FMD) are ultrasound-based biomarkers that emerge into scientific and clinical practice indicating cardiovascular effects of medical and lifestyle-based treatment beyond classical risk factors. This study is the first to provide reference values and to assess the predictive value of L-FMC, FMD and their composite endpoint vasoactive range (VAR) in healthy adults.

METHODS

L-FMC, FMD and VAR were measured in 457 nonsmoking adults of 20-91 years without chronic diseases, medication, with normal heart function and very low cardiovascular risk. Sex-specific percentiles were calculated and predictive ability for elevated cardiovascular risk was assessed using receiver-operating characteristic (ROC) curves.

RESULTS

From 20 to 91 years of age, L-FMC increased 86.1 and 105.3%, FMD decreased 63.6 and 47.1% and VAR decreased 58.3 and 55.2% in women and men, respectively. Area under the ROC curves was 0.54 (95% CI = 0.49-0.54) for L-FMC, 0.67 (95% CI = 0.62-0.67) for FMD and 0.72 (95% CI = 0.67-0.72) for VAR (P < 0.001). Discriminatory cut-offs for elevated risk were 0.24% for L-FMC (sensitivity = 0.42, specificity = 0.67), 6.4% for FMD (sensitivity = 0.71, specificity = 0.60) and 6.3% for VAR (sensitivity = 0.62, specificity = 0.73).

CONCLUSION

This study demonstrates reduced endothelial function with aging in healthy men and women with very low cardiovascular risk. Percentiles crossed cut-offs for elevated cardiovascular risk between 50 and 55 years in men and 70 and 75 years in women, indicating higher risk for cardiovascular disease in men. VAR showed the highest ability to identify individuals with elevated cardiovascular risk, and should be included in the monitoring and treatment of accelerated vascular aging even in healthy individuals.

Influence of prostaglandins and endothelial-derived hyperpolarizing factors on brachial and popliteal endothelial-dependent function in young adults

Heterogeneous flow-mediated dilation (FMD) and low-flow-mediated constriction (L-FMC) responses have been reported between upper- and lower-limb arteries. Radial artery L-FMC, but not FMD, responses are blunted when endothelial-derived hyperpolarizing factors (EDHFs) or prostaglandin production is inhibited in young adults. However, it is unknown if these mechanisms similarly impact endothelial-dependent responses in the brachial (BA) and popliteal (POP) arteries. We tested whether BA- and POP-L-FMC and FMD would be influenced by independent EDHF and prostaglandin inhibition. Eighteen participants (23 ± 3 yr; 6♀) completed three randomized and double-blinded ultrasound assessments following ingestion of an opaque capsule containing maltodextrin (control), 150 mg of fluconazole (EDHF inhibition), or 500 mg of aspirin (prostaglandin inhibition). POP resting diameter was reduced following fluconazole administration (6.13 ± 0.63 mm vs. 6.19 ± 0.65 mm in control, P = 0.03). Compared with control, fluconazole also blunted the relative L-FMC responses in both the BA (-2.1 ± 0.8% vs. -0.8 ± 1.0%, P = 0.001) and POP (-1.7 ± 1.1% vs. -0.8 ± 0.9%, P = 0.009). In contrast, aspirin did not impact either the BA (-1.9 ± 0.7%) or POP-L-FMC (-1.3 ± 0.6%) responses (both, P > 0.35). The FMD response was unchanged following fluconazole or aspirin administration in either artery (both, P > 0.36). Our findings demonstrate that EDHF mediates L-FMC responses in both the brachial and popliteal arteries. Complementary to the nitric oxide-mediated FMD response, L-FMC appears to provide information regarding the EDHF pathway. Future research should uncover if these mechanisms impact older adults and/or patient populations characterized by vascular endothelial dysfunction associated with low aerobic fitness and habitual physical activity levels.NEW & NOTEWORTHY We compared changes in upper- and lower-limb artery endothelial-dependent vasodilatory and vasoconstrictor responses between control, prostaglandin inhibition, and endothelial-derived hyperpolarizing factor inhibition conditions. Neither prostaglandins nor endothelial-derived hyperpolarizing factor influenced flow-mediated dilation responses in either the brachial or popliteal artery. In contrast, endothelial-derived hyperpolarizing factor, but not prostaglandins, reduced resting brachial artery blood flow and shear rate and resting popliteal artery diameter, as well as low-flow-mediated constriction responses in both the popliteal and brachial arteries.

The impact of age and sex on popliteal artery endothelial-dependent vasodilator and vasoconstrictor function

Lower-limb arteries, such as the popliteal artery, are a common site of atherosclerosis. These arteries are habitually exposed to large fluctuations in blood flow during physical and sedentary activities. Low-flow-mediated constriction (L-FMC) and flow-mediated dilation (FMD) provide indices of endothelial-dependent vasoconstriction and vasodilation, respectively. Age and sex both impact upper-limb FMD. However, it is unclear whether these factors also influence popliteal endothelial-dependent function. Popliteal endothelial function was compared between younger and older males and females (n=14 per group) matched for age- and sex-specific relative aerobic fitness levels (each group's normative percentile: ~45%). Nitroglycerin-mediated dilation (NMD) was also assessed as a measure of endothelial-independent vasodilation. Ageing reduced relative popliteal FMD in both males (older: 4.3±1.8% versus younger 5.7±1.9%) and females (older: 2.9±1.8% versus younger: 6.1±1.6%, both: P<0.046). FMD was also lower in older females versus older males (P=0.04). Popliteal NMD findings followed the same pattern as FMD. Compared to younger adults, relative L-FMC responses were blunted among older males (older: -1.2±1.1% versus younger: -2.2±1.0%) and females (older: -1.0±1.2% versus younger: -2.1±1.3%, both P<0.03) with no sex-differences observed in either age group (all, P>0.60). The adverse age- and sex-related (older adults only) declines in popliteal FMD were mediated, in part, by reduced vascular smooth muscle sensitivity to nitric oxide. Endothelial-dependent vasoconstriction was also attenuated with age, but unaffected by sex. Despite similar normative aerobic fitness percentiles (~45%), older adults exhibited attenuated popliteal endothelial function than their younger counterparts. This was particularly evident in older females who exhibited the lowest endothelial-dependent vasodilatory responses.

Popliteal flow-mediated dilatory responses to an acute bout of prolonged sitting between earlier and later phases of natural menstrual and oral contraceptive pill cycles

We compared changes in popliteal artery endothelial function to a 3-h bout of sitting in females across their natural menstrual or oral contraceptive pill cycles. Pre-sitting endothelial-dependent vasodilation was greater in females who naturally menstruate during the later versus earlier phase but unchanged among contraceptive pill phases. Neither menstrual nor oral contraceptive pill phases attenuated the robust decline in conduit artery health following an acute period of uninterrupted sitting in young females.

Sex does not influence impairments in popliteal endothelial-dependent vasodilator or vasoconstrictor responses following prolonged sitting

An acute bout of prolonged sitting (PS) impairs the popliteal artery flow-mediated dilation (FMD) response. Despite equivocal reductions in mean shear rate, young women demonstrate an attenuated decline in popliteal FMD versus young men. However, it is uncertain whether popliteal endothelial-dependent vasoconstrictor responses [low-flow-mediated constriction (L-FMC)] are similarly affected by PS and/or whether sex differences exist. We tested the hypothesis that women would have attenuated reductions in both popliteal FMD and L-FMC responses following an acute bout of PS. Popliteal FMD and L-FMC responses were assessed via duplex ultrasonography before and after a 3-h bout of PS. These responses were then compared between 10 men (24 ± 2 yr) and 10 women (23 ± 2 yr) with similar ( P > 0.13) levels of objectively measured habitual physical activity (via PiezoRx) and sedentary time (via activPAL). At baseline, men and women exhibited similar ( P > 0.46) popliteal FMD (4.8 ± 1.2 vs. 4.5 ± 0.6%) and L-FMC (–1.7 ± 1.0 vs. –1.9 ± 0.9%) responses. Both sexes experienced identical (group: P > 0.76; time: P < 0.001) PS-induced impairments in popliteal FMD (–2.8 ± 1.4 vs. –2.6 ± 0.9%) and L-FMC (1.3 ± 0.7% vs. 1.4 ± 0.7%). In young adults, sex did not influence the negative PS-induced FMD, L-FMC, or microvascular responses in the lower limb. As such, our findings suggest that young men and women are similarly susceptible to the acute negative vascular effects of PS. Future studies should extend these findings to older, less physically active adults and/or patients with vascular disease.

NEW & NOTEWORTHY We compared changes in popliteal artery endothelial function to a single 3-h bout of sitting between young men and women. Both groups exhibited similar endothelial-dependent vasodilation (i.e., flow-mediated dilation) and endothelial-dependent vasoconstrictor responses (i.e., low-flow-mediated constriction) at baseline and equivocal impairments in these measures of endothelial function following prolonged sitting. These findings demonstrate that acute impairments in conduit artery endothelial health associated with uninterrupted sitting are not influenced by sex in young, healthy adults.

Relationship between brachial and popliteal artery low-flow-mediated constriction in older adults: impact of aerobic fitness on vascular endothelial function

We previously observed that brachial artery (BA) low-flow-mediated constriction (L-FMC) is inversely related to aerobic fitness (i.e., V̇o2peak) in older adults (OA). However, it is unclear if an L-FMC response is elicited in the popliteal artery (POP) or if a similar inverse relationship with aerobic fitness exists. Considering that the POP experiences larger shear stress fluctuations during sedentary behaviors and traditional lower limb modes of aerobic exercise, we tested the hypotheses that 1) heterogeneous L-FMC responses exist between the BA versus POP of OA, and 2) that aerobic fitness will be inversely related to POP L-FMC. L-FMC was assessed in 47 healthy OA (30 women, 67 ± 5 yr) using duplex ultrasonography and quantified as the percent decrease in diameter (from baseline) during the last 30 s of a 5-min distal cuff occlusion period. When allometrically scaled to baseline diameter, the BA exhibited a greater L-FMC response than the POP (–1.3 ± 1.6 vs. –0.4 ± 1.6%; P = 0.03). Furthermore, L-FMC responses in the BA and POP were not correlated ( r = 0.22; P = 0.14). V̇o2peak was strongly correlated to POP L-FMC ( r = –0.73; P < 0.001). The heterogeneous BA versus POP L-FMC data indicate that upper limb L-FMC responses do not represent a systemic measure of endothelial-dependent vasoconstrictor capacity in OA. The strong association between V̇o2peak and POP L-FMC suggests that localized shear stress patterns, perhaps induced by lower limb dominant modes of aerobic exercise, may result in greater vasoconstrictor responsiveness in healthy OA.

NEW & NOTEWORTHY We compared low-flow-mediated constriction responses between the brachial and popliteal arteries of healthy older adults. Vasoconstrictor responses were not correlated between arteries. A strong relationship between aerobic fitness and low-flow-mediated vasoconstriction was observed in the popliteal artery. These findings suggest that brachial vasoconstrictor responsiveness is not reflective of the popliteal artery, which is exposed to larger shear stress fluctuations during bouts of sedentary behavior and traditional lower limb modes of exercise.