How should flow-mediated dilation be normalized to its stimulus?

Assessment of power spectral density of microvascular hemodynamics in skeletal muscles at very low and low-frequency via near-infrared diffuse optical spectroscopies

In this work, we used a hybrid time domain near-infrared spectroscopy (TD-NIRS) and diffuse correlation spectroscopy (DCS) device to retrieve hemoglobin and blood flow oscillations of skeletal muscle microvasculature. We focused on very low (VLF) and low-frequency (LF) oscillations (i.e., frequency lower than 0.145 Hz), that are related to myogenic, neurogenic and endothelial activities. We measured power spectral density (PSD) of blood flow and hemoglobin concentration in four muscles (thenar eminence, plantar fascia, sternocleidomastoid and forearm) of 14 healthy volunteers to highlight possible differences in microvascular hemodynamic oscillations. We observed larger PSDs for blood flow compared to hemoglobin concentration, in particular in case of distal muscles (i.e., thenar eminence and plantar fascia). Finally, we compared the PSDs measured on the thenar eminence of healthy subjects with the ones measured on a septic patient in the intensive care unit: lower power in the endothelial-dependent frequency band, and larger power in the myogenic ones were observed in the septic patient, in accordance with previous works based on laser doppler flowmetry.

Endothelial Function Assessment by Flow-Mediated Dilation Method: A Valuable Tool in the Evaluation of the Cardiovascular System

Cardiovascular diseases (CVDs) in the course of atherosclerosis are one of the most critical public health problems in the world. Endothelial cells synthesize numerous biologically active substances involved in regulating the functions of the cardiovascular system. Endothelial dysfunction is an essential element in the pathogenesis of atherosclerosis. Thus, the assessment of endothelial function in people without overt CVD allows for a more accurate estimate of the risk of developing CVD and cardiovascular events. The assessment of endothelial function is primarily used in scientific research, and to a lesser extent in clinical practice. Among the tools for assessing endothelial function, we can distinguish biochemical and physical methods, while physical methods can be divided into invasive and non-invasive methods. Flow-mediated dilation (FMD) is based on the ultrasound assessment of changes in the diameter of the brachial artery as a result of increased blood flow. FMD is a non-invasive, safe, and repeatable test, but it must be performed by qualified and experienced medical staff. The purpose of this paper is to present the literature review results on the assessment of endothelial function using the FMD method, including its methodology, applications in clinical practice and research, limitations, and future perspectives.

Predictors of arterial stiffness in adolescents and adults with type 1 diabetes: a cross-sectional study

INTRODUCTION

Individuals with type 1 diabetes have increased arterial stiffness compared with age-matched healthy controls. Our aim was to determine which hemodynamic and demographic factors predict arterial stiffness in this population.

RESEARCH DESIGN AND METHODS

Carotid-femoral pulse wave velocity (cfPWV) was examined in 41 young adults and adolescents with type 1 diabetes without microvascular complications. Two ordinary least squares regression analyses were performed to determine multivariate relationships between cfPWV (loge) and (1) age, duration of diabetes, sex, and hemoglobin A1c and (2) augmentation index (AIx), mean arterial pressure, flow-mediated dilation (FMD), and heart rate. We also examined differences in macrovascular outcome measures between sexes.

RESULTS

Age, sex, and FMD provided unique predictive information about cfPWV in these participants with type 1 diabetes. Despite having similar cardiovascular risk factors, men had higher cfPWV compared with women but no differences were observed in other macrovascular outcomes (including FMD and AIx).

CONCLUSIONS

Only age, sex, and FMD were uniquely associated with arterial stiffness in adolescents and adults with uncomplicated type 1 diabetes. Women had less arterial stiffness and similar nitric oxide-dependent endothelial function compared with men. Larger, prospective investigation is warranted to determine the temporal order of and sex differences in arterial dysfunction in type 1 diabetes.

A Primer on Repeated Sitting Exposure and the Cardiovascular System: Considerations for Study Design, Analysis, Interpretation, and Translation

Sedentary behavior, particularly sitting, is ubiquitous in many contemporary societies. This is a major societal concern considering the evidence for a strong association between sitting behavior and cardiovascular disease morbidity and mortality. Unsurprisingly, leading public health agencies have begun to advocate “reduction” in sitting behavior. Though, the guidelines are typically vague and non-specific. The lack of specific guidelines for prolonged sitting is attributable to the absence of available evidence to facilitate guideline development. To inform policy, well-designed randomized controlled trials are required to test the efficacy of specific and translatable sitting reduction strategies. To guide the design of randomized controlled trials, this review postulates that several gaps in the literature first need to be filled. Following a general discussion of the importance of sitting behavior to contemporary societies, each of the following are discussed: (i) acute sitting exposure and systems physiology; (ii) recommendations for a systems physiology toolbox; (iii) study design considerations for acute sitting exposure; and (iv) translation of sitting-focused research.

The Effects of Acute Exposure to Prolonged Sitting, With and Without Interruption, on Vascular Function Among Adults: A Meta-analysis

BACKGROUND

Exposure to acute prolonged sitting can result in vascular dysfunction, particularly within the legs. This vascular dysfunction, assessed using flow-mediated dilation (FMD), is likely the consequence of decreased blood flow-induced shear stress. With mixed success, several sitting interruption strategies have been trialled to preserve vascular function.

OBJECTIVES

The objectives of this meta-analysis were to (1) assess the effects of acute prolonged sitting exposure on vascular function in the upper- and lower-limb arteries, and (2) evaluate the effectiveness of sitting interruption strategies in preserving vascular function. Sub-group analyses were conducted to determine whether artery location or interruption modality explain heterogeneity.

DATA SOURCES

Electronic databases (PubMed, Web of Science, SPORTDiscus, and Google Scholar) were searched from inception to January 2020. Reference lists of eligible studies and relevant reviews were also checked.

STUDY SELECTION

Inclusion criteria for objective (1) were: (i) FMD% was assessed pre- and post-sitting; (ii) studies were either randomised-controlled, randomised-crossover, or quasi-experimental trials; (iii) the sitting period was ≥ 1 h; and (iv) participants were healthy non-smoking adults (≥ 18 years), and free of vascular-acting medication and disease at the time of testing. Additional inclusion criteria for objective (2) were: (i) the interruption strategy must have been during the sitting period; (ii) there was a control (uninterrupted sitting) group/arm; and (iii) the interruption strategy must have involved the participants actively moving their lower- or upper-limbs.

APPRAISAL AND SYNTHESIS METHODS

One thousand eight hundred and two articles were identified, of which 17 (22 trials, n = 269) met inclusion criteria for objective (1). Of those 17 articles, 6 studies (9 trials, n = 127) met the inclusion criteria for objective (2). Weighted mean differences (WMD), 95% confidence intervals (95% CI), and standardised mean difference (SMD) were calculated for all trials using random-effects meta-analysis modelling. SMD was used to determine the magnitude of effect, where < 0.2, 0.2, 0.5, and 0.8 was defined as trivial, small, moderate, and large respectively.

RESULTS

(1) Random-effects modelling showed uninterrupted bouts of prolonged sitting resulted in a significant decrease in FMD% (WMD = - 2.12%, 95% CI - 2.66 to - 1.59, SMD = 0.84). Subgroup analysis revealed reductions in lower- but not upper-limb FMD%. (2) Random-effects modelling showed that interrupting bouts of sitting resulted in a significantly higher FMD% compared to uninterrupted sitting (WMD = 1.91%, 95% CI 0.40 to 3.42, SMD = 0.57). Subgroup analyses failed to identify an optimum interruption strategy but revealed moderate non-significant effects for aerobic interventions (WMD = 2.17%, 95% CI - 0.34 to 4.67, SMD = 0.69) and simple resistance activities (WMD = 2.40%, 95% CI - 0.08 to 4.88, SMD = 0.55) and a trivial effect for standing interruptions (WMD = 0.24%, 95% CI - 0.90 to 1.38, SMD = 0.16).

CONCLUSIONS

Exposure to acute prolonged sitting leads to significant vascular dysfunction in arteries of the lower, but not upper, limbs. The limited available data indicate that vascular dysfunction can be prevented by regularly interrupting sitting, particularly with aerobic or simple resistance activities.

Effects of compression stockings on lower-limb venous and arterial system responses to prolonged sitting: A randomized cross-over trial

Exposure to prolonged sitting increases blood pooling and the risk of lower-extremity vascular arterial and venous complications, including deep vein thrombosis. Compression garment stockings (CGS) may prevent pooling, thereby mitigating the associated vascular complications. Three aims were addressed: (i) does use of CGS help to prevent blood pooling; (ii) does blood pooling correlate with decreased stroke volume; and (iii) does use of CGS preserve leg arterial flow-mediated dilation and reactive hyperemia response. Twenty inactive participants (22 ± 4 years, 30% female, 22.1 ± 2.0 kg/m2) were randomized to sit for 3 hours with (CGS) and without use of CGS. Blood pooling was determined using medial gastrocnemius total hemoglobin. Stroke volume was estimated using finger photoplethysmography, respectively. Macrovascular and microvascular function were measured using popliteal artery flow-mediated dilation and reactive hyperemia, respectively. In response to 3 hours of sitting: (i) there was an interaction effect for total hemoglobin ( p < 0.001); the condition without use of CGS increased 10.5% (95% CI: 6.7 to 14.3) and CGS increased 4.3% (95% CI: 0.1 to 7.7); (ii) blood pooling was negatively associated with stroke volume ( r = −0.58, 95% CI: −0.68 to −0.45); (iii) reactive hyperemia and flow-mediated arterial dilation was impaired after prolonged sitting, and use of CGS was not associated with attenuation of this impairment. Use of CGS decreases blood pooling but does not preserve arterial macrovascular and microvascular responses to prolonged sitting. Further study is warranted to determine whether CGS has additive benefits when combined with sitting interruption strategies.

Longitudinal association among endothelial function, arterial stiffness and subclinical organ damage in hypertension

OBJECTIVES

To examine the longitudinal mutual association between endothelial dysfunction and arterial stiffness, and also to determine which of the two variables was more closely associated with the progression of subclinical organ damage.

METHODS

The brachial-ankle pulse wave velocity (baPWV), carotid intima-media thickness (CIMT), estimated glomerular filtration rate, microalbuminuria and flow-mediated vasodilatation of the brachial artery (FMD) were measured three times at 1.5-year intervals in 674 Japanese patients receiving antihypertensive treatment.

RESULTS

The change of the baPWV during the study period was larger in the subjects with baseline FMD values in the lowest tertile as compared to those with baseline FMD values in the highest tertile. The change of the CIMT was smaller in the subjects with baseline baPWV values in the lowest tertile than in those with baseline baPWV values in the highest tertile. After the adjustment, the FMD value at the baseline was inversely associated with the baPWV at the end of the study period (beta=-0.07, p=0.01), although, the reverse association was not significant. The baPWV, but not the FMD value, at the baseline was associated with the CIMT (beta=0.06, p=0.04) measured at the end of the study period.

CONCLUSIONS

In hypertension, endothelial dysfunction was associated with the progression of arterial stiffness, although the reverse association was not confirmed. The increased arterial stiffness rather than endothelial dysfunction may be more closely associated with the progression of atherosclerotic vascular damage, and the endothelial dysfunction-arterial stiffness-atherosclerosis continuum may be important in hypertension.

Ultrasound Assessment of Endothelial Function: A Technical Guideline of the Flow-mediated Dilation Test

Cardiovascular disease is the primary cause of mortality and a major cause of disability worldwide. The dysfunction of the vascular endothelium is a pathological condition characterized mainly by a disruption in the balance between vasodilator and vasoconstrictor substances and is proposed to play an important role in the development of atherosclerotic cardiovascular disease. Therefore, a precise evaluation of endothelial function in humans represents an important tool that could help better understand the etiology of multiple cardio-centric pathologies. Over the past twenty-five years, many methodological approaches have been developed to provide an assessment of endothelial function in humans. Introduced in 1989, the FMD test incorporates a forearm occlusion and subsequent reactive hyperemia that promotes nitric oxide production and vasodilation of the brachial artery. The FMD test is now the most widely utilized, non-invasive, ultrasonic assessment of endothelial function in humans and has been associated with future cardiovascular events. Although the FMD test could have clinical utility, it is a physiological assessment that has inherited several confounding factors that need to be considered. This article describes a standardized protocol for determining FMD including the recommended methodology to help minimize the physiological and technical issues and improve the precision and reproducibility of the assessment.

The use of shear rate-diameter dose-response curves as an alternative to the flow-mediated dilation test

The brachial artery flow-mediated dilation test (FMD) is the non-invasive gold-standard used to test endothelial function. Reduced FMD precedes the development of atherosclerosis and provides an early marker for predicting future cardiovascular disease events. Although, this test is of high potential, it is somewhat limited by poor reproducibility. By utilizing hand warming and grip exercise combined with hierarchical linear modeling, shear rate-diameter dose-response curves may provide a novel and more accurate way to assess endothelial function in humans. Shear rate-diameter dose-response curves could potentially improve upon the traditional FMD measurement and serve as a superior clinical and research tool for assessing cardiovascular disease risk in a variety of populations. The current paper presents testable hypotheses and methodology for assessing the validity and reliability of an alternative to the current FMD test.

Effect of repeated sprints on postprandial endothelial function and triacylglycerol concentrations in adolescent boys

This study investigated whether repeated, very short duration sprints influenced endothelial function (indicated by flow-mediated dilation) and triacylglycerol concentrations following the ingestion of high-fat meals in adolescent boys. Nine adolescent boys completed two, 2-day main trials (control and exercise), in a counter-balanced, cross-over design. Participants were inactive on day 1 of the control trial but completed 40 × 6 s maximal cycle sprints on day 1 of the exercise trial. On day 2, capillary blood samples were collected and flow-mediated dilation measured prior to, and following, ingestion of a high-fat breakfast and lunch. Fasting flow-mediated dilation and plasma triacylglycerol concentration were similar in the control and exercise trial (P > 0.05). In the control trial, flow-mediated dilation was reduced by 20% and 27% following the high-fat breakfast and lunch; following exercise these reductions were negated (main effect trial, P < 0.05; interaction effect trial × time, P < 0.05). The total area under the plasma triacylglycerol concentration versus time curve was 13% lower on day 2 in the exercise trial compared to the control trial (8.65 (0.97) vs. 9.92 (1.16) mmol · l(-1) · 6.5 h, P < 0.05). These results demonstrate that repeated 6 s maximal cycle sprints can have beneficial effects on postprandial endothelial function and triacylglycerol concentrations in adolescent boys.

Flow-mediated dilation is associated with endothelial oxidative stress in human venous endothelial cells

Flow-mediated dilation (FMD) is recognized as a non-invasive endothelial function bioassay. However, FMD's relationship with endothelial cell oxidative stress in humans is yet to be determined. Here, we sought to determine if FMD was associated with endothelial nitric oxide synthase (eNOS) and endothelial oxidative stress in humans. Twenty-seven apparently healthy young men (26.5±5.9 years) underwent brachial artery FMD testing and endothelial cell biopsy from a forearm vein. Non-normalized FMD (%) and three different brachial artery FMD normalizations were performed: (1) peak shear rate (%/SR); (2) area under the SR curve until peak dilation (%/AUC); and (3) AUC 30 seconds before peak dilation (%/AUC30). Immunofluorescence quantification was used to assess eNOS expression and nitrotyrosine (NT), a criterion marker of endothelial oxidative stress. Values for eNOS and NT expression were reported as a ratio of endothelial cell to human umbilical vein endothelial cell average pixel intensity. NT expression was significantly correlated with FMD normalized by AUC30 (r = -0.402, p<0.05). Other FMD normalizations and non-normalized FMD were not significantly correlated with NT expression (r range = -0.364 to -0.142, all p>0.05). There were no significant correlations between eNOS expression and normalized and non-normalized FMD (r range = -0.168 to -0.066, all p>0.05). In conclusion, brachial artery FMD is associated with venous endothelial cell oxidative stress. However, this association is observed only when FMD is normalized by AUC30.

Cardiovascular disease among breast cancer survivors: the call for a clinical vascular health toolbox

With better detection and treatment, breast cancer is less likely to be the primary cause of death in the majority of breast cancer survivors; mortality due to cardiovascular disease (CVD) is now more common. Given the long latency periods between cancer treatment completion and potential symptomatic CVD, there is a need to detect vascular changes before symptoms appear. This short review provides an overview of non-invasive, widely available, and relatively inexpensive techniques for assessing endothelial function, central and regional arterial stiffness, central blood pressures, and intima-media thickness. These tools exhibit acceptable reliability and validity, and are relatively practical. Clinical assessment recommendations are also provided. There is sufficient evidence to encourage the use of these techniques as a component of routine serial assessments, and to help guide appropriate treatment strategies.