New Noninvasive Methods to Evaluate Microvascular Structure and Function

Blood pressure measurement and assessment of arterial structure and function: an expert group position paper

Measuring blood pressure (BP) and investigating arterial hemodynamics are essential in understanding cardiovascular disease and assessing cardiovascular risk. Several methods are used to measure BP in the doctor's office, at home, or over 24 h under ambulatory conditions. Similarly, several noninvasive methods have been introduced for assessing arterial structure and function; these methods differ for the large arteries, the small ones, and the capillaries. Consequently, when studying arterial hemodynamics, the clinician is faced with a multitude of assessment methods whose technical details, advantages, and limitations are sometimes unclear. Moreover, the conditions and procedures for their optimal implementation, and/or the reference normality values for the parameters they yield are not always taken into sufficient consideration. Therefore, a practice guideline summarizing the main methods and their use in clinical practice is needed. This expert group position paper was developed by an international group of scientists after a two-day meeting during which each of the most used methods and techniques for blood pressure measurement and arterial function and structure evaluation were presented and discussed, focusing on their advantages, limitations, indications, normal values, and their pragmatic clinical application.

Is the peripheral microcirculation a window into the human coronary microvasculature?

An increasing body of evidence suggests a pivotal role for the microvasculature in the development of cardiovascular disease. A dysfunctional coronary microvascular network, specifically within endothelial cells-the inner most cell layer of vessels-is considered a strong, independent risk factor for future major adverse cardiac events. However, challenges exist with evaluating this critical vascular bed, as many of the currently available techniques are highly invasive and cost prohibitive. The more easily accessible peripheral microcirculation has surfaced as a potential surrogate in which to study mechanisms of coronary microvascular dysfunction and likewise may be used to predict poor cardiovascular outcomes. In this review, we critically evaluate a variety of prognostic, physiological, and mechanistic studies in humans to answer whether the peripheral microcirculation can add insight into coronary microvascular health. A conceptual framework is proposed that the health of the endothelium specifically may link the coronary and peripheral microvascular beds. This is supported by evidence showing a correlation between human coronary and peripheral endothelial function in vivo. Although not a replacement for investigating and understanding coronary microvascular function, the microvascular endothelium from the periphery responds similarly to (patho)physiological stress and may be leveraged to explore potential therapeutic pathways to mitigate stress-induced damage.

Retinal microcirculation: A window into systemic circulation and metabolic disease

The retinal microcirculation system constitutes a unique terminal vessel bed of the systemic circulation, and its perfusion status is directly associated with the neural function of the retina. This vascular network, essential for nourishing various layers of the retina, comprises two primary microcirculation systems: the retinal microcirculation and the choroidal microcirculation, with each system supplying blood to distinct retinal layers and maintaining the associated neural function. The blood flow of those capillaries is regulated via different mechanisms. However, a range of internal and external factors can disrupt the normal architecture and blood flow within the retinal microcirculation, leading to several retinal pathologies, including diabetic retinopathy, macular edema, and vascular occlusions. Metabolic disturbances such as hyperglycemia, hypertension, and dyslipidemia are known to modify retinal microcirculation through various pathways. These alterations are observable in chronic metabolic conditions like diabetes, coronary artery disease, and cerebral microvascular disease due to advances in non-invasive or minimally invasive retinal imaging techniques. Thus, examination of the retinal microcirculation can provide insights into the progression of numerous chronic metabolic disorders. This review discusses the anatomy, physiology and pathophysiology of the retinal microvascular system, with a particular emphasis on the connections between retinal microcirculation and systemic circulation in both healthy states and in the context of prevalent chronic metabolic diseases.

Prognostic significance of the wall to lumen ratio of retinal arterioles evaluated by adaptive optics

OBJECTIVE

Microvascular structural alterations may be considered an important form of hypertension-mediated organ damage. An increased media-to-lumen ratio of subcutaneous small arteries evaluated with locally invasive techniques (micromyography) predicts the development of cardiovascular (CV) events. However, it is not known whether retinal arteriole structural alterations evaluated with a noninvasive approach (Adaptive Optics) may have a prognostic significance.

DESIGN AND METHODS

Two-hundred and thirty-seven subjects (mean age 58.7 ± 16.1 years, age range 13-89 years; 116 males) were included in the study: 65 normotensive subjects (27.4 %) and 172 patients with essential hypertension or primary aldosteronism (72.6 %). All subjects underwent a non-invasive evaluation of retinal arteriolar wall-to-lumen ratio (WLR) by Adaptive Optics. Subjects were re-evaluated after an average follow-up time of 4.55 years in order to assess the occurrence of clinical events (non CV and/or CV death or events).

RESULTS

Fifty-four events occurred in the study population:26 were cardio-cerebrovascular events (ischemic or hemorragic stroke, atrial fibrillation, heart failure, coronary artery disease, peripheral artery disease, cardiac valvular disease) while the remaining were deaths for any cause, or neoplastic diseases. Subjects with events were older and had a WLR of retinal arterioles significantly greater than those without events. The event-free survival was significantly worse in those with a baseline WLR above the median value of the population (0.28) according to Kaplan-Mayer survival curves and multivariate analysis (Cox's proportional hazard model). The evidence was confirmed after restricting the analysis to CV events.

CONCLUSIONS

Structural alterations of retinal arterioles evaluated by Adaptive Optics may predict total and CV events.

Retinal Microcirculation Measurements in Response to Endurance Exercises Analysed by Adaptive Optics Retinal Camera

This study aimed to precisely investigate the effects of intensive physical exercise on retinal microvascular regulation in healthy volunteers through adaptive optics retinal camera (AO) measurement. We included healthy volunteers (11 men and 14 women) aged 20.6 ± 0.9. The heart rate (HR) and systolic and diastolic blood pressures (SBP, DBP) were recorded before and after a submaximal physical exertion of continuously riding a training ergometer. The superior temporal retinal artery measurements were captured using the AO-rtx1TM (Imagine Eyes, Orsay, France) without pupil dilation. We compared measures of vessel diameter (VD), lumen diameter (LD), two walls (Wall 1, 2), wall-to-lumen ratio (WLR), and wall cross-sectional analysis (WCSA) before and immediately after the cessation of exercise. Cardiovascular parameter results: After exercise, SBP, DBP, and HR changed significantly from 130.2 ± 13.2 to 159.7 ± 15.6 mm Hg, 81.2 ± 6.3 to 77.1 ± 8.2 mm Hg, and 80.8 ± 16.1 to 175.0 ± 6.2 bpm, respectively (p < 0.002). Retinal microcirculation analysis showed no significant decrease in LD, Wall 1 after exercise: from 96.0 ± 6.8 to 94.9 ± 6.7 (p = 0.258), from 11.0 ± 1.5 to 10.4 ± 1.5 (p = 0.107), respectively, and significant reduction in VD from 118.5 ± 8.3 to 115.9 ± 8.3 (p = 0.047), Wall 2 from 11.5 ± 1.0 to 10.7 ± 1.3 (p = 0.017), WLR from 0.234 ± 0.02 to 0.222 ± 0.010 (p = 0.046), WCSA from 3802.8 ± 577.6 to 3512.3 ± 535.3 (p = 0.016). The AO is a promising technique for investigating the effects of exercise on microcirculation, allowing for the tracking of changes throughout the observation. Intensive dynamic physical exertion increases blood pressure and heart rate and causes the vasoconstriction of small retinal arterioles due to the autoregulation mechanism.

History of the cutaneous microcirculation from antiquity to modern times

 This review spans a wide arc from the first observations of the early anatomists to the present day. William Harvey was the first to describe the heart as the centre of the large and small circulatory system. He thus replaced the previously valid system of Galenos, It was Marcello Malpighi who first described that the capillary system connects the arteries with the veins. In 1688 Antoni van Leeuwenhoek (1632-1686) confirmed these results with a paper on capillary perfusion in the caudal fin of the glass eel. It was then Hermann Boerhave (1668-1738, Leiden) who was the first to carry out microcirculation tests on patients. He studied the microcirculation in the human bulbar conjunctiva. Even today, microcirculation studies in the conjunctiva bulbi of patients are carried out today. Until 1831, it was never quite clear whether the observations reported belonged mainly to the field of microcirculation, which had not yet been defined. This was done in Great Britain by Marshall Hall (1790-1857). Technical Improvements allowed increasingly sophisticated studies of the morphological structure of the terminal vasculature. According to Gustav Ricker (1870-1948, Vienna), the terminal vasculature comprises the functional unit of the smallest arteries, arterioles, capillaries and venules. In 1921 it was still thought that the blood circulation was the sole response to the pumping action of the heart. Even the classic work by Bayliss on the myogenic hypothesis (later referred to as "blood flow autoregulation") initially received little attention. More strikingly, even the findings of August Krogh, for which he received the Nobel Prize in Medicine in 1920 (for his discovery of the mechanisms of capillary motor regulation), were ignored. During an outstanding autoregulation symposium held in 1963 a broad consensus was reached on active and passive mechanisms, which is more or less valid till today. The mechanisms of regulation of capillary blood flow are now largely understood, although not completely resolved. The development of video systems with recording capability and automated off-line recording of capillary erythrocyte velocities allowed the application of morphological and dynamic studies of cutaneous capillaries in humans. These reopened the field of physiological or pathophysiological questions again for many groups worldwide. Since 1955, many publications on "microcirculation (5423)" and "capillary microscopy (2195)" have been listed in pubmed.

Microvascular Dysfunction in Obesity-Hypertension

PURPOSE OF REVIEW

This review aims to explore the role of microvascular dysfunction in obesity-hypertension, discuss the effects obesity has on renal microvasculature, review the current methods for assessing microvascular dysfunction and available therapeutic options, and identify critical areas for further research.

RECENT FINDINGS

There is a strong association between obesity and hypertension. However, the pathophysiology of obesity-hypertension is not clear. Microvascular dysfunction has been linked to hypertension and obesity and could be an important mediator of obesity-related hypertension. Newer therapies for hypertension and obesity could have ameliorating effects on microvascular dysfunction, including GLP-1 agonists and SGLT-2 inhibitors. There is still much progress to be made in our understanding of the complex interplay between obesity, hypertension, and microvascular dysfunction. Continued efforts to understand microvascular dysfunction and its role in obesity-hypertension are crucial to develop precision therapy to target obesity-hypertension.

The importance of microvascular inflammation in ageing and age-related diseases: a position paper from the ESH working group on small arteries, section of microvascular inflammation

Microcirculation is pervasive and orchestrates a profound regulatory cross-talk with the surrounding tissue and organs. Similarly, it is one of the earliest biological systems targeted by environmental stressors and consequently involved in the development and progression of ageing and age-related disease. Microvascular dysfunction, if not targeted, leads to a steady derangement of the phenotype, which cumulates comorbidities and eventually results in a nonrescuable, very high-cardiovascular risk. Along the broad spectrum of pathologies, both shared and distinct molecular pathways and pathophysiological alteration are involved in the disruption of microvascular homeostasis, all pointing to microvascular inflammation as the putative primary culprit. This position paper explores the presence and the detrimental contribution of microvascular inflammation across the whole spectrum of chronic age-related diseases, which characterise the 21st-century healthcare landscape. The manuscript aims to strongly affirm the centrality of microvascular inflammation by recapitulating the current evidence and providing a clear synoptic view of the whole cardiometabolic derangement. Indeed, there is an urgent need for further mechanistic exploration to identify clear, very early or disease-specific molecular targets to provide an effective therapeutic strategy against the otherwise unstoppable rising prevalence of age-related diseases.

Microcirculation in Hypertension: A Therapeutic Target to Prevent Cardiovascular Disease?

Arterial hypertension is a common condition worldwide and an important risk factor for cardio- and cerebrovascular events, renal diseases, as well as microvascular eye diseases. Established hypertension leads to the chronic vasoconstriction of small arteries as well as to a decreased lumen diameter and the thickening of the arterial media or wall with a consequent increased media-to-lumen ratio (MLR) or wall-to-lumen ratio (WLR). This process, defined as vascular remodeling, was firstly demonstrated in small resistance arteries isolated from subcutaneous biopsies and measured by micromyography, and this is still considered the gold-standard method for the assessment of structural alterations in small resistance arteries; however, microvascular remodeling seems to represent a generalized phenomenon. An increased MLR may impair the organ flow reserve, playing a crucial role in the maintenance and, probably, also in the progressive worsening of hypertensive disease, as well as in the development of hypertension-mediated organ damage and related cardiovascular events, thus possessing a relevant prognostic relevance. New non-invasive techniques, such as scanning laser Doppler flowmetry or adaptive optics, are presently under development, focusing mainly on the evaluation of WLR in retinal arterioles; recently, also retinal microvascular WLR was demonstrated to have a prognostic impact in terms of cardio- and cerebrovascular events. A rarefaction of the capillary network has also been reported in hypertension, which may contribute to flow reduction in and impairment of oxygen delivery to different tissues. These microvascular alterations seem to represent an early step in hypertension-mediated organ damage since they might contribute to microvascular angina, stroke, and renal dysfunction. In addition, they can be markers useful in monitoring the beneficial effects of antihypertensive treatment. Additionally, conductance arteries may be affected by a remodeling process in hypertension, and an interrelationship is present in the structural changes in small and large conductance arteries. The review addresses the possible relations between structural microvascular alterations and hypertension-mediated organ damage, and their potential improvement with antihypertensive treatment.

Evaluation of systemic endothelial-dependent and endothelial-independent microvascular reactivity in metabolically healthy obesity: An observational study

BACKGROUND

Metabolically healthy obesity (MHO), a phenotype of obesity considered to be of lower cardiovascular risk, is still a controversial concept. This study aimed to investigate the presence of subclinical systemic microvascular dysfunction in individuals with MHO.

METHODS

This was a cross-sectional study in which 112 volunteers were allocated into three groups: metabolically healthy normal weight (MHNW), MHO, or metabolically unhealthy obesity (MUO). Obesity was defined as a body mass index (BMI) ≥ 30 kg/m². MHO was defined as the absence of any component of metabolic syndrome, except waist circumference. Microvascular reactivity was evaluated using cutaneous laser speckle contrast imaging.

RESULTS

Mean age was 33.2 ± 7.66 years. The median BMI in the MHNW, MHO and MUO groups was 23.6, 32.8, and 35.8 kg/m², respectively. Baseline microvascular conductance values were lower in the MUO group (0.25 ± 0.08 APU/mmHg) than in MHO (0.30 ± 0.10 APU/mmHg) and MHNW groups (0.33 ± 0.12 APU/mmHg) (P = 0.0008). There were no significant differences regarding endothelial-dependent (acetylcholine stimulation or postocclusive reactive hyperemia) or endothelial-independent (sodium nitroprusside stimulation) microvascular reactivity among the groups.

CONCLUSIONS

Individuals with MUO had lower baseline systemic microvascular flow than those with MHNW or MHO, but endothelium-dependent or endothelium-independent microvascular reactivity were not changed in any of the groups. The relatively young age of the study population, the low frequency of class III obesity, or the strict definition of MHO (absence of any metabolic syndrome criteria) might account for the lack of difference of microvascular reactivity among MHNW, MHO or MUO.

Retinal Vascular Geometry in Hypertension: cSLO-Based Method

INTRODUCTION

We aim to introduce a method using confocal scanning laser ophthalmoscopy (cSLO) images for measuring retinal vascular geometry, including vessel branch angle (BA), vessel diameter, vessel tortuosity, and fractal dimension (D_f), and to elucidate the relationship between hypertension and these metrics.

METHODS

A total of 119 participants (119 eyes) were enrolled, among which 72 were normotensive and 47 were hypertensive. Infrared cSLO images were extracted from the circular scan around the optics disc using a commercial cSLO + optical coherence tomography instrument. Preprocessed cSLO images were further analyzed using the appropriate tool/macro/plugin of ImageJ.

RESULTS

Intraclass correlation coefficients of selected methods used for conducting the cSLO-based geometric analyses were all higher than 0.80. Arterial/arteriolar BA, arteriolar vessel diameter, and total D_f in normotensive subjects were 85.80 ± 7.79°, 116.80 ± 12.58 μm, and 1.430 ± 0.037, respectively, significantly higher than those of hypertensive subjects (82.13 ± 10.83°, 108.2 ± 11.12 μm, and 1.361 ± 0.044, all P < 0.05). The aforementioned metrics remained negatively correlated with hypertension even after adjusting for age alone or age and gender (P < 0.05). However, the difference between arteriolar tortuosity and all studied venous/venular geometric parameters in both subjects was insignificant (all P > 0.05).

CONCLUSION

Proposed cSLO-based methods for assessing various vascular geometric parameters were highly repeatable and reproducible. Arterial/arteriolar BA, arteriolar vessel diameter, and total D_f were retinal vascular parameters significantly correlated with hypertension in a negative manner.

Microvascular wall-to-lumen ratio in patients with arterial hypertension: A randomized controlled exercise trial

BACKGROUND AND AIMS

High blood pressure is one of the main cardiovascular disease risk factors that contribute to vascular remodeling and dysfunction. We aimed to investigate I) group differences of the retinal microstructure between patients with hypertension and healthy individuals and II) the effects of a high-intensity interval training (HIIT) on hypertension-induced microvascular remodeling in patients with hypertension in a randomized controlled trial.

METHODS

Arteriolar and venular retinal vessel microstructure including retinal vessel wall (RVW), lumen diameter and wall-to-lumen ratio (WLR) of 41 hypertensive patients, treated with anti-hypertensive medication, and 19 normotensive healthy controls were screened based on high-resolution fundoscopies. Patients with hypertension were randomized to a control group receiving standard physical activity recommendations and an intervention group receiving a supervised and walking-based HIIT for eight weeks. Measurements were repeated after the intervention period.

RESULTS

Hypertensive patients showed thicker arteriolar RVW (28.0 ± 7.7mu vs. 21.4 ± 4.4mu, p = 0.003) and higher arteriolar WLR (58.5 ± 14.8 % vs. 42.5 ± 8.2 %, p < 0.001) compared to normotensive controls. The intervention group showed reductions in arteriolar RVW (β -3.1 (95 % CI, -4.38, -1.78) p < 0.001) and arteriolar WLR (-5.3 (-10.14, -0.39) p = 0.035) compared to the control group. The intervention effects were independent of age, sex, change in blood pressure and change in cardiorespiratory fitness.

CONCLUSIONS

HIIT in patients with hypertension improves retinal vessel microvascular remodeling after eight weeks of training. In patients with hypertension, screening retinal vessel microstructure by fundoscopy and monitoring efficacy of short-term exercise treatment are sensitive diagnostic approaches to quantify microvascular health in these patients.

Epigenetic Signatures in Arterial Hypertension: Focus on the Microvasculature

Systemic arterial hypertension (AH) is a multifaceted disease characterized by accelerated vascular aging and high cardiometabolic morbidity and mortality. Despite extensive work in the field, the pathogenesis of AH is still incompletely understood, and its treatment remains challenging. Recent evidence has shown a deep involvement of epigenetic signals in the regulation of transcriptional programs underpinning maladaptive vascular remodeling, sympathetic activation and cardiometabolic alterations, all factors predisposing to AH. After occurring, these epigenetic changes have a long-lasting effect on gene dysregulation and do not seem to be reversible upon intensive treatment or the control of cardiovascular risk factors. Among the factors involved in arterial hypertension, microvascular dysfunction plays a central role. This review will focus on the emerging role of epigenetic changes in hypertensive-related microvascular disease, including the different cell types and tissues (endothelial cells, vascular smooth muscle cells and perivascular adipose tissue) as well as the involvement of mechanical/hemodynamic factors, namely, shear stress.

Challenges in the evaluation of endothelial cell dysfunction: a statement from the European Society of Hypertension Working Group on Endothelin and Endothelial Factors

Endothelial cell function is mediated by different mechanisms in different vascular beds. Moreover, in humans, endothelial cell dysfunction triggers and accelerates the progression of cardiovascular and chronic kidney diseases. Progression of such diseases can be in part mitigated by the control of cardiovascular risk factors and drugs targeting different systems, including endothelin receptor antagonists (ERAs), renin-angiotensin aldosterone antagonists and agents affecting glucose metabolism, all of which were shown to improve endothelial cell function. In recent years, the microRNAs, which are endogenous regulators of gene expression, have been identified as transmitters of information from endothelial cells to vascular smooth muscle cells, suggesting that they can entail tools to assess the endothelial cell dysfunction in arterial hypertension and target for pharmacologic intervention. This article critically reviews current challenges and limitations of available techniques for the invasive and noninvasive assessment of endothelial cell function, and also discusses therapeutic aspects as well as directions for future research in the areas of endothelial cell biology and pathophysiology in humans.

State of the Art Review: Vascular Remodeling in Hypertension

Although the gold-standard method for the assessment of structural alteration in small resistance arteries is the evaluation of the MLR by micromyography in bioptic tissues, new, noninvasive techniques are presently under development, focusing mainly on the evaluation of WLR in retinal arterioles. These approaches represent a promising and interesting future perspective. Appropriate antihypertensive treatment is able to prevent the development of microvascular alterations or to induce their regression. Also, conductance arteries may be affected by a remodeling process in hypertension, and a cross-talk may exist between structural changes in the small and large arteries. In conclusion, the evaluation of microvascular structure is ready for clinical prime time, and it could, in the future, represent an evaluation to be performed in the majority of hypertensive patients, to better stratify cardiovascular risk and better evaluate the effects of antihypertensive therapy. However, for this purpose, we need a clear demonstration of the prognostic relevance of noninvasive measures of microvascular structure, in basal conditions and during treatment. Vascular remodeling may be frequently observed in hypertension, as well as in obesity and diabetes mellitus. An increased media to lumen ratio (MLR) or wall to lumen ratio (WLR) in microvessels is the hallmark of hypertension, and may impair organ flow reserve, being relevant in the maintenance and, probably, also in the progressive worsening of hypertensive disease, as well as in the development of hypertension-mediated organ damage/cardiovascular events. The molecular mechanisms underlying the development of vascular remodeling are only partly understood.

Unattended versus Attended Blood Pressure Measurement: Relationship with Retinal Microcirculation

Though the relationship between both “attended” and “unattended” BP and several forms of target organ damage have been evaluated, data on retinal arteriolar alterations are lacking. The aim of our study was to evaluate the relationship between “attended” or “unattended” BP values and retinal arteriolar changes in consecutive individuals undergoing a clinical evaluation and assessment of retinal fundus at an ESH Excellence Centre. An oscillometric device programmed to perform 3 BP measurements, at 1 min intervals and after 5 min of rest was used on all individuals to measure BP with the patient alone in the room (“unattended”) or in the presence of the physician (“attended”) in the same day in a random order. The retinal arteriole’s wall thickness (WT) was measured automatically by a localization algorithm as the difference between external (ED) and internal diameter (ID) by adaptive optics (RTX-1, Imagine Eyes, Orsay, Francia). Media-to-lumen ratio (WLR) of the retinal arterioles and cross-sectional area (WCSA) of the vascular wall were calculated. Results: One-hundred-forty-two patients were examined (mean age 57 ± 12 yrs, 48% female, mean BMI 26 ± 4). Among them, 60% had hypertension (84% treated) and 11% had type 2 diabetes mellitus. Unattended systolic BP (SBP) was lower as compared to attended SBP (129 ± 14.8. vs. 122.1 ± 13.6 mmHg, p < 0.0001). WLR was similarly correlated with unattended and attended SBP (r = 0.281, p < 0.0001 and r = 0.382, p < 0.0001) and with unattended and attended diastolic BP (r = 0.34, p < 0.001 and r = 0.29, p < 0.0001). The differences between correlations were not statistically significant (Steiger’s Z test). Conclusion: The measurement of “unattended” or “attended” BP provides different values, and unattended BP is lower as compared to attended BP. In this study a similar correlation was observed between attended and unattended BP values and structural changes of retinal arterioles.