Reproducibility of four frequently used local heating protocols to assess cutaneous microvascular function

Reproducibility of Laser Doppler Flowmetry in gingival microcirculation. A study on six different protocols

OBJECTIVES

Laser Doppler Flowmetry (LDF) is a non-invasive technique for the assessment of tissue blood flow, but increased reproducibility would facilitate longitudinal studies. The aim of the study was to assess the interday reproducibility of Laser Doppler Flowmetry (LDF) at rest, at elevated local temperatures, and with the use of the vasodilator Methyl Nicotinate (MN) in six interconnected protocols for the measurement of the blood supply to the microvascular bed of the gingiva.

METHODS

Ten healthy volunteers were included. Interweek LDF measurements with custom-made acrylic splints were performed. Six protocols were applied in separate regions of interest (ROI): 1; basal LDF, 2; LDF with thermoprobe 42 °C, 3; LDF with thermoprobe 45 °C, 4; LDF with thermoprobe 42 °C and MN, 5; LDF with thermoprobe 45 °C and MN and 6; LDF with MN.

RESULTS

Intra-individual reproducibility was assessed by the within-subject coefficient of variation (wCV) and the intraclass correlation coefficient (ICC). Basal LDF measurements demonstrated high reproducibility with wCV 11.1 in 2 min and 10.3 in 5 min. ICC was 0.9 and 0.92. wCV after heat and MN was 4.9-10.3 and ICC 0.82-0.93. The topically applied MN yielded increased blood flow.

CONCLUSION

This is the first study evaluating the reproducibility of basal LDF compared to single or multiple vasodilatory stimuli in gingiva. Multiple collector fibers probe and stabilizing acrylic splints are recommended. Vasodilatory stimulation showed a tendency toward higher reproducibility. Furthermore, MN yields vasodilation in gingiva.

Reliability of laser-Doppler flowmetry derived measurements of forearm and calf cutaneous vasodilation during gradual local heating in young adults

OBJECTIVES

Evaluate reliability of laser-Doppler flowmetry derived cutaneous vasodilation on the upper and lower limbs during gradual local heating.

METHODS

In twenty-eight young adults (21 (SD 3) years, 14 females), absolute cutaneous vascular conductance (CVC_abs) and CVC normalized to maximum vasodilation at 44 °C (%CVC_max) were assessed at two adjacent sites on each of the forearm and calf during gradual local skin heating (33-42 °C at 1 °C·5 min^-1) for two identical trials (∼1 week apart). Responses were assessed for baseline, the steady-state heating plateau at 42 °C and the span (i.e. plateau-baseline).

RESULTS

Between-day reliability was characterized as measurement consistency across trials. Within-day reliability was characterized as within-limb measurement consistency across adjacent skin sites. Between- and within-day absolute reliability (coefficient of variation) generally improved with heating, from poor (>25 %) at baseline to good (<10 %) for %CVC_max and moderate (10-25 %) for CVC_abs for plateau and span. However, relative reliability (intraclass correlation coefficient) was generally not acceptable (<0.70) for any condition. Responses were generally consistent for females and males and there were no major forearm and calf differences.

CONCLUSIONS

Consistency of CVC estimates improved during gradual local heating with negligible limb and sex differences, which are important considerations for experimental design and interpretation.

Acute impact of aerobic exercise on local cutaneous thermal hyperaemia

Little is known about the acute changes in cutaneous microvascular function that occur in response to exercise, the accumulation of which may provide the basis for beneficial chronic cutaneous vascular adaptations. Therefore, we examined the effects of acute exercise on cutaneous thermal hyperaemia. Twelve healthy, recreationally active participants (11 male, 1 female) performed 30-minute cycling at 50 % (low-intensity exercise, LOW) or 75 % (high-intensity exercise, HIGH) maximum heart rate. Laser Doppler flowmetry (LDF) and rapid local skin heating were used to quantify cutaneous thermal hyperaemia before (PRE), immediately following (IMM) and 1-h (1HR) after exercise. Baseline, axon reflex peak, axon reflex nadir, plateau, maximum skin blood flow responses to rapid local heating (42 °C for 30-min followed by 44 °C for 15-min) at each stage were assessed and indexed as cutaneous vascular conductance [CVC = flux / mean arterial blood pressure (MAP), PU·mm Hg^-1], and expressed as a percentage of maximum (%CVC_max). Exercise increased heart rate (HR), MAP and skin blood flow (all P < 0.001), and to a greater extent during HIGH (all P < 0.001). The axon reflex peak and nadir were increased immediately and 1-h after exercise (all comparisons P < 0.01 vs. PRE), which did not differ between intensities (peak: P = 0.34, axon reflex nadir: P = 0.91). The endothelium-dependent plateau response was slightly elevated after exercise (P = 0.06), with no effect of intensity (P = 0.58) nor any interaction effect (P = 0.55). CONCLUSION: Exercise increases cutaneous microvascular axonal responses to local heating for up to 1-h, suggesting an augmented sensory afferent function post-exercise. Acute exercise may only modestly affect endothelial function in cutaneous microcirculation.

Novel Method to Assess Endothelial Function via Monitoring of Perfusion Response to Local Heating by Imaging Photoplethysmography

Endothelial dysfunction is one of the most important markers of the risk of cardiovascular complications. This study is aimed to demonstrate the feasibility of imaging photoplethysmography to assess microcirculation response to local heating in order to develop a novel technology for assessing endothelial function. As a measure of vasodilation, we used the relative dynamics of the pulsatile component of the photoplethysmographic waveform, which was assessed in a large area of the outer surface of the middle third of the subject's forearm. The perfusion response was evaluated in six healthy volunteers during a test with local skin heating up to 40-42 °C and subsequent relaxation. The proposed method is featured by accurate control of the parameters affecting the microcirculation during the prolonged study. It was found that in response to local hyperthermia, a multiple increase in the pulsation component, which has a biphasic character, was observed. The amplitude of the first phase of the perfusion reaction depends on both the initial skin temperature and the difference between the basal and heating temperatures. The proposed method allows the assessment of a reproducible perfusion increase in response to hyperthermia developed due to humoral factors associated with the endothelium, thus allowing detection of its dysfunction.

The effects of exercise training in the cold on cerebral blood flow and cerebrovascular function in young healthy individuals

Exercise elicits acute increases in cerebral blood flow velocity (CBFv) and provokes long-term beneficial effects on CBFv, thereby reducing cerebrovascular risk. Acute exposure to a cold stimulus also increases CBFv. We compared the impact of exercise training in cold and thermoneutral environments on CFBv, cerebrovascular function and peripheral endothelial function. Twenty-one (16 males, 22 ± 5 years) individuals were randomly allocated to either a cold (5 °C) or thermoneutral (15 °C) exercise intervention. Exercise consisted of 50-min cycling at 70% heart rate max, three times per week for eight weeks. Transcranial Doppler was used to determine pre and post intervention CBFv, dynamic cerebral autoregulation (dCA) and cerebrovascular reactivity (CVRCO2). Conduit endothelial function, microvascular function and cardiorespiratory fitness were also assessed. Cardiorespiratory fitness improved (2.91 ml.min.kg-1, 95%CI 0.49, 5.3; P = 0.02), regardless of exercise setting. Neither intervention had an impact on CBFv, CVRCO2, FMD or microvascular function (P > 0.05). There was a significant interaction between time and condition for dCA normalised gain with evidence of a decrease by 0.192%cm.s-1.%mmHg-1 (95%CI -0.318, -0.065) following training in the cold and increase (0.129%cm.s-1.%mmHg-1, 95%CI 0.011, 0.248) following training in the thermoneutral environment (P = 0.001). This was also evident for dCA phase with evidence of an increase by 0.072 rad (95%CI -0.007, 0.152) following training in the cold and decrease by 0.065 (95%CI -0.144, 0.014) radians following training in the thermoneutral environment (P = 0.02). Both training interventions improved fitness but CBFv, CVRCO2 and peripheral endothelial function were unaltered. Exercise training in the cold improved dCA whereas thermoneutral negated dCA.

Nitric oxide-mediated cutaneous microvascular function is not altered in young adults following mild-to-moderate SARS CoV-2 infection

Vascular dysfunction has been reported in adults who have recovered from COVID-19. To date, no studies have investigated the underlying mechanisms of persistent COVID-19-associated vascular dysfunction. Our purpose was to quantify nitric oxide (NO)-mediated vasodilation in healthy adults who have recovered from SARS-CoV-2 infection. We hypothesized that COVID-19-recovered adults would have impaired NO-mediated vasodilation compared with adults who have not had COVID-19. In methods, we performed a cross-sectional study including 10 (5 men/5 women, 24 ± 4 yr) healthy control (HC) adults who were unvaccinated for COVID-19, 11 (4 men/7 women, 25 ± 6 yr) healthy vaccinated (HV) adults, and 12 (5 men/7 women, 22 ± 3 yr) post-COVID-19 (PC, 19 ± 14 wk) adults. COVID-19 symptoms severity (survey) was assessed. A standardized 39°C local heating protocol was used to assess NO-dependent vasodilation via perfusion (intradermal microdialysis) of 15 mM NG-nitro-l-arginine methyl ester during the plateau of the heating response. Red blood cell flux was measured (laser-Doppler flowmetry) and cutaneous vascular conductance (CVC = flux/mmHg) was expressed as a percentage of maximum (28 mM sodium nitroprusside + 43°C). In results, the local heating plateau (HC: 61 ± 20%, HV: 60 ± 19%, PC: 67 ± 19%, P = 0.80) and NO-dependent vasodilation (HC: 77 ± 9%, HV: 71 ± 7%, PC: 70 ± 10%, P = 0.36) were not different among groups. Neither symptom severity (25 ± 12 AU) nor time since diagnosis correlated with the NO-dependent vasodilation (r = 0.46, P = 0.13; r = 0.41, P = 0.19, respectively). In conclusion, healthy adults who have had mild-to-moderate COVID-19 do not have altered NO-mediated cutaneous microvascular function.NEW & NOTEWORTHY Healthy young adults who have had mild-to-moderate COVID-19 do not display alterations in nitric oxide-mediated cutaneous microvascular function. In addition, healthy young adults who have COVID-19 antibodies from the COVID-19 vaccinations do not display alterations in nitric oxide-mediated cutaneous microvascular function.

Hydrogen sulfide-dependent microvascular vasodilation is improved following chronic sulfhydryl-donating antihypertensive pharmacotherapy in adults with hypertension

Hypertension is characterized by systemic microvascular endothelial dysfunction, in part due to a functional absence of hydrogen sulfide (H₂S)-mediated endothelium-dependent dilation. Treatment with a sulfhydryl-donating ACE inhibitor (SH-ACE inhibitor) improves endothelial function in preclinical models of hypertension. To date, no studies have directly assessed the effects of SH-ACE-inhibitor treatment on H₂S-dependent vasodilation in humans with hypertension. We hypothesized that SH-ACE-inhibitor treatment would improve H₂S-mediated endothelium-dependent vasodilation. Ten adults with hypertension [1 woman and 9 men; 56 ± 9 yr; systolic blood pressure (SBP): 141 ± 8.5 mmHg; diastolic blood pressure (DBP): 90.3 ± 6 mmHg] were treated (16 wk) with the SH-ACE-inhibitor captopril. Red blood cell flux (laser-Doppler flowmetry) was measured continuously during graded intradermal microdialysis perfusion of the endothelium-dependent agonist acetylcholine (ACh; 10^-10 to 10^-1 M) alone (control) and in combination with an inhibitor of enzymatic H₂S production [10^-3 M aminooxyacetate (AOAA)] preintervention and postintervention. Cutaneous vascular conductance (CVC; flux/mmHg) was calculated and normalized to the site-specific maximal CVC (0.028 M sodium nitroprusside and local heat to 43°C). Area under the curve was calculated using the trapezoid method. The 16-wk SH-ACE-inhibitor treatment resulted in a reduction of blood pressure (systolic BP: 129 ± 10 mmHg; diastolic BP: 81 ± 9 mmHg, both P < 0.05). Preintervention, inhibition of H₂S production had no effect on ACh-induced vasodilation (316 ± 40 control vs. 322 ± 35 AU AOAA; P = 0.82). Captopril treatment improved ACh-induced vasodilation (316 ± 40 pre vs. 399 ± 55 AU post; P = 0.04) and increased the H₂S-dependent component of ACh-induced vasodilation (pre: -6.6 ± 65.1 vs. post: 90.2 ± 148.3 AU, P = 0.04). These data suggest that SH-ACE-inhibitor antihypertensive treatment improves cutaneous microvascular endothelium-dependent vasodilation in adults with hypertension, in part via H₂S-dependent mechanisms.NEW & NOTEWORTHY This is the first study to prospectively assess the effects of sulfhydryl antihypertensive treatment on microvascular endothelial function in adults with hypertension. Our data suggest that 16 wk of SH-ACE-inhibitor antihypertensive treatment improves cutaneous microvascular endothelium-dependent vasodilation in middle-aged adults with hypertension, in part via H₂S-dependent mechanisms.

Regional cutaneous vasodilator responses to rapid and gradual local heating in young adults

PURPOSE

To examine the extent of regional variations in cutaneous vasodilatation during rapid and gradual local thermal hyperaemia (LTH) in young adults.

METHODS

In thirty young adults (21 ± 3 years, 15 females), cutaneous vascular conductance, normalized to maximum local skin heating at 44 °C (%CVC_max), was assessed at the upper chest, abdomen, dorsal arm, dorsal forearm, thigh, and medial calf during rapid (33-42 °C at 1 °C·20 s^-1) and gradual (33-42 °C at 1 °C·5 min^-1) LTH on separate days. For both protocols, local temperatures were held at 42 °C for up to 35 min, followed by 20-30 min at 44 °C. During rapid LTH, between-region responses were evaluated at baseline, the initial vasodilator peak, and 42 °C plateau. During gradual LTH, responses were assessed at baseline and the 42 °C plateau.

RESULTS

There were significant main effects of body region on %CVC_max for the initial peak and plateau during rapid LTH and for the plateau during gradual LTH (all P < 0.001) Conversely, main effects of sex and the sex by region interaction were not significant (all P > 0.05). The magnitudes of between-region differences varied across the body (~1-17% range). The greatest effects were observed for the abdomen, wherein responses were consistently lower compared to other regions. Further, responses were consistent between males and females across all body regions and heating phases.

CONCLUSION

Regional variations in the cutaneous vasodilator response to local heating are evident for rapid and gradual LTH in young adults, with the largest effects observed for the abdomen, albeit regional differences were similar between sexes.

Regional variation in nitric oxide-dependent cutaneous vasodilatation during local heating in young adults

NEW FINDINGS

What is the central question of this study? Are regional differences in nitric oxide (NO)-dependent cutaneous vasodilatation during local skin heating present in young adults? What is the main finding and its importance? NO-dependent cutaneous vasodilatation varied across the body. The abdomen demonstrated larger NO contributions, while the chest demonstrated smaller NO contributions, compared to other regions. This exploratory work is an important first step in characterizing regional heterogeneity of cutaneous microvascular control across the torso and limbs. Equally, it serves to generate hypotheses for future studies examining regional cutaneous microvascular control in ageing and disease.

ABSTRACT

Regional variations in cutaneous vasodilatation during local skin heating exist across the body. While nitric oxide (NO) is a well-known modulator of this response, the extent of regional differences in NO-dependent cutaneous vasodilatation during local skin heating remains uncertain. In 16 habitually active young adults (8 females; 25 ± 5 years), cutaneous vascular conductance, normalized to maximum vasodilatation (% CVC_max ), was assessed at the upper chest, abdomen, dorsal forearm, thigh and lateral calf during local skin heating. Across all regions, local skin temperatures were simultaneously increased from 33 to 42°C (1°C per 10 s), and held until a stable heating plateau was achieved (∼40 min). Next, with local skin temperature maintained at 42°C, 20 mM of N^G -nitro-l-arginine methyl ester (l-NAME) was continuously infused at each site until a stable l-NAME plateau was achieved (∼40 min). The difference between heating and l-NAME plateaus was identified as the NO contribution for each region. There was no evidence for region-specific responses at baseline (P = 0.561), the heating plateau (P = 0.351) or l-NAME plateau (P = 0.082), but there was for the NO contribution (P = 0.048). Overall, point estimates for between-region differences in the NO contribution varied across the body from 0 to 19% CVC_max . The greatest effects were observed for the abdomen, wherein the NO contribution was consistently greater than for the other regions (range: 9-19% CVC_max ). The chest was consistently lower than the other regions (range: 7-19% CVC_max ). The smallest effects were observed between limb regions (range: 0-2% CVC_max ). These findings advance our understanding of the mechanisms influencing regional variations in the cutaneous vasodilator response to local skin heating in young adults.

Effect of sinusoidal leg cycling exercise period on brachial artery blood flow dynamics in humans

Purpose

To quantify the dynamics of blood flow in brachial artery (BF-BA) in response to sinusoidal work rate (WR) leg cycling exercises of 2-, 4-, and 6-min periods and to examine their relationship with the forearm skin blood flow (SBF).

Methods

Seven healthy young male subjects performed upright leg ergometer exercise with a constant WR (mean sinusoidal WR) for 30 min followed by sinusoidal WR exercise of three different periods (number of repetitions): 2 min (7), 4 min (4), and 6 min (3). The WR fluctuated from 20 W to a peak WR corresponding to 60% peak oxygen uptake (VO₂). We continuously measured pulmonary gas exchange, heart rate (HR), blood velocity and cross-sectional area of BA, and forearm SBF and sweating rate (SR).

Results

All variables were followed by the sinusoidal WR. The phases of the variables for gas exchange and central circulation, such as VO₂ and HR with WR forcing were similar (e.g., phase shift (θ) in HR [°]: 2 min, 60 ± 7; 4 min, 45 ± 10; 6 min, 37 ± 8; mean ± SD) to previous study results, that is, a longer period showed a shorter θ and larger amplitude of responses. Contrarily, the BF-BA response showed anti-phase (approximately 180°) regardless of the period, whereas the θ of forearm SBF and SR were similar to gas exchange and central circulation.

Conclusions

Inactive limb BF-BA during sinusoidal leg cycling exercise was out of phase relative to the regulation of O₂-delivery to active muscles and thermoregulation. The response of BF-BA seems to not always reflect the response of forearm SBF in the downstream area.

Novel Noninvasive Assessment of Microvascular Structure and Function in Humans

INTRODUCTION

Optical coherence tomography (OCT) is a novel high-resolution imaging technique capable of visualizing in vivo structures at a resolution of ~10 μm. We have developed specialized OCT-based approaches that quantify diameter, speed, and flow rate in human cutaneous microvessels. In this study, we hypothesized that OCT-based microvascular assessments would possess comparable levels of reliability when compared with those derived using conventional laser Doppler flowmetry (LDF).

METHODS

Speckle decorrelation images (OCT) and red blood cell flux (LDF) measures were collected from adjacent forearm skin locations on 2 d (48 h apart), at baseline, and after a 30-min rapid local heating protocol (30°C-44°C) in eight healthy young individuals. OCT postprocessing quantified cutaneous microvascular diameter, speed, flow rate, and density (vessel recruitment) within a region of interest, and data were compared between days.

RESULTS

Forearm skin LDF (13 ± 4 to 182 ± 31 AU, P < 0.05) and OCT-derived diameter (41.8 ± 6.6 vs 64.5 ± 6.9 μm), speed (68.4 ± 9.5 vs 89.0 ± 7.3 μm·s), flow rate (145.0 ± 60.6 vs 485 ± 132 pL·s), and density (9.9% ± 4.9% vs 45.4% ± 5.9%) increased in response to local heating. The average OCT-derived microvascular flow response (pL·s) to heating (234% increase) was lower (P < 0.05) than the LDF-derived change (AU) (1360% increase). Pearson correlation was significant for between-day local heating responses in terms of OCT flow (r = 0.93, P < 0.01), but not LDF (P = 0.49). Bland-Altman analysis revealed that between-day baseline OCT-derived flow rates were less variable than LDF-derived flux.

CONCLUSIONS

Our findings indicate that OCT, which directly visualizes human microvessels, not only allows microvascular quantification of diameter, speed, flow rate, and vessel recruitment but also provides outputs that are highly reproducible. OCT is a promising novel approach that enables a comprehensive assessment of cutaneous microvascular structure and function in humans.

Historical reviews of the assessment of human cardiovascular function: interrogation and understanding of the control of skin blood flow

Several techniques exist for the determination of skin blood flow that have historically been used in the investigation of thermoregulatory control of skin blood flow, and more recently, in clinical assessments or as an index of global vascular function. Skin blood flow measurement techniques differ in their methodology and their strengths and limitations. To examine the historical development of techniques for assessing skin blood flow by describing the origin, basic principles, and important aspects of each procedure and to provide recommendations for best practise. Venous occlusion plethysmography was one of the earliest techniques to intermittently index a limb’s skin blood flow under conditions in which local muscle blood flow does not change. The introduction of laser Doppler flowmetry provided a method that continuously records an index of skin blood flow (red cell flux) (albeit from a relatively small skin area) that requires normalisation due to high site-to-site variability. The subsequent development of laser Doppler and laser speckle imaging techniques allows the mapping of skin blood flow from larger surface areas and the visualisation of capillary filling from the dermal plexus in two dimensions. The use of iontophoresis or intradermal microdialysis in conjunction with laser Doppler methods allows for the local delivery of pharmacological agents to interrogate the local and neural control of skin blood flow. The recent development of optical coherence tomography promises further advances in assessment of the skin circulation via three-dimensional imaging of the skin microvasculature for quantification of vessel diameter and vessel recruitment.

Is core temperature the trigger of a menopausal hot flush?

OBJECTIVE

Menopausal hot flushes negatively impact quality of life and may be a biomarker of cardiovascular and metabolic disease risk; therefore understanding the physiology of hot flushes is important. Current thinking is that a small elevation (∼0.03-0.05C) in core temperature surpasses a sweating threshold (that is reduced in the menopause), sweating is activated, and a hot flush ensues. Nevertheless, more recent studies examining thermoregulatory control question whether core temperature per se can explain the trigger for a hot flush. The primary aim of this study was to assess the contribution of increases in core temperature on the occurrence of menopausal hot flushes.

METHODS

For this purpose, 108 hot flushes were objectively assessed in a laboratory setting in 72 symptomatic postmenopausal women (aged 45.8 ± 5.1 years; body mass index 25.9 ± 4.5 kg/m) from five previously reported studies. Women rested, wearing a tube-lined suit (or trousers), which was perfused with 34C water. A subset then underwent mild heat stress (48°C water). Sweat rate, skin blood flow, blood pressure, heart rate, skin, and core temperature were measured continuously throughout. A hot flush was objectively identified during rest (spontaneous hot flush) or mild heating as an abrupt increase in sternal sweat rate. Further, a subset of symptomatic postmenopausal women (n = 22) underwent whole-body passive heating for 60 minutes to identify core temperature thresholds and sensitivities for sweat rate and cutaneous vasodilation, which were compared to a subset of premenopausal women (n = 18). Data were analyzed using t tests and/or general linear modeling, and are presented as mean (95% confidence interval).

RESULTS

In the 20 minutes before a spontaneous hot flush, core temperature increased by 0.03 ± 0.12C (P < 0.05), but only 51% of hot flushes were preceded by an increase in core temperature. During mild heating, 76% of hot flushes were preceded by an increase in core temperature. The temperature thresholds for sweating were similar, but the vasodilatory threshold was higher in postmenopausal compared with premenopausal women (37.1 ± 0.2 vs 36.8 ± 0.3°C; P = 0.06).

CONCLUSION

We provide new evidence that menopausal hot flushes are unlikely triggered by an increase in core temperature. These findings provide important information about the physiology of hot flushes that have implications for treatment and management options for menopausal hot flushes.

Spectral analysis of the blood flow in the foot microvascular bed during thermal testing in patients with diabetes mellitus

Timely diagnostics of microcirculatory system abnormalities, which are the most severe diabetic complications, is one of the major problems facing modern health care. Functional abnormalities manifest themselves earlier than the structural ones, and therefore their assessment is the issue of primary importance. In this study Laser Doppler flowmetry, a noninvasive technique for the cutaneous blood flow monitoring, was utilized together with local temperature tests and wavelet analysis. The study of the blood flow in the microvascular bed of toes was carried out in the control group of 40 healthy subjects and in two groups of 17 type 1 and 23 type 2 diabetic patients. The local temperature tests demonstrated that the diabetic patients have impaired vasodilation in response to local heating. The tendency for impaired low frequency pulsations of the blood flow associated with endothelial and neurogenic activities in both diabetes groups was observed. Local thermal tests induced variations in perfusion and its spectral characteristics, which were different in the groups under study. In our opinion, the obtained preliminary results can be a basis for further research and provide a deeper understanding of pathological processes that drive microvascular abnormalities caused by diabetes mellitus.