Noninvasive examination of endothelial, sympathetic, and myogenic contributions to regional differences in the human cutaneous microcirculation

Wireless Dynamic Light Scattering Sensors Detect Microvascular Changes Associated With Ageing and Diabetes

This article presents clinical results of wireless portable dynamic light scattering sensors that implement laser Doppler flowmetry signal processing. It has been verified that the technology can detect microvascular changes associated with diabetes and ageing in volunteers. Studies were conducted primarily on wrist skin. Wavelet continuous spectrum calculation was used to analyse the obtained time series of blood perfusion recordings with respect to the main physiological frequency ranges of vasomotions. In patients with type 2 diabetes, the area under the continuous wavelet spectrum in the endothelial, neurogenic, myogenic, and cardio frequency ranges showed significant diagnostic value for the identification of microvascular changes. Aside from spectral analysis, autocorrelation parameters were also calculated for microcirculatory blood flow oscillations. The groups of elderly volunteers and patients with type 2 diabetes, in comparison with the control group of younger healthy volunteers, showed a statistically significant decrease of the normalised autocorrelation function in time scales up to 10 s. A set of identified parameters was used to test machine learning algorithms to classify the studied groups of young controls, elderly controls, and diabetic patients. Our conclusion describes and discusses the classification metrics that were found to be most effective.

Effect of Low- and High-Frequency Auricular Stimulation with Electro-Acupuncture on Cutaneous Microcirculation: A Cross-Over Study in Healthy Subjects

Background: The regulation of microcirculation depends on the dynamic interaction of different factors: the autonomic nervous system plays a pivotal role in the blood flow and acupuncture can modulate it, obtaining different results depending on the site, the frequency, and the intensity of the stimulation. Methods: 18 healthy subjects have been enrolled and have undergone two sessions of electroacupuncture stimulations: one session using high frequency and one with low frequency. Microcirculation has been monitored continuously during stimulation using the laser Doppler method. Results: The microcirculatory parameters have shown a significant difference between high and low-frequency stimulation, suggesting that low-frequency stimulation is more effective for obtaining a vasodilator effect. Discussion: Our results show that low-frequency stimulation can increase the cutaneous microcirculatory flux, without significantly modifying blood pressure and heart rate. The auricular stimulation causes an increase in the activity of the vagus nerve, increasing the cholinergic activity without acting on post-junctional muscarinic receptors. Conclusion: Auricular acupuncture has a significant impact on the regulation of microcirculation.

Wavelet analysis of laser Doppler microcirculatory signals: Current applications and limitations

Laser Doppler flowmetry (LDF) has long been considered a gold standard for non-invasive assessment of skin microvascular function. Due to the laser Doppler (LD) microcirculatory signal's complex biological and physiological context, using spectral analysis is advisable to extract as many of the signal's properties as feasible. Spectral analysis can be performed using either a classical Fourier transform (FT) technique, which has the disadvantage of not being able to localize a signal in time, or wavelet analysis (WA), which provides both the time and frequency localization of the inspected signal. So far, WA of LD microcirculatory signals has revealed five characteristic frequency intervals, ranging from 0.005 to 2 Hz, each of which being related to a specific physiological influence modulating skin microcirculatory response, providing for a more thorough analysis of the signals measured in healthy and diseased individuals. Even though WA is a valuable tool for analyzing and evaluating LDF-measured microcirculatory signals, limitations remain, resulting in a lack of analytical standardization. As a more accurate assessment of human skin microcirculation may better enhance the prognosis of diseases marked by microvascular dysfunction, searching for improvements to the WA method is crucial from the clinical point of view. Accordingly, we have summarized and discussed WA application and its limitations when evaluating LD microcirculatory signals, and presented insight into possible future improvements. We adopted a novel strategy when presenting the findings of recent studies using WA by focusing on frequency intervals to contrast the findings of the various studies undertaken thus far and highlight their disparities.

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.

Body Position Affects Capillary Blood Flow Regulation Measured with Wearable Blood Flow Sensors

In this study we demonstrate what kind of relative alterations can be expected in average perfusion and blood flow oscillations during postural changes being measured in the skin of limbs and on the brow of the forehead by wearable laser Doppler flowmetry (LDF) sensors. The aims of the study were to evaluate the dynamics of cutaneous blood perfusion and the regulatory mechanisms of blood microcirculation in the areas of interest, and evaluate the possible significance of those effects for the diagnostics based on blood perfusion monitoring. The study involved 10 conditionally healthy volunteers (44 ± 12 years). Wearable laser Doppler flowmetry monitors were fixed at six points on the body: two devices were fixed on the forehead, on the brow; two were on the distal thirds of the right and left forearms; and two were on the distal thirds of the right and left lower legs. The protocol was used to record three body positions on the tilt table for orthostatic test for each volunteer in the following sequence: (a) supine body position; (b) upright body position (+75°); (c) tilted with the feet elevated above the head and the inclination of body axis of 15° (−15°, Trendelenburg position). Skin blood perfusion was recorded for 10 min in each body position, followed by the amplitude–frequency analysis of the registered signals using wavelet decomposition. The measurements were supplemented with the blood pressure and heart rate for every body position analysed. The results identified a statistically significant transformation in microcirculation parameters of the average level of skin blood perfusion and oscillations of amplitudes of neurogenic, myogenic and cardiac sensors caused by the postural changes. In paper, we present the analysis of microcirculation in the skin of the forehead, which for the first time was carried out in various positions of the body. The area is supplied by the internal carotid artery system and can be of particular interest for evaluation of the sufficiency of blood supply for the brain.

Effect of orthostasis on the regulation of skin blood flow in upper and lower extremities in human

OBJECTIVE

The research is aimed to investigate interactions between cardiovascular signals and to assess contributions of central and local mechanisms to skin blood flow regulation in upper and lower extremities at rest and under orthostasis.

METHODS

Heart rate variability, respiration, forearm, and foot skin blood flow were assessed at rest and during postural test in 25 healthy volunteers. Spectral analysis was performed. Phase synchronization degree of analyzed signals was determined by group phase wavelet coherence function.

RESULTS

Skin blood flow was lower on foot at rest and during postural test than on forearm. High-frequency component of heart rate variability was higher at ~0.3 Hz during postural test versus rest. Blood flow oscillation amplitudes on the foot were lower in frequency range including respiratory interval at rest than on forearm. Postural exposure increased amplitude of foot blood flow oscillations in respiratory interval and decreased amplitudes in cardiac interval versus rest. Orthostasis increased group wavelet phase coherence between foot blood flow and heart rate variability or respiration, as well as between forearm and foot blood flow at 0.3 Hz corresponding to respiration.

CONCLUSIONS

The contribution of central mechanisms associated with respiration to blood flow regulation increased in lower extremities during orthostasis.

Stable laser-Doppler flow-motion patterns in the human cutaneous microcirculation: Implications for prospective geroscience studies

Microvascular function has been assessed by determining the rhythmic oscillations in blood flow induced by the vasomotion of resistance vessels. Although laser-Doppler flowmetry (LDF) allows simple, non-invasive evaluation of this flow-motion in the cutaneous microcirculation, the temporal and spatial reproducibility of such assessments remains unclear.

In the present study, we investigated cutaneous flow-motion in three consecutive years in eight skin regions using LDF in six healthy young volunteers. The characteristic flow-motion frequency was determined using fast-Fourier transformation. Additionally, in two years a more traditional measure of microvascular reactivity, postocclusive reactive hyperemia (PORH) was evoked in the forearm after transient brachial artery occlusion (1–2–3 min) induced by cuff inflation.

Well-defined flow-motion was found in six regions showing significant differences in frequency: the highest flow-motion frequency was found in the frontal and temporal regions (8.0 ± 1.1 and 8.5 ± 1.0 cycles/min, cpm, respectively, mean ± SD) followed by the scapular, infraclavicular and coxal regions (7.5 ± 1.3; 6.7 ± 1.1 and 6.5 ± 1.2 cpm, respectively). The lowest, stable flow-motion was found in the posterior femoral region (5.5 ± 1.0 cpm), whereas flow-motion was detectable only sporadically in the limbs. The region-dependent flow-motion frequencies were very stable within individuals either between the body sides, or among the three measurements, only the infraclavicular region showed a small difference (114 ± 17%∗, % of value in 1st year; ∗P < 0.05). However, PORH indices differed after 2–3 min occlusions significantly in consecutive years.

We report that flow-motion frequencies determined from LDF signals show both region-specificity and excellent intra-individual temporal and spatial reproducibility suggesting their usefulness for non-invasive follow-up of microvascular reactivity.

Noninvasive assessment of increases in microvascular endothelial function following repeated bouts of hyperaemia

OBJECTIVE

Spectral analyses of laser-Doppler signal can delineate underlying mechanisms in response to pharmacological agents and in cross-sectional studies of healthy and clinical populations. We tested whether spectral analyses can detect acute changes in endothelial function in response to a 6-week intervention of repeated bouts of hyperaemia.

METHODS

Eleven males performed forearm occlusion (5 s with 10 s rest) for 30 min, 5 times/week for 6 weeks on one arm; the other was an untreated control. Skin blood flow was measured using laser-Doppler fluxmetry (LDF), and endothelial function was assessed with and without nitric oxide (NO) synthase-inhibition with L-NAME in response to local heating (42 °C and 44 °C) and acetylcholine. A wavelet transform was used for spectral analysis of frequency intervals associated with physiological functions.

RESULTS

Basal measures were all unaffected by the hyperaemia intervention (all P > 0.05). In response to local skin heating to 42 °C, the 6 weeks hyperaemia intervention increased LDF, endothelial NO-independent and NO-dependent activity (all P ≤ 0.038). In response to peak local heating (44 °C) endothelial NO-independent and NO-dependent activity increased (both P ≤ 0.01); however, LDF did not (P > 0.2). In response to acetylcholine, LDF, endothelial NO-independent and NO-dependent activity all increased (all P ≤ 0.003) post-intervention.

CONCLUSIONS

Spectral analysis appears sufficiently sensitive to measure changes over time in cutaneous endothelial activity that are consistent with standard physiological (local heating) and pharmacological (acetylcholine) interventions of assessing cutaneous endothelial function, and may be useful not only in research but also clinical diagnosis and treatment.

A new approach to the analysis of skin blood flow oscillations in human

It is considered that there are six non-overlapping frequency intervals with constant boundaries. These intervals correspond to different mechanisms of skin blood flow regulation. The boundaries do not depend on functional conditions but this statement should be verified. Also it remains unclear how the oscillatory components of skin blood flow are related. Thus the study is aimed to verify statistically the boundaries of frequency intervals, to test the hypothesis that the boundaries depend on age and to search for relationships between spectral components of skin blood flow. The study involved 105 healthy volunteers aged from 20 to 65 years, which were divided into two age groups. Skin blood flow was registered with laser Doppler flowmeter (LDF). Assessments of frequency interval boundaries and relationship between the frequency oscillatory components of blood flow were conducted with histogram approach, bootstrap method and correlation analysis. New frequency interval boundaries were found. They were different in two groups. A linear correlation and frequency areas with moderate (0.5-0.7) and high (>0.7) correlation coefficients were found between spectral components of blood flow. The dependence of these correlations on the age was shown. Thus we proposed a conceptually new approach to analysis of spectral components of skin microhemodynamics and interpretation of results obtained by laser Doppler techniques. This approach is the result of the development of modern understanding of relationships between skin blood flow regulation mechanisms and spectral components of LDF signals. It allows one to have a new look at these relationships as well as demonstrates their dependence on the functional state of the organism as a whole.

The reliability of cutaneous low-frequency oscillations in young healthy males

OBJECTIVE

Spectral analyses of laser-Doppler flowmetry measures enable a simple and non-invasive method to investigate mechanisms regulating skin blood flow. We assessed within-day and day-to-day variability of cutaneous spectral analyses.

METHODS

Eleven young, healthy males were tested twice in three identical sessions, with 19 to 24 days between visits, for a total of six tests. Wavelet data were analyzed at rest, in response to local skin heating to 42 and 44°C, and during 5-minutes PORH. We did this for six frequency bands commonly associated with physiological functions. To assess reliability, we calculated CV and ICC scores.

RESULTS

At rest, mean CV for the wavelet data ranged from 21% to 24% and ICC scores ranged from 0.67 to 0.91. During local heating, mean CV scores ranged from 17% to 22% and mean ICC scores ranged from 0.71 to 0.95. For peak PORH, CV ranged from 14% to 23% and the ICC range was 0.88 to 0.97. For the area under the curve of the PORH, CV range was 12% to 21% and ICC range was 0.81 to 0.92.

CONCLUSIONS

These analyses indicate good-to-excellent reliability of the wavelet data in healthy young males.

Cutaneous vasomotor responses in boys and men

Few studies have investigated skin blood flow in children and age-related differences in the underlying mechanisms. We examined mechanisms of skin blood flow responses to local heating, postocclusive reactive hyperaemia (PORH), and isometric handgrip exercise in adult and prepubescent males, hypothesizing that skin blood flow responses would be greater in children compared with adults. We measured skin blood flow in 12 boys (age, 9 ± 1 years) and 12 men (age, 21 ± 1 years) using laser-Doppler flowmetry at rest, in response to 3-min PORH, 2-min isometric handgrip exercise, and local skin heating to 39 °C (submaximal) and 44 °C (maximal). Using wavelet analysis we assessed endothelial, neural, and myogenic activities. At rest and in response to local heating to 39 °C, children had higher skin blood flow and endothelial activity compared with men (d ≥ 1.1, p < 0.001) and similar neurogenic and myogenic activities (d < 0.2, p > 0.05). Maximal responses to 44 °C local skin heating, PORH, and isometric handgrip exercise did not differ between boys and men (all d ≤ 0.2, p > 0.05). During PORH children demonstrated greater endothelial activity compared with men (d ≥ 0.6, p < 0.05); in contrast, men had higher neurogenic activity (d = 1.0, p < 0.01). During isometric handgrip exercise there were no differences in endothelial, neurogenic, and myogenic activities (d < 0.2, p > 0.3), with boys and men demonstrating similar increases in endothelial activity and decreases in myogenic activity (d ≥ 0.8, p < 0.05). These data suggest that boys experience greater levels of skin blood flow at rest and in response to submaximal local heating compared with men, while maximal responses appear to be similar. Additionally, endothelial mediators seem to contribute more to vasodilatation in boys than in men.

A comparison of the cutaneous microvascular properties of the Spontaneously Hypertensive and the Wistar-Kyoto rats by Spectral analysis of Laser Doppler

This work was aimed to study skin blood perfusion, vasomotion and vascular responses of the Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) in different stages of age using spectral. Laser-Doppler flowmetry (LDF) was used to examine the ears and limbs of WKY (12 and 48 weeks old) and SHR (12 and 48 weeks old). The skin blood flow oscillations (SBFOs) were studied by wavelet spectral analysis of LDF tracings. Then, we observed that old groups showed decreased perfusion and SBFO in the ears of both SHR and WKY. The SHR showed obviously lower postocclusive reactive hypera (PORH) ratio at the same age. A decreased peak-time occurred in the SHR of old age group. After PORH test, a statistically significant increase was observed within all subintervals in the absolute amplitude of 12-week WKY and only within IV and III subintervals in the absolute amplitude of 12-week SHR. But, the absolute amplitude of 48-week WKY and SHR showed no statistically significant increase within all subintervals. Results indicated that local regulating function of peripheral vascular was impaired in rat with hypertension and aging. Abbreviations LDF: Laser-Doppler flowmetry; SBF: Skin blood flow; SBFO: Skin blood flow oscillation; PORH: Postocclusive reactive hyperemia; SHR: Spontaneously hypertensive rats; WKY: Wistar-Kyoto rats; LDF: Laser-Doppler flowmetry; LDI: Laser Doppler Imaging; BP: Blood pressure.

Bridging the gap between basic research on microcirculation and clinical world: The translational marriage between engineering and medicine

The words "Translational" and "Medicine" have been recently coupled to indicate a combination of disciplines, resources, expertise, and techniques aimed at enhancing prevention, diagnosis, and therapies. As stated in 2015 by the European Society for Translational Medicine, translational medicine is "an interdisciplinary branch of the biomedical field supported by three main pillars: benchside, bedside and community". By definition, Translational Medicine is a highly interdisciplinary field, which gathers several specialties aimed at improving the global healthcare system.With regard to the assessment of the microcirculatory function, it is worthwhile to mention the growing interest from both basic research and clinical practice. Microcirculation is where the exchange of substances between blood and tissues takes place. Thus, it plays a key role in the pathophysiology of many diseases. Nonetheless, a gap does exist between the theoretical analysis of the microcirculatory function and its clinical exploitation. This gap can be due to the weak dissemination of analytical methods and theoretical results within the clinical community, which also delays the establishment of specific operative guidelines.This paper aims at encouraging, and possibly accelerating, the translation of basic research outcomes on microcirculatory function assessment into clinical applications.

Anesthesia effects on the low frequency blood flow oscillations in mouse skin

BACKGROUND

When laboratory animals are used one needs to anesthetize them before recording. However, the influence of anesthesia on animal blood flow oscillations has not been studied. The effects of two ways of anesthesia, zoletil-xylazine, and zoletil-nitrous oxide mixtures, on mouse skin perfusion using laser Doppler flowmetry (LDF) technique were studied.

METHODS

BALB/c mice were used. LDF probe was placed on the ventral surface of the left hind paw. Spectral analysis of LDF signals was performed with continuous adaptive wavelet transform to identify and describe peripheral blood flow oscillations in mouse skin.

RESULTS

Low-frequency oscillation interval boundaries (myogenic, neurogenic, and endothelial) for mice were shown to coincide with the boundaries determined for human and rats, that demonstrate their independence from the body size. Zoletil-xylazine anesthesia significantly decreased neurogenic and endothelial oscillation amplitudes by 29% and 50% respectively and increased the amplitude of cardiac oscillations by 23% compared to zoletyl-nitrous oxide anesthesia. There were no significant changes of the amplitudes of myogenic and respiratory oscillations with zoletil-nitrous oxide anesthesia compared to the zoletil-xylazine mixture.

CONCLUSION

We suggest that the different influence of anesthesia modes on the amplitudes of skin blood flow oscillations is associated with sympathetic activity suppressed by zoletil-xylazine anesthesia.

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.

The role of shear stress on cutaneous microvascular endothelial function in humans

PURPOSE

Previous studies suggest that exercise and heat stress improve cutaneous endothelial function, caused by increases in shear stress. However, as vasodilatation in the skin is primarily a thermogenic phenomenon, we investigated if shear stress alone without increases in skin temperature that occur with exercise and heat stress increases endothelial function. We examined the hypothesis that repeated bouts of brief occlusion would improve cutaneous endothelial function via shear stress-dependent mechanisms.

METHODS

Eleven males underwent a shear stress intervention (forearm occlusion 5 s rest 10 s) for 30 min, five times·week^-1 for 6 weeks on one arm, the other was an untreated control. Skin blood flow was measured using laser-Doppler flowmetry, and endothelial function was assessed with and without NOS-inhibition with L-NAME in response to three levels of local heating (39, 42, and 44 °C), ACh administration, and reactive hyperaemia. Data are cutaneous vascular conductance (CVC, laser-Doppler/blood pressure).

RESULTS

There were no changes in the control arm (all d ≤ 0.2, p > 0.05). In the experimental arm, CVC to 39 °C was increased after 3 and 6 weeks (d = 0.6; p ≤ 0.01). Nitric oxide contribution was increased after 6 weeks compared to baseline (d = 0.85, p < 0.001). Following skin heating to 42 °C and 44 °C, CVC was not different at weeks 3 or 6 (d ≤ 0.8, p > 0.05). For both 42 and 44 °C, nitric oxide contribution was increased after weeks 3 and 6 (d ≥ 0.4, p < 0.03). Peak and area-under-the-curve responses to ACh increased following 6 weeks (p < 0.001).

CONCLUSIONS

Episodic increases in shear stress, without changes in skin or core temperature, elicit an increase in cutaneous microvascular reactivity and endothelial function.

Spectral analysis of reflex cutaneous vasodilatation during passive heat stress

Previous work has demonstrated that spectral analysis is a useful tool to non-invasively ascertain the mechanisms of control of the cutaneous circulation. The majority of work using spectral analysis has focused on local control mechanisms, with none examining reflex control. Skin blood flow was analysed using spectral analysis on the dorsal aspect of the forearm of 7 males and 7 females during passive heat stress, with mean forearm and local temperature at the site of measurement maintained at thermoneutral (33°C) to minimize the effect of local control mechanisms. Participants were passively heated to ~1.2±0.1°C above baseline rectal temperature (d=4.0, P<0.001) using a water-perfused, tube lined suit, with skin blood flow assessed using a laser-Doppler probe with an integrated temperature monitor. Spectral analysis was performed using a Morlet wavelet on the entire data set, with median power extracted during 20min of data during baseline (normothermia) and hyperthermia. Passive heat stress significantly increased laser-Doppler flux above baseline (d=4.7, P<0.001). Spectral power of the endothelial nitric oxide-independent (0.005-0.01Hz; d=1.1, P=0.004), neurogenic (0.2-0.05Hz; d=0.6, P=0.025), myogenic (0.05-0.15Hz; d=1.5, P=0.002), respiratory (0.15-0.4Hz; d=1.4 P=0.002), and cardiac (0.4-2.0Hz; d=1.1, P=0.012) frequency intervals increased with passive heat stress. In contrast, the endothelial nitric oxide-dependent frequency interval did not change (0.01-0.02Hz; d=0.3, P=0.09) with passive heat stress. These data suggest that cutaneous reflex vasodilatation is neurogenic in origin and not mediated by endothelial-nitric oxide synthase, and are congruent with invasive examinations of reflex cutaneous vasodilatation.

The effect of heating rate on the cutaneous vasomotion responses of forearm and leg skin in humans

We examined skin blood flow (SkBF) and vasomotion in the forearm and leg using laser-Doppler fluxmetry (LDF) and spectral analysis to investigate endothelial, sympathetic, and myogenic activities in response to slow (0.1 °C·10 s(-1)) and fast (0.5 °C·10 s(-1)) local heating. At 33 °C (thermoneutral) endothelial activity was higher in the legs than the forearms (P ≤ 0.02). Fast-heating increased SkBF more than slow heating (P=0.037 forearm; P=0.002 leg). At onset of 42 °C, endothelial (P=0.043 forearm; P=0.48 leg) activity increased in both regions during the fast-heating protocol. Following prolonged heating (42 °C) endothelial activity was higher in both the forearm (P=0.002) and leg (P<0.001) following fast-heating. These results confirm regional differences in the response to local heating and suggest that the greater increase in SkBF in response to fast local heating is initially due to increased endothelial and sympathetic activity. Furthermore, with sustained local skin heating, greater vasodilatation was observed with fast heating compared to slow heating. These data indicate that this difference is due to greater endothelial activity following fast heating compared to slow heating, suggesting that the rate of skin heating may alter the mechanisms contributing to cutaneous vasodilatation.

Shoulder skin and muscle hemodynamics during backpack carriage

The purpose of this study was to quantify the effects of loaded backpacks on shoulder muscle oxygenation, skin blood flow, and pain. We hypothesized that backpack load carriage is associated with lower shoulder muscle oxygenation and skin microvascular flow. Near-infrared spectroscopy quantified shoulder tissue oxygenation and laser Doppler flow measured skin microvascular flow. Eight adult volunteers donned a standard backpack without added load, 5 kg load, and 10 kg load for 5 min while standing. An 8 min rest period before each backpack donning condition ensured that all measured parameters returned to baseline. Data were analyzed using a repeated measures ANOVA and significance set at p < 0.05. Donning a 10 kg backpack significantly reduced shoulder muscle oxygenation by 22 ± 23% as compared to the empty backpack control condition (p = 0.023). In addition, a 10 kg backpack load reduced skin microvascular flow by 82 ± 22%, as compared to the empty backpack control condition (p = 0.024). Perceived pain was significantly higher when wearing the 10 kg backpack (level 4 on a 10-maximal pain scale) as compared to the empty backpack (0, 0-no pain) (p < 0.05). In conclusion, backpack loads of 10 kg decrease shoulder muscle oxygenation and skin microvascular flow.

Accessing to arteriovenous blood flow dynamics response using combined laser speckle contrast imaging and skin optical clearing

Laser speckle contrast imaging (LSCI) shows a great potential for monitoring blood flow, but the spatial resolution suffers from the scattering of tissue. Here, we demonstrate the capability of a combination method of LSCI and skin optical clearing to describe in detail the dynamic response of cutaneous vasculature to vasoactive noradrenaline injection. Moreover, the superior resolution, contrast and sensitivity make it possible to rebuild arteries-veins separation and quantitatively assess the blood flow dynamical changes in terms of flow velocity and vascular diameter at single artery or vein level.

The contribution of sensory nerves to cutaneous vasodilatation of the forearm and leg to local skin heating

PURPOSE

The initial cutaneous vasodilatory response to local skin heating is larger in the forearm than the leg. While the initial vasodilatation of the forearm to local heating is primarily dependent on sensory nerves, their role in the leg is unknown. We compared the contribution of sensory nerves in driving the cutaneous vasodilatory response of the forearm and leg to local heating using local anaesthetic (EMLA) cream.

METHOD

In seven participants, two skin sites were selected on both the dorsal forearm and anterolateral calf; one site on each region received EMLA, with the other an untreated control. All sites were controlled at 33 °C and then locally heated to 42 °C with integrated laser-Doppler local heating probes.

RESULTS

Cutaneous vascular conductance (CVC) during the initial vasodilatation to local heating was smaller in the leg (47 ± 9% max) compared to the forearm (62 ± 7 % max) (P = 0.012). EMLA reduced the initial vasodilatation at both the leg (27 ± 13 % max) (P = 0.02) and forearm (33 ± 14% max) (P < 0.001). The times to onset of vasodilatation, initial vasodilatory peak, and plateau phase were longer in the leg compared to the forearm (all P < 0.05), and EMLA increased these times in both regions (both P < 0.05). CVC during the plateau phase to sustained local skin heating was higher in the leg compared to the forearm at both the untreated (93 ± 6 vs. 85 ± 4% max) (P = 0.33) and EMLA-treated (94 ± 5 vs. 86 ± 6% max) (P = 0.001) sites; EMLA did not affect CVC (P > 0.05).

CONCLUSION

The differences in the initial vasodilatory peak to local skin heating between the forearm and the leg are due to the contribution of sensory nerves.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Increased microvascular vasodilation and cardiovascular risk following a pre-eclamptic pregnancy

Women who develop pre-eclampsia are at high-risk for premature cardiovascular disease and death. The aim of this study was to assess microvascular function and cardiovascular risk in the early postpartum period for women who did/did not have a pregnancy complicated by pre-eclampsia. Peripheral microvascular function was assessed in women in the third trimester of uncomplicated pregnancies, with re-evaluation at 2 and 6 months postpartum. The effect of pre-eclampsia on postpartum microvascular function was assessed 2 and 6 months after delivery. Never-pregnant, naturally cycling women served for comparison. Cutaneous microvascular reactivity to acetylcholine and sodium nitroprusside, delivered locally by iontophoresis, was measured by laser Doppler flowmetry. 30-year and lifetime risk estimates for cardiovascular disease were established. Acetylcholine-mediated vasodilation was enhanced by normotensive pregnancy, and declined to nonpregnant levels by 6 months postpartum. Acetylcholine-mediated vasodilation remained high in pre-eclamptic subjects from 2 to 6 months postpartum compared to normotensive and never-pregnant controls. Pre-eclamptic subjects exhibited elevated 30-year and lifetime risk at 6 months postpartum. This study provides in vivo evidence of microvascular and cardiovascular risk implications of pre-eclampsia as early as 6 months postpartum, and suggests that the development of pre-eclampsia may be used to identify women at risk and eligible for risk screening and intervention.

Sex- and limb-specific differences in the nitric oxide-dependent cutaneous vasodilation in response to local heating

Local heating of the skin is commonly used to assess cutaneous microvasculature function. Controversy exists as to whether there are limb or sex differences in the nitric oxide (NO)-dependent contribution to this vasodilation, as well as the NO synthase (NOS) isoform mediating the responses. We tested the hypotheses that 1) NO-dependent vasodilation would be greater in the calf compared with the forearm; 2) total NO-dependent dilation would not be different between sexes within limb; and 3) women would exhibit greater neuronal NOS (nNOS)-dependent vasodilation in the calf. Two microdialysis fibers were placed in the skin of the ventral forearm and the calf of 19 (10 male and 9 female) young (23 ± 1 yr) adults for the local delivery of Ringer solution (control) or 5 mM N(ω)-propyl-l-arginine (NPLA; nNOS inhibition). Vasodilation was induced by local heating (42°C) at each site, after which 20 mM N(G)-nitro-l-arginine methyl ester (l-NAME) was perfused for within-site assessment of NO-dependent vasodilation. Cutaneous vascular conductance (CVC) was calculated as laser-Doppler flux/mean arterial pressure and normalized to maximum (28 mM sodium nitroprusside, 43°C). Total NO-dependent vasodilation in the calf was lower compared with the forearm in both sexes (Ringer: 42 ± 5 vs. 62 ± 4%; P < 0.05; NPLA: 37 ± 3 vs. 59 ± 5%; P < 0.05) and total NO-dependent vasodilation was lower in the forearm for women (Ringer: 52 ± 6 vs. 71 ± 4%; P < 0.05; NPLA: 47 ± 6 vs. 68 ± 5%; P < 0.05). NPLA did not affect total or NO-dependent vasodilation across limbs in either sex (P > 0.05). These data suggest that the NO-dependent component of local heating-induced cutaneous vasodilation is lower in the calf compared with the forearm. Contrary to our original hypothesis, there was no contribution of nNOS to NO-dependent vasodilation in either limb during local heating.