Wavelet analysis of oscillations in the peripheral blood circulation measured by laser Doppler technique

Assessment of cutaneous microcirculation by laser Doppler flowmetry in type 1 diabetes

BACKGROUND

The burden of type 1 diabetes (T1D) is growing worldwide, stressing the requirement to limit the threat of its long-term complications. In this regard, the development of methods for the early diagnosis and non-invasive monitoring of vascular abnormalities is widely recognized as one of the greatest priorities of the clinical research in this field.

OBJECTIVE

To assess the deterioration of physiological properties extracted from laser Doppler flowmetry (LDF) signals of microvascular perfusion and, secondly, to investigate their association with the quality of long-term metabolic control.

METHODS

Microvascular perfusion was recorded at the hallux of 63 control subjects and 47 T1D patients, whose glycaemic control was characterized in terms of the annual average levels of glycosylated haemoglobin (HbA1c). Pulse Decomposition Analysis was applied to the LDF data, in order to derive non-invasive markers of vascular stiffness based on a multi-Gaussian representation of the peripheral pulse waveforms; furthermore, wavelet transform analysis was used to evaluate the microvascular myogenic vasomotion and, finally, a physiological model of the reactive hyperaemia to a local thermal stimulus at 43 was used to test the integrity of the neurovascular pathways.

RESULTS

Compared to the control group, T1D patients showed a lower microvascular perfusion at baseline, and a larger vasodilatory reserve upon local heating, but no significant difference in myogenic activity. Moreover, the results of the PDA carried out on the LDF pulse waves, indicate the presence of a significant strong relation between large artery stiffness and the overall loss of glycaemic control over the past year.

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.

Multi-domain analysis of microvascular flow motion dynamics in NAFLD

OBJECTIVE

To determine whether analysis of microvascular network perfusion using complexity-based methods can discriminate between groups of individuals at an increased risk of developing CVD.

METHODS

Data were obtained from laser Doppler recordings of skin blood flux at the forearm in 50 participants with non-alcoholic fatty liver disease grouped for absence (n = 28) or presence (n = 14) of type 2 diabetes and use of calcium channel blocker medication (n = 8). Power spectral density was evaluated and Lempel-Ziv complexity determined to quantify signal information content at single and multiple time-scales to account for the different processes modulating network perfusion.

RESULTS

Complexity was associated with dilatory capacity and respiration and negatively with baseline blood flux and cardiac band power. The relationship between the modulators of flowmotion and complexity of blood flux is shown to change with time-scale improving discrimination between groups. Multiscale Lempel-Ziv achieved best classification accuracy of 86.1%.

CONCLUSIONS

Time and frequency domain measures alone are insufficient to discriminate between groups. As cardiovascular disease risk increases, the degree of complexity of the blood flux signal reduces, indicative of a reduced temporal activity and heterogeneous distribution of blood flow within the microvascular network sampled. Complexity-based methods, particularly multiscale variants, are shown to have good discriminatory capabilities.

Effects of Mandibular Extension on Pial Arteriolar Diameter Changes in Glucocorticoid-Induced Hypertensive Rats

Previously, in normotensive rats, it has been observed that a repetitive sub-maximal mouth opening (mandibular extension, ME) obtained by placing a home-made U-shaped dilator between the superior and inferior dental arches of the rat caused modulation of pial arteriolar tone. The present study was aimed to characterize pial microcirculation in two different cortical brain regions and to assess the hemodynamic effects of a single or double ME on pial arteriolar rhythmic diameter changes in rats rendered hypertensive by dexamethasone administrations. Cranial windows were prepared on parietal and frontal region. Pial arterioles were classified by Strahler method in five orders by in vivo fluorescence microscopy technique associated with a computerized system that permits off-line measurements of arteriolar diameter changes. Two 10 min ME at 10 min interval were applied; then the animals were monitored for further 240 min. Dexamethasone-treated rats exhibited a marked arterial rarefaction and asymmetry of bifurcation in the pial microvascular networks more evident in the frontal region. Starting from ME1, in both cortical areas, the arterioles dilated, and the vasodilation became significant compared to baseline after ME2 for the entire observation period. The spectral analysis carried out on order 2 arteriolar diameter change tracings, showed that double ME increased the spectral density of the frequency components related to endothelial, neuronal and myogenic activities in both the cortical regions studied. In conclusion, double ME has a generalized effect in the cortical areas by restoring the physiological vasomotion of the pial arterioles that was severely impaired by the experimentally hypertension.

Bariatric surgery in adolescents: a prospective randomized controlled trial comparing laparoscopic gastric banding to combined lifestyle interventions in adolescents with severe obesity (BASIC trial)

Background

Obesity in children and adolescents is an increasing problem associated with multiple co-morbidities including metabolic and endocrine changes, cardiovascular abnormalities, and impaired quality of life. Combined lifestyle interventions are the current standard treatment for severe obesity in children. However, the medium- and long-term results of these interventions are relatively poor. Bariatric surgery shows substantial weight loss and health improvement in adults and retrospective studies in adolescents show similar outcomes. However, well-designed prospective studies in this young age group are rare. Our objectives are to determine whether combining surgery with lifestyle interventions in severely obese adolescents leads to a significant additional weight reduction compared to lifestyle interventions solely, and to assess its effect on obesity-associated co-morbidities in a prospective randomized controlled setting.

Methods

Patients aged 14–16 years with sex- and age-adjusted BMI > 40 kg/m² (or > 35 kg/m² with comorbidity) and failure to achieve weight reduction > 5% during at least one year of combined lifestyle interventions are included in this trial. Randomization determines whether laparoscopic adjustable gastric banding will be added to combined lifestyle intervention throughout the trial period. Sixty children will be included in this trial. Follow-up visits are planned at 6 months, 1,2 and 3 years. Primary endpoints are percentage of total weight loss, and change of BMI. Secondary endpoints include body composition, pubertal development, metabolic and endocrine changes, inflammatory status, cardiovascular abnormalities, non-alcoholic steatohepatitis, quality of life and changes in behaviour.

Discussion

This randomized controlled trial is designed to provide important information about the safety and efficacy of laparoscopic adjustable gastric banding treatment in severely obese adolescents with unsuccessful combined lifestyle interventions. The reversibility of this surgical procedure forms a strong argument to decide for gastric banding over other surgical procedures, since bariatric surgery in adolescents is still in its infancy.

Trial registration

The BASIC trial is registered in the register of ClinicalTrials.gov since July 2010, Identifier: NCT01172899

Electronic supplementary material

The online version of this article (10.1186/s12887-019-1395-9) contains supplementary material, which is available to authorized users.

Using Multiscale Entropy to Assess the Efficacy of Local Cooling on Reactive Hyperemia in People with a Spinal Cord Injury

Pressure ulcers are one of the most common complications of a spinal cord injury (SCI). Prolonged unrelieved pressure is thought to be the primary causative factor resulting in tissue ischemia and eventually pressure ulcers. Previous studies suggested that local cooling reduces skin ischemia of the compressed soft tissues based on smaller hyperemic responses. However, the effect of local cooling on nonlinear properties of skin blood flow (SBF) during hyperemia is unknown. In this study, 10 wheelchair users with SCI and 10 able-bodied (AB) controls underwent three experimental protocols, each of which included a 10-min period as baseline, a 20-min intervention period, and a 20-min period for recovering SBF. SBF was measured using a laser Doppler flowmetry. During the intervention period, a pressure of 60 mmHg was applied to the sacral skin, while three skin temperature settings were tested, including no temperature change, a decrease by 10 °C, and an increase by 10 °C, respectively. A multiscale entropy (MSE) method was employed to quantify the degree of regularity of blood flow oscillations (BFO) associated with the SBF control mechanisms during baseline and reactive hyperemia. The results showed that under pressure with cooling, skin BFO both in people with SCI and AB controls were more regular at multiple time scales during hyperemia compared to baseline, whereas under pressure with no temperature change and particularly pressure with heating, BFO were more irregular during hyperemia compared to baseline. Moreover, the results of surrogate tests indicated that changes in the degree of regularity of BFO from baseline to hyperemia were only partially attributed to changes in relative amplitudes of endothelial, neurogenic, and myogenic components of BFO. These findings support the use of MSE to assess the efficacy of local cooling on reactive hyperemia and assess the degree of skin ischemia in people with SCI.

Comparison of complexity and spectral indices of skin-surface laser-doppler signals in patients with breast cancer receiving chemotherapy and Kuan-Sin-Yin

This study tested the hypothesis that measuring and analyzing skin-surface blood flow dynamics can be used to noninvasively discriminate the different microcirculatory and physiological function states of breast-cancer patients with chemotherapy between receiving and not receiving Kuan-Sin-Yin (KSY) treatment. The 17 included patients were assigned randomly to 2 comparison groups: Group K (n = 10) received KSY treatment, while Group NK (n = 7) did not receive KSY treatment. Beat-to-beat, spectral, and approximate-entropy (ApEn) analyses were applied to the 20-minute laser-Doppler sequences. The self-reported quality of life and cancer-related symptoms of patients were also investigated. In posttests, Group NK had a significantly larger ApEn ratio than that in Group K, significantly smaller values of laser-Doppler-flowmetry variability indices, and a slightly higher relative energy contribution of the neural-related frequency band compared to those in the pretests. Almost all cancer-related symptoms showed improvements in Group K compared to in Group NK. The present findings indicated that the present analysis can be used to detect the significantly different responses in the laser-Doppler indices between taking and not taking KSY. The KSY effect was also noted to be accompanied with improvement of EORTC QLQ-C30 scores. These could lead to a rapid, inexpensive, and objective technique for enhancing clinical applications in quality-of-life monitoring of breast cancer therapy.

Glabrous and non-glabrous vascular responses to mild hypothermia

This study examined cutaneous vasoconstriction to whole-body hypothermia, specifically contributions of neural and endothelial vasomotor responses in glabrous and non-glabrous skin. Eleven participants were semi-recumbent at an ambient temperature of 22 °C for 30 min, after which ambient temperature was decreased to 0 °C until rectal temperature (T_re) had decreased by 0.5 °C. Laser-Doppler fluxmetry was measured at the forehead and thigh for measures of glabrous and non-glabrous skin, respectively; wavelet analysis was performed on the laser-Doppler signal to determine endothelial and neural activities. Hypothermia took on average 97 ± 7 min and caused marked decreases at glabrous (42 ± 5%baseline, p < 0.001) and non-glabrous (69 ± 4%baseline, p < 0.001) skin. In glabrous skin, neural activity increased from 11 ± 1% at thermoneutral to 18 ± 1% (p < 0.001). In non-glabrous skin there was an initial decrease (p = 0.001) in neural activity from 13 ± 2% to 9 ± 1% (-0.2 °C decrease in T_re) and then increased (p = 0.002) to 21 ± 2% baseline at -0.5 °C T_re. Endothelial activity decreased in both glabrous (16 ± 3% to 6 ± 1%, p < 0.001) and non-glabrous (15 ± 1% to 7 ± 1%, p = 0.003) skin. Hypothermia elicits large decreases in skin blood flow in both glabrous and non-glabrous skin that are related to increases in neural activity and a reduction of endothelial activity.

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.

Sustained vasomotor control of skin microcirculation in Sherpas versus altitude-naive lowlanders: Experimental evidence from Xtreme Everest 2

NEW FINDINGS

What is the central question of this study? Do Sherpa highlanders, when exposed to graded hypobaric hypoxia, exhibit enhanced vasomotor and neurovascular control to maintain microcirculatory flux, and thus tissue oxygenation, when compared with altitude-naive lowlanders? What is the main finding and its importance? Sherpas, when exposed to hypobaric hypoxia at high altitude, demonstrated superior preservation of their peripheral microcirculatory perfusion, a greater oxygen unloading rate and sustained microvascular reactivity with enhanced vasomotion, when compared with altitude-naive lowlanders. These differences have not been reported previously and may improve our understanding of the multifactorial responses to sustained environmental hypoxia.

ABSTRACT

Enhanced oxygen delivery, consequent to an increased microvascular perfusion, has been postulated to play a key role in the physiological adaptation of Tibetan highlanders to the hypobaric hypoxia encountered at high altitude. We tested the hypothesis that Sherpas, when exposed to graded hypobaric hypoxia, demonstrate enhanced vasomotor and neurovascular control to maintain microcirculatory flux, and thus tissue oxygenation, when compared with altitude-naive lowlanders. Eighty-three lowlanders [39 men and 44 women, 38.8 (13.1) years old; mean (SD)] and 61 Sherpas [28 men and 33 women, 27.9 (6.9) years old] were studied on ascent to Everest Base Camp over 11 days. Skin blood flux and tissue oxygen saturation were measured simultaneously using combined laser Doppler fluximetry and white light spectroscopy at baseline, 3500 and 5300 m. In both cohorts, ascent resulted in a decline in the sympathetically mediated microvascular constrictor response (P < 0.001), which was more marked in lowlanders than in Sherpas (P < 0.001). The microvascular dilator response evaluated by postocclusive reactive hyperaemia was significantly greater in Sherpas than in lowlanders at all sites (P < 0.002). Spectral analysis of the blood flux signals revealed enhanced myogenic (vasomotion) activity in Sherpas, which was unaffected by ascent to 5300 m. Although skin tissue oxygenation was lower in Sherpas than in lowlanders, the oxygen unloading rate was faster, and deoxyhaemoglobin levels higher, at all altitudes. Together, these data suggest that Sherpas, when exposed to hypobaric hypoxia, demonstrated superior preservation of peripheral microcirculatory perfusion compared with altitude-naive lowlanders. The physiological differences in local microvasculature vasomotor and neurovascular control may play a key role in Sherpa adaptation to high-altitude hypobaric hypoxia by sustaining local perfusion and tissue oxygenation.

Physiological fluctuations show frequency-specific networks in fNIRS signals during resting state

Physiological fluctuations are commonly present in functional studies of hemodynamic response such as functional magnetic resonance imaging (fMRI) and functional near-infrared spectroscopy (fNIRS). However, the effects of these signals in neural mechanisms are not fully understood. Thus, the aim of this study is to propose that frequency-specific networks exist in the somatosensory region within the frequency range of physiological fluctuations. We used a wavelet coherence approach to identify functional connectivity between cortical regions. Based on the spectral response, four frequency bands were identified: cardiac (0.8-1.5 Hz), respiration (0.16-0.6 Hz), low frequency oscillations (LFO) (0.04-0.15 Hz), and very low frequency oscillations (VLFO) (0.0130.04 Hz). Eight cortical networks were revealed after ipsilateral and contralateral analysis to evaluate connectivity in each frequency band. The ANOVA analysis proved the adequacy of the connectivity map for all frequencies bands. Finally, these findings suggest possible frequency-specific organizations within the frequency bands of physiological fluctuations in the resting human brain.

Impact of Healthy Aging on Multifractal Hemodynamic Fluctuations in the Human Prefrontal Cortex

Fluctuations in resting-state cerebral hemodynamics show scale-free behavior over two distinct scaling ranges. Changes in such bimodal (multi) fractal pattern give insight to altered cerebrovascular or neural function. Our main goal was to assess the distribution of local scale-free properties characterizing cerebral hemodynamics and to disentangle the influence of aging on these multifractal parameters. To this end, we obtained extended resting-state records (N = 2¹⁴) of oxyhemoglobin (HbO), deoxyhemoglobin (HbR) and total hemoglobin (HbT) concentration time series with continuous-wave near-infrared spectroscopy technology from the brain cortex. 52 healthy volunteers were enrolled in this study: 24 young (30.6 ± 8.2 years), and 28 elderly (60.5 ± 12.0 years) subjects. Using screening tests on power-law, multifractal noise, and shuffled data sets we evaluated the presence of true multifractal hemodynamics reflecting long-range correlation (LRC). Subsequently, scaling-range adaptive bimodal signal summation conversion (SSC) was performed based on standard deviation (σ) of signal windows across a range of temporal scales (s). Building on moments of different order (q) of the measure, σ(s), multifractal SSC yielded generalized Hurst exponent function, H(q), and singularity spectrum, D(h) separately for a fast and slow component (the latter dominating the highest temporal scales). Parameters were calculated reflecting the estimated measure at s = N (focus), degree of LRC [Hurst exponent, H(2) and maximal Hölder exponent, h_max] and measuring strength of multifractality [full-width-half-maximum of D(h) and ΔH₁₅ = H(−15)−H(15)]. Correlation-based signal improvement (CBSI) enhanced our signal in terms of interpreting changes due to neural activity or local/systemic hemodynamic influences. We characterized the HbO-HbR relationship with the aid of fractal scale-wise correlation coefficient, r_σ(s) and SSC-based multifractal covariance analysis. In the majority of subjects, cerebral hemodynamic fluctuations proved bimodal multifractal. In case of slow component of raw HbT, h_max, and Ĥ(2) were lower in the young group explained by a significantly increased r_σ(s) among elderly at high temporal scales. Regarding the fast component of CBSI-pretreated HbT and that of HbO-HbR covariance, h_max, and focus were decreased in the elderly group. These observations suggest an attenuation of neurovascular coupling reflected by a decreased autocorrelation of the neuronal component concomitant with an accompanying increased autocorrelation of the non-neuronal component in the elderly group.

Increased skin blood flow during low intensity vibration in human participants: Analysis of control mechanisms using short-time Fourier transform

Aim

Investigate the immediate effect of low intensity vibration on skin blood flow and its underlying control mechanisms in healthy human participants.

Materials and methods

One-group pre-post design in a university laboratory setting. Nine adults underwent two bouts of 10-minute vibration (30Hz, peak acceleration 0.4g). Outcome measures include skin blood flow, and skin temperature on the right foot. To examine the control mechanisms underlying the vibration-induced blood flow response, SHORT-TIME Fourier analyses were computed to obtain the spectral densities for three frequency bands: metabolic (0.0095–0.02Hz), neurogenic (0.02–0.06Hz), and myogenic (0.06–0.15Hz). Non-parametric Friedman’s tests were computed to compare changes of the outcome measures and control mechanisms over the course of vibration.

Results

Vibration increased skin blood flow during both bouts of vibration, however the effect did not last after vibration was terminated. Myogenic spectral density increased during both bouts of vibration, whereas the metabolic and neurogenic spectral densities increased only during the 2^nd bout of vibration. Interestingly, only the metabolic spectral density remained elevated after vibration ended.

Conclusion

Low intensity vibration produced acute increases in skin blood flow mediated in part by vascular control mechanisms of myogenic origin. Further investigation is warranted to determine whether low intensity vibration induces similar increases in skin blood flow in populations prone to developing chronic non-healing wounds, such as spinal cord injury and diabetes.

Application of cmOCT and continuous wavelet transform analysis to the assessment of skin microcirculation dynamics

Correlation mapping optical coherence tomography (cmOCT) is a powerful technique for the imaging of skin microvessels structure, based on the discrimination of the static and dynamic regions of the tissue. Although the suitability of cmOCT to visualize the microcirculation has been proved in humans and animal models, less evidence has been provided about its application to examine functional dynamics. Therefore, the goal of this research was validating the cmOCT method for the investigation into microvascular function and vasomotion. A spectral domain optical coherence tomography (SD-OCT) device was employed to image 90 sequential three-dimensional (3-D) OCT volumes from the forearm of 12 volunteers during a 25-min postocclusive reactive hyperemia (PORH) test. The volumes were processed using cmOCT to generate blood flow maps at selected cutaneous depths. The maps clearly trace flow variations during the PORH response for both capillaries and arterioles/venules microvascular layers. Continuous blood flow signals were reconstructed from cmOCT maps to study vasomotion by applying wavelet transform spectral analysis, which revealed fluctuations of flow during PORH, reflecting the regulation of microvascular tone mediated by endothelial cells and sympathetic nerves. The results clearly demonstrate that cmOCT allows the generation of functional information that may be used for diagnostic applications.

Assessing low-frequency oscillations in cerebrovascular diseases and related conditions with near-infrared spectroscopy: a plausible method for evaluating cerebral autoregulation?

BACKGROUND

Cerebral autoregulation (CA) is the brain's ability to always maintain an adequate and relatively constant blood supply, which is often impaired in cerebrovascular diseases. Near-infrared spectroscopy (NIRS) examines oxygenated hemoglobin (OxyHb) in the cerebral cortex. Low- and very low-frequency oscillations ( LFOs ≈ 0.1    Hz and VLFOs ≈ 0.05 to 0.01 Hz) in OxyHb have been proposed to reflect CA.

AIM

To systematically review published results on OxyHb LFOs and VLFOs in cerebrovascular diseases and related conditions measured with NIRS.

APPROACH

A systematic search was performed in the MEDLINE database, which generated 36 studies relevant for inclusion.

RESULTS

Healthy people have relatively stable LFOs. LFO amplitude seems to reflect myogenic CA being decreased by vasomotor paralysis in stroke, by smooth muscle damage or as compensatory action in other conditions but can also be influenced by the sympathetic tone. VLFO amplitude is believed to reflect neurogenic and metabolic CA and is lower in stroke, atherosclerosis, and with aging. Both LFO and VLFO synchronizations appear disturbed in stroke, while the former is also altered in internal carotid stenosis and hypertension.

CONCLUSION

We conclude that amplitudes of LFOs and VLFOs are relatively robust measures for evaluating mechanisms of CA and synchronization analyses can show temporal disruption of CA. Further research and more coherent methodologies are needed.

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.

Increase in slow-wave vasomotion by hypoxia and ischemia in lowlanders and highlanders

The physiological relevance of slow-wave vasomotion is still unclear, even though it has been hypothesized that it could be a compensatory mechanism for enhancing tissue oxygenation in conditions of reduced oxygen supply. The aim of our study was to explore the effects of hypoxia and ischemia on slow-wave vasomotion in microcirculation. Peripheral oxygen saturation and forearm microcirculation flow (laser-Doppler flowmetry) were recorded at baseline and during postocclusive reactive hyperemia in the Himalaya region from 8 European lowlanders (6 men; aged 29-39 yr) at 1,350, 3,400, and 5,050 m and from 10 Nepalese male highlanders (aged 21-39 yr) at 3,400 and 5,050 m of altitude. The same measurements were also performed at sea level in 16 healthy volunteers (aged 23-61 yr) during a short-term exposure to normobaric hypoxia. In lowlanders, exposure to progressively higher altitude under baseline flow conditions progressively increased 0.06-0.15 Hz vasomotion amplitude [power spectral density % was expressed as geometric means (geometric standard deviation) = 14.0 (3.6) at 1,350 m; 87.0(2.3) at 3,400 m and 249.8 (3.6) at 5,050 m; P = 0.006 and P < 0.001 vs. 1,350 m, respectively]. In highlanders, low frequency vasomotion amplitude was similarly enhanced at different altitudes [power spectral density % = 183.4 (4.1) at 3,400 m vs. 236.0 (3.0) at 5,050 m; P = 0.139]. In both groups at altitude, it was further increased after ischemic stimulus ( P < 0.001). At baseline, acute short lasting normobaric hypoxia did not induce low frequency vasomotion, which was conversely induced by ischemia, even under normal oxygenation and barometric pressure. This study offers the demonstration of a significant increase in slow-wave vasomotion under prolonged hypobaric-hypoxia exposure at high altitude, with a further enhancement after ischemia induction. NEW & NOTEWORTHY This study offers the demonstration in humans of the occurrence of enhanced slow-wave vasomotion in microcirculation induced by exposure to hypobaric hypoxia, ischemia, and their combination. This phenomenon, where vasomotion can be hypothesized to behave as a "peripheral heart," may represent a compensating adaptive change aimed at improving peripheral flow and tissue oxygenation in conditions of reduced oxygen supply, such as altitude-induced hypobaric hypoxia and postocclusion ischemia.

Exploring the Relationship between Blood Flux Signals and HRV following Different Thermal Stimulations using Complexity Analysis

To investigate the relationship between local blood flux and heart rate variability following different thermal stimulations, healthy subjects were recruited and subject to different thermal stimulations on the right forearm. Multiscale entropy and multiscale fuzzy entropy were used to measure the complexity of the local blood flux, and the approximate entropy was calculated to evaluate the HRV complexity. The results indicated that thermal stimulation significantly increased local blood flux and that different temperature stimulations resulted in different complexities in local blood flux. A 42 °C or 44 °C thermal stimulation, other than stimulations below 42 °C, resulted in a moderate correlation between local blood flux and heart rate variability complexity. The results provide a new perspective in terms of complexity to explore the relationship between skin blood flux signals and cardiac function.

Peripheral Resistance Baroreflex During Incremental Bicycle Ergometer Exercise: Characterization and Correlation With Cardiac Baroreflex

The arm of the baroreflex (BR) controlling peripheral resistances (PR), labeled as BR of PR (prBR), was characterized through an extension of the cardiac BR (cBR) sequence analysis. The method exploits recordings of skin blood flow (SBF) from the palm of the non-dominant hand via a laser Doppler flowmeter and of arterial pressure (AP) from the middle finger of the same hand via a plethysmographic device. PR was estimated beat-by-beat as the ratio of mean AP to mean SBF computed over the same heart period (HP). Peripheral resistances-diastolic arterial pressure (PR-DAP) sequences featuring simultaneous increases of PR and decreases of diastolic AP (DAP) or vice versa were identified and the slope of the regression line in the (DAP, PR) plane was taken as an estimate of prBR sensitivity (BRS_prBR). The percentage of prBR sequences (SEQ%_prBR) was taken as a measure of prBR involvement and the prBR effectiveness index (EI_prBR) was computed as the fraction of DAP sequences capable to drive antiparallel PR variations. Analogous markers were computed over cBR from HP and systolic AP (SAP) variability [i.e., cBR sensitivity (BRS_cBR), percentage of cBR sequences (SEQ%_cBR)_, and effectiveness index of the cBR (EI_cBR)]. prBR and cBR were typified during incremental light-to-moderate bicycle ergometer exercise at 10, 20, and 30% of the maximum effort in 16 healthy subjects (aged from 22 to 58 years, six males). We found that: (i) BRS_cBR decreased gradually with the challenge, while BRS_prBR declined only at the heaviest workload; (ii) SEQ%_cBR decreased solely at the lightest workload, while the decline of SEQ%_prBR was significant regardless of the intensity of the challenge; (iii) EI_prBR and EI_cBR were not affected by exercise; (iv) after pooling together all the data regardless of the experimental conditions, BRS_prBR and BRS_cBR were uncorrelated, while SEQ%_cBR and SEQ%_prBR as well as EI_cBR and EI_prBR, were significantly and positively correlated; (v) when the correlation between SEQ%_cBR and SEQ%_prBR and between EI_cBR and EI_prBR was assessed separately in each experimental condition, it was not systematically detected. This study suggests that prBR characterization provides information complementary to cBR that might be fruitfully exploited to improve patients' risk stratification.

Using Laser Doppler Imaging and Monitoring to Analyze Spinal Cord Microcirculation in Rat

Laser Doppler flowmetry (LDF) is a noninvasive method for blood flow (BF) measurement, which makes it preferable for measuring microcirculatory alterations of the spinal cord. In this article, our goal was to use both Laser Doppler imaging and monitoring to analyze the change of BF after spinal cord injury. Both the laser Doppler image scanner and the probe/monitor were being employed to obtain each readout. The data of LDPI provided a local distribution of BF, which gave an overview of perfusion around the injury site and made it accessible for comparative analysis of BF among different locations. By intensely measuring the probing area over a period of time, a combined probe was used to simultaneously measure the BF and oxygen saturation of the spinal cord, showing overall spinal cord perfusion and oxygen supply. LDF itself has a few limitations, such as relative flux, sensitivity to movement, and biological zero signal. However, the technology has been applied in clinical and experimental study due to its simple setup and rapid measurement of BF.

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.

Human subarachnoid space width oscillations in the resting state

Abnormal cerebrospinal fluid (CSF) pulsatility has been implicated in patients suffering from various diseases, including multiple sclerosis and hypertension. CSF pulsatility results in subarachnoid space (SAS) width changes, which can be measured with near-infrared transillumination backscattering sounding (NIR-T/BSS). The aim of this study was to combine NIR-T/BSS and wavelet analysis methods to characterise the dynamics of the SAS width within a wide range of frequencies from 0.005 to 2 Hz, with low frequencies studied in detail for the first time. From recordings in the resting state, we also demonstrate the relationships between SAS width in both hemispheres of the brain, and investigate how the SAS width dynamics is related to the blood pressure (BP). These investigations also revealed influences of age and SAS correlation on the dynamics of SAS width and its similarity with the BP. Combination of NIR-T/BSS and time-frequency analysis may open up new frontiers in the understanding and diagnosis of various neurodegenerative and ageing related diseases to improve diagnostic procedures and patient prognosis.

Application of Multiscale Entropy in Assessing Plantar Skin Blood Flow Dynamics in Diabetics with Peripheral Neuropathy

Diabetic foot ulcer (DFU) is a common complication of diabetes mellitus, while tissue ischemia caused by impaired vasodilatory response to plantar pressure is thought to be a major factor of the development of DFUs, which has been assessed using various measures of skin blood flow (SBF) in the time or frequency domain. These measures, however, are incapable of characterizing nonlinear dynamics of SBF, which is an indicator of pathologic alterations of microcirculation in the diabetic foot. This study recruited 18 type 2 diabetics with peripheral neuropathy and eight healthy controls. SBF at the first metatarsal head in response to locally applied pressure and heating was measured using laser Doppler flowmetry. A multiscale entropy algorithm was utilized to quantify the regularity degree of the SBF responses. The results showed that during reactive hyperemia and thermally induced biphasic response, the regularity degree of SBF in diabetics underwent only small changes compared to baseline and significantly differed from that in controls at multiple scales (p < 0.05). On the other hand, the transition of regularity degree of SBF in diabetics distinctively differed from that in controls (p < 0.05). These findings indicated that multiscale entropy could provide a more comprehensive assessment of impaired microvascular reactivity in the diabetic foot compared to other entropy measures based on only a single scale, which strengthens the use of plantar SBF dynamics to assess the risk for DFU.

Tibial Nerve Stimulation to Drive Genital Sexual Arousal in an Anesthetized Female Rat

BACKGROUND

There is clinical evidence that percutaneous tibial nerve stimulation can positively benefit women with female sexual interest/arousal disorder, yet no studies have explored the potential mechanisms further.

AIM

To investigate the effect of tibial nerve stimulation on vaginal blood perfusion (VBP) in an anesthetized rat model.

METHODS

16 ketamine-anesthetized rats were surgically implanted with a nerve cuff electrode on 1 tibial nerve. The tibial nerve was stimulated for 30 minutes continuously or non-continuously at a frequency of 10 to 25 Hz.

OUTCOMES

VBP was measured with laser Doppler flowmetry and analyzed using a wavelet transform of time-frequency representations with a focus on the neurogenic energy range (0.076-0.200 Hz).

RESULTS

25 of 33 (75.8%) stimulation periods had at least a 500% increase in laser Doppler flowmetry neurogenic energy compared with baseline. This increase was most common within 20 to 35 minutes after the start of stimulation. There was no statistically significant difference for frequency used or estrous cycle stage.

CLINICAL TRANSLATION

The results of this study provide further support for percutaneous tibial nerve stimulation as an alternative treatment option for women with genital arousal aspects of female sexual interest/arousal disorder.

STRENGTHS AND LIMITATIONS

This study successfully demonstrates the ability of tibial nerve stimulation to increase VBP. However, further studies to determine parameter optimization and to illuminate neural mechanisms are needed. Further studies also are necessary to determine effects of repeated stimulation sessions.

CONCLUSION

Long-duration tibial stimulation was successful at driving increases in the neurogenic component of VBP, providing evidence that tibial nerve stimulation could be used to treat genital arousal aspects of female sexual interest/arousal disorder by improving pelvic blood flow. Zimmerman LL, Rice IC, Berger MB, Bruns TM. Tibial Nerve Stimulation to Drive Genital Sexual Arousal in an Anesthetized Female Rat. J Sex Med 2018;15:296-303.

Flow motion dynamics of microvascular blood flow and oxygenation: Evidence of adaptive changes in obesity and type 2 diabetes mellitus/insulin resistance

An altered spatial heterogeneity and temporal stability of network perfusion can give rise to a limited adaptive ability to meet metabolic demands. Derangement of local flow motion activity is associated with reduced microvascular blood flow and tissue oxygenation, and it has been suggested that changes in flow motion activity may provide an early indicator of declining, endothelial, neurogenic, and myogenic regulatory mechanisms and signal the onset and progression of microvascular pathophysiology. This short conference review article explores some of the evidence for altered flow motion dynamics of blood flux signals acquired using laser Doppler fluximetry in the skin in individuals at risk of developing or with cardiometabolic disease.

Frequency-specific Effective Connectivity in Subjects with Cerebral Infarction as Revealed by NIRS Method

A connectivity-based approach can highlight the network reorganization in the chronic phases after stroke and contributes to the development of therapeutic interventions. Using dynamic Bayesian inference, this study aimed to assess the effective connectivity (EC) in various frequency bands through the near-infrared spectroscopy (NIRS) method in subjects with cerebral infarction (CI). A phase-coupling model was established based on phase information extracted using the wavelet transform of NIRS signals. Coupling strength and the main coupling direction were estimated using dynamic Bayesian inference. Wilcoxon test and chi-square test were used to determine the significant difference in EC between two groups. Results showed that the coupling strength of the EC in the CI group significantly decreased relative to that in the healthy group. The decrease was most significant in the frequency intervals IV (0.021 Hz-0.052 Hz; p = 0.0006) and VI (0.005 Hz-0.095 Hz; p = 0.0028). The main coupling direction changed from the right prefrontal cortex to the right motor cortex and left motor cortex in the frequency intervals IV (p₁ = 0.041, p₂ = 0.047) and II (p₁ = 0.0017, p₂ = 0.0036), respectively. The EC decreased or was even lost significantly in the EC map of the CI group. Experimental results indicated that information propagation was blocked in the CI group than in the healthy group and resulted in the decreased coupling strength and connectivity loss. The main coupling direction of the motor section changed from driving into being driven because of the degradation of limb movement function.

Nonlinear dynamics of skin blood flow response to mechanical and thermal stresses in the plantar foot of diabetics with peripheral neuropathy

BACKGROUND

Diabetic foot ulcers (DFU) are a major complication in diabetics. Impaired microvascular reactivity is a major contributor to the development of DFU and has been traditionally quantified by time-domain or frequency-domain measures of skin blood flow (SBF). These measures, however, are unable to characterize the changes of nonlinear dynamics of SBF associated with diabetes and peripheral neuropathy.

OBJECTIVE

The objective of this study was to investigate altered nonlinear dynamics of skin blood flow in the plantar foot of diabetics with peripheral neuropathy.

METHODS

18 type 2 diabetics with peripheral neuropathy and 8 healthy controls were recruited. SBF at the first metatarsal head in response to a loading pressure of 300 mmHg and a local heating was measured using laser Doppler flowmetry. A sample entropy approach was used to quantify the degree of regularity of SBF.

RESULTS

Our results showed that the regularity degree of SBF in the diabetic foot underwent only small changes during post-occlusive reactive hyperemia and thermally induced biphasic response compared to non-diabetics. SBF of the diabetic foot has higher degree of irregularity during reactive hyperemia because of attenuated myogenic activity, and demonstrated higher regularity during the biphasic response largely due to significantly enhanced cardiac activities.

CONCLUSIONS

This study suggests that the regularity degree of SBF at the first metatarsal head could be used to assess impaired microvascular reactivity and thus may be used to assess the risk for DFU in diabetics with peripheralneuropathy.

Oscillations of Subarachnoid Space Width as a Potential Marker of Cerebrospinal Fluid Pulsatility

In the cerebrospinal fluid (CSF) circulation, two components can be distinguished: bulk flow (circulation) and pulsatile flow (back and forth motion). CSF pulsatile flow is generated by both cardiac and respiratory cycles. Recent years have seen increased interest in cardiac- and respiratory-driven CSF pulsatility as an important component of cerebral homeostasis. CSF pulsatility is affected by cerebral arterial inflow and jugular outflow and potentially linked to white matter abnormalities in various diseases, such as multiple sclerosis or hypertension. In this review, we discuss the physiological mechanisms associated with CSF pulsation and its clinical significance. Finally, we explain the concept of using the oscillations of subarachnoid space width as a surrogate for CSF pulsatility.

Coupling of Blood Pressure and Subarachnoid Space Oscillations at Cardiac Frequency Evoked by Handgrip and Cold Tests: A Bispectral Analysis

The aim of the study was to assess blood pressure-subarachnoid space (BP-SAS) width coupling properties using time-frequency bispectral analysis based on wavelet transforms during handgrip and cold tests. The experiments were performed on a group of 16 healthy subjects (F/M; 7/9) of the mean age 27.2 ± 6.8 years and body mass index of 23.8 ± 4.1 kg/m². The sequence of challenges was first handgrip and then cold test. The handgrip challenge consisted of a 2-min strain, indicated by oral communication from the investigator, at 30% of maximum strength. The cold test consisted of 2 min of hand immersion to approximately wrist level in cold water of 4 °C, verified by a digital thermometer. Each test was preceded by 10 min at baseline and was followed by 10-min recovery recordings. BP and SAS were recorded simultaneously. Three 2-min stages of the procedure, baseline, test, and recovery, were analyzed. We found that BP-SAS coupling was present only at cardiac frequency, while at respiratory frequency both oscillators were uncoupled. Handgrip and cold test failed to affect BP-SAS cardiac-respiratory coupling. We showed similar handgrip and cold test cardiac bispectral coupling for individual subjects. Further studies are required to establish whether the observed intersubject variability concerning the BP-SAS coupling at cardiac frequency has any potential clinical predictive value.

The effect of dim light at night on cerebral hemodynamic oscillations during sleep: A near-infrared spectroscopy study

Recent studies have reported that dim light at night (dLAN) is associated with risks of cardiovascular complications, such as hypertension and carotid atherosclerosis; however, little is known about the underlying mechanism. Here, we evaluated the effect of dLAN on the cerebrovascular system by analyzing cerebral hemodynamic oscillations using near-infrared spectroscopy (NIRS). Fourteen healthy male subjects underwent polysomnography coupled with cerebral NIRS. The data collected during sleep with dim light (10 lux) were compared with those collected during sleep under the control dark conditions for the sleep structure, cerebral hemodynamic oscillations, heart rate variability (HRV), and their electroencephalographic (EEG) power spectrum. Power spectral analysis was applied to oxy-hemoglobin concentrations calculated from the NIRS signal. Spectral densities over endothelial very-low-frequency oscillations (VLFOs) (0.003-0.02 Hz), neurogenic VLFOs (0.02-0.04 Hz), myogenic low-frequency oscillations (LFOs) (0.04-0.15 Hz), and total LFOs (0.003-0.15 Hz) were obtained for each sleep stage. The polysomnographic data revealed an increase in the N2 stage under the dLAN conditions. The spectral analysis of cerebral hemodynamics showed that the total LFOs increased significantly during slow-wave sleep (SWS) and decreased during rapid eye movement (REM) sleep. Specifically, endothelial (median of normalized value, 0.46 vs. 0.72, p = 0.019) and neurogenic (median, 0.58 vs. 0.84, p = 0.019) VLFOs were enhanced during SWS, whereas endothelial VLFOs (median, 1.93 vs. 1.47, p = 0.030) were attenuated during REM sleep. HRV analysis exhibited altered spectral densities during SWS induced by dLAN, including an increase in very-low-frequency and decreases in low-frequency and high-frequency ranges. In the EEG power spectral analysis, no significant difference was detected between the control and dLAN conditions. In conclusion, dLAN can disturb cerebral hemodynamics via the endothelial and autonomic systems without cortical involvement, predominantly during SWS, which might represent an underlying mechanism of the increased cerebrovascular risk associated with light exposure during sleep.

Prospects of Laser Doppler flowmetry application in assessment of skin microcirculation in diabetes

This review includes results of scientific and clinical use of laser Doppler flowmetry (LDF) in patients with diabetes mellitus. LDF is a non-invasive method for the quantitative evaluation of microcirculation, which can assess microcirculatory rhythms and conduct functional tests with various impacts, allowing the exploration of regulatory mechanisms of microcirculation. LDF reveals specific diabetes changes in the regulatory function of microcirculation. Microcirculation disturbances, which are traditionally associated with the pathogenesis of complications, also occur in patients with early disorders of carbohydrate metabolism and may precede the manifestation of diabetes. However, this method is still not applied in clinical practice. In this review, we analysed factors limiting the implementation of LDF in practical medicine and suggest ways to improve its clinical significance.

Low-Frequency Components in Rat Pial Arteriolar Rhythmic Diameter Changes

This study aimed to analyze the frequency components present in spontaneous rhythmic diameter changes in rat pial arterioles. Pial microcirculation was visualized by fluorescence microscopy. Rhythmic luminal variations were evaluated via computer-assisted methods. Spectral analysis was carried out on 30-min recordings under baseline conditions and after administration of acetylcholine (Ach), papaverine (Pap), Nω-nitro-L-arginine (L-NNA) prior to Ach, indomethacin (INDO), INDO prior to Ach, charybdotoxin and apamin, and charybdotoxin and apamin prior to Ach. Under baseline conditions all arteriolar orders showed 3 frequency components in the ranges of 0.0095-0.02, 0.02-0.06, and 0.06-0.2 Hz, another 2 in the ranges of 0.2-2.0 and 2.5-4.5 Hz, and another ultra-low-frequency component in the range of 0.001-0.0095 Hz. Ach caused a significant increase in the spectral density of the frequency components in the range of 0.001-0.2 Hz. Pap was able to slightly increase spectral density in the ranges of 0.001-0.0095 and 0.0095-0.02 Hz. L-NNA mainly attenuated arteriolar responses to Ach. INDO prior to Ach did not affect the endothelial response to Ach. Charybdotoxin and apamin, suggested as endothelium-derived hyperpolarizing factor inhibitors, reduced spectral density in the range of 0.001-0.0095 Hz before and after Ach administration. In conclusion, regulation of the blood flow distribution is due to several mechanisms, one of which is affected by charibdotoxin and apamin, modulating the vascular tone.

Coherence and Coupling Functions Reveal Microvascular Impairment in Treated Hypertension

The complex interactions that give rise to heart rate variability (HRV) involve coupled physiological oscillators operating over a wide range of different frequencies and length-scales. Based on the premise that interactions are key to the functioning of complex systems, the time-dependent deterministic coupling parameters underlying cardiac, respiratory and vascular regulation have been investigated at both the central and microvascular levels. Hypertension was considered as an example of a globally altered state of the complex dynamics of the cardiovascular system. Its effects were established through analysis of simultaneous recordings of the electrocardiogram (ECG), respiratory effort, and microvascular blood flow [by laser Doppler flowmetry (LDF)]. The signals were analyzed by methods developed to capture time-dependent dynamics, including the wavelet transform, wavelet-based phase coherence, non-linear mode decomposition, and dynamical Bayesian inference, all of which can encompass the inherent frequency and coupling variability of living systems. Phases of oscillatory modes corresponding to the cardiac (around 1.0 Hz), respiratory (around 0.25 Hz), and vascular myogenic activities (around 0.1 Hz) were extracted and combined into two coupled networks describing the central and peripheral systems, respectively. The corresponding spectral powers and coupling functions were computed. The same measurements and analyses were performed for three groups of subjects: healthy young (Y group, 24.4 ± 3.4 y), healthy aged (A group, 71.1 ± 6.6 y), and aged treated hypertensive patients (ATH group, 70.3 ± 6.7 y). It was established that the degree of coherence between low-frequency oscillations near 0.1 Hz in blood flow and in HRV time series differs markedly between the groups, declining with age and nearly disappearing in treated hypertension. Comparing the two healthy groups it was found that the couplings to the cardiac rhythm from both respiration and vascular myogenic activity decrease significantly in aging. Comparing the data from A and ATH groups it was found that the coupling from the vascular myogenic activity is significantly weaker in treated hypertension subjects, implying that the mechanisms of microcirculation are not completely restored by current anti-hypertension medications.

Assessing Microvascular Function in Humans from a Chronic Disease Perspective

Microvascular dysfunction (MVD) is considered a crucial pathway in the development and progression of cardiometabolic and renal disease and is associated with increased cardiovascular mortality. MVD often coexists with or even precedes macrovascular disease, possibly due to shared mechanisms of vascular damage, such as inflammatory processes and oxidative stress. One of the first events in MVD is endothelial dysfunction. With the use of different physiologic or pharmacologic stimuli, endothelium-dependent (micro)vascular reactivity can be studied. This reactivity depends on the balance between various mediators, including nitric oxide, endothelin, and prostanoids, among others. The measurement of microvascular (endothelial) function is important to understand the pathophysiologic mechanisms that contribute to MVD and the role of MVD in the development and progression of cardiometabolic/renal disease. Here, we review a selection of direct, noninvasive techniques for measuring human microcirculation, with a focus on methods, interpretation, and limitations from the perspective of chronic cardiometabolic and renal disease.

Determination of Skeletal Muscle Microvascular Flowmotion with Contrast-Enhanced Ultrasound

Most methods of assessing flowmotion (rhythmic oscillation of blood flow through tissue) are limited to small sections of tissue and are invasive in tissues other than skin. To overcome these limitations, we adapted the contrast-enhanced ultrasound (CEUS) technique to assess microvascular flowmotion throughout a large region of tissue, in a non-invasive manner and in real time. Skeletal muscle flowmotion was assessed in anaesthetised Sprague Dawley rats, using CEUS and laser Doppler flowmetry (LDF) for comparison. Wavelet transformation of CEUS and LDF data was used to quantify flowmotion. The α-adrenoceptor antagonist phentolamine was infused to predictably blunt the neurogenic component of flowmotion. Both techniques identified similar flowmotion patterns, validating the use of CEUS to assess flowmotion. This study demonstrates for the first time that the novel technique of CEUS can be adapted for determination of skeletal muscle flowmotion in large regions of skeletal muscle.

Repeated Mandibular Extension in Rat: A Procedure to Modulate the Cerebral Arteriolar Tone

Previous data have shown both in the rat and in the human that a single mandibular extension lasting 10 min induces a significant important and prolonged reduction in blood pressure and heart rate, affecting also rat pial microcirculation by the release of endothelial factors. In the present work, we assessed whether repeated mandibular extension could further prolong these effects. We performed two mandibular extensions, the second mandibular extension being applied 10 min after the first one. The second mandibular extension produced a reduction in blood pressure and heart rate for at least 240 min. As in the case of a single mandibular extension, pial arterioles dilated persisting up to 140 min after the second extension. Spectral analysis on 30 min recordings under baseline conditions and after repetitive mandibular extensions showed that the pial arterioles dilation was associated with rhythmic diameter changes sustained by an increase in the frequency components related to endothelial, neurogenic, and myogenic activity while a single mandibular extension caused, conversely, an increase only in the endothelial activity. In conclusion, repetitive mandibular extension prolonged the effects of a single mandibular extension on blood pressure, heart rate and vasodilation and induced a modulation of different frequency components responsible of the pial arteriolar tone, in particular increasing the endothelial activity.