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

Dynamic Microcirculation Characteristics of Plantar Skin Under Metatarsal Head of Human Foot in Response to Life-Like Pressure Stimulus

OBJECTIVE

Diabetic foot ulcer (DFU) is a severe complication with high mortality. High plantar pressure and poor microcirculation are considered main causes of DFU. The specific aims were to provide a novel technique for real-time measurement of plantar skin blood flow (SBF) under walking-like pressure stimulus and delineate the first plantar metatarsal head dynamic microcirculation characteristics because of life-like loading conditions in healthy individuals.

METHODS

Twenty young healthy participants (14 male and 6 female) were recruited. The baseline (i.e., unloaded) SBF of soft tissue under the first metatarsal head were measured using laser Doppler flowmetry (LDF). A custom-made machine was utilized to replicate daily walking pressure exertion for 5 min. The exerted plantar force was adjusted from 10 N (127.3 kPa) to 40 N (509.3 kPa) at an increase of 5 N (63.7 kPa). Real-time SBF was acquired using the LDF. After each pressure exertion, postload SBF was measured for comparative purposes. Statistical analysis was performed using the R software.

RESULTS

All levels of immediate-load and postload SBF increased significantly compared with baseline values. As the exerted load increased, the postload and immediate-load SBF tended to increase until the exerted load reached 35 N (445.6 kPa). However, in immediate-load data, the increasing trend tended to level off as the exerted pressure increased from 15 N (191.0 kPa) to 25 N (318.3 kPa). For postload and immediate-load SBF, they both peaked at 35 N (445.6 kPa). However, when the exerted force exceeds 35 N (445.6 kPa), both the immediate-load and postload SBF values started to decrease.

CONCLUSIONS

Our study offered a novel real-time plantar soft tissue microcirculation measurement technique under dynamic conditions. For the first metatarsal head of healthy people, 20 N (254.6 kPa)-plantar pressure has a fair microcirculation stimulus compared with higher pressure. There might be a pressure threshold at 35 N (445.6 kPa) for the first metatarsal head, and soft tissue microcirculation may decrease when local pressure exceeds it.

Definition of thermal indicators for the study of thermoregulation alterations in the foot of people living within diabetic peripheral neuropathy: A proof of concept

AIMS

This study investigates how diabetic peripheral neuropathy is linked to impairment of thermoregulatory mechanisms using a thermal camera, spectral thermal analysis and a physical test.

METHODS

The plantar skin temperature of all participants was measured using a thermal camera following a 6-min walking exercise. The data were subjected to frequency decomposition, resulting in two frequency ranges corresponding to endothelial and neurogenic mechanisms. Then, 40 thermal indicators were evaluated for each participant. ROC curve and statistical tests allowed to identify indicators able to detect the presence or absence of diabetic peripheral neuropathy.

RESULTS

The study included 33 participants living with diabetes. The results revealed that a 6-min walk exercise increased plantar foot temperature and highlighted a significant difference between people living with diabetes with and without peripheral neuropathy (p < 0.01). The results also revealed the advantages of using thermal images rather than single point measurements.

CONCLUSIONS

Diabetic peripheral neuropathy is linked to impairment of thermoregulatory mechanisms. This link can be highlighted after a dedicated 6-min walk exercise, enabling to activate these mechanisms, and measuring with a thermal camera the temporal plantar skin temperature. Assessment of this link gave best results by filtering the thermal signal in the neurogenic range.

Application of Multiscale Sample Entropy in Assessing Effects of Exercise Training on Skin Blood Flow Oscillations in People with Spinal Cord Injury

Spinal cord injury (SCI) causes a disruption of autonomic nervous regulation to the cardiovascular system, leading to various cardiovascular and microvascular diseases. Exercise training is an effective intervention for reducing risk for microvascular diseases in healthy people. However, the effectiveness of exercise training on improving microvascular function in people with SCI is largely unknown. The purpose of this study was to compare blood flow oscillations in people with spinal cord injury and different physical activity levels to determine if such a lifestyle might influence skin blood flow. A total of 37 participants were recruited for this study, including 12 athletes with SCI (ASCI), 9 participants with SCI and a sedentary lifestyle (SSCI), and 16 healthy able-bodied controls (AB). Sacral skin blood flow (SBF) in response to local heating at 42 °C for 50 min was measured using laser Doppler flowmetry. The degree of the regularity of blood flow oscillations (BFOs) was quantified using a multiscale entropy approach. The results showed that BFO was significantly more irregular in ASCI and AB compared to SSCI during the maximal vasodilation period. Our results also demonstrate that the difference in the regularity of BFOs between original SBF signal and phase-randomized surrogate time series was larger in ASCI and AB compared to SSCI. Our findings indicate that SCI causes a loss of complexity of BFOs and exercise training may improve complexity in people with SCI. This study demonstrates that multiscale entropy is a sensitive method for detecting differences between different categories of people with SCI and might be able to detect effects of exercise training related to skin blood flow.

Effect of Walking Speeds on Complexity of Plantar Pressure Patterns

Various walking speeds may induce different responses on the plantar pressure patterns. Current methods used to analyze plantar pressure patterns are linear and ignore nonlinear features. The purpose of this study was to analyze the complexity of plantar pressure images after walking at various speeds using nonlinear bidimensional multiscale entropy (MSE2D). Twelve participants (age: 27.1 ± 5.8 years; height: 170.3 ± 10.0 cm; and weight: 63.5 ± 13.5 kg) were recruited for walking at three speeds (slow at 1.8 mph, moderate at 3.6 mph, and fast at 5.4 mph) for 20 minutes. A plantar pressure measurement system was used to measure plantar pressure patterns. Complexity index (CI), a summation of MSE2D from all time scales, was used to quantify the changes of complexity of plantar pressure images. The analysis of variance with repeated measures and Fisher’s least significant difference correction were used to examine the results of this study. The results showed that CI of plantar pressure images of 1.8 mph (1.780) was significantly lower compared with 3.6 (1.790) and 5.4 mph (1.792). The results also showed that CI significantly increased from the 1st min (1.780) to the 10th min (1.791) and 20th min (1.791) with slow walking (1.8 mph). Our results indicate that slow walking at 1.8 mph may not be good for postural control compared with moderate walking (3.6 mph) and fast walking (5.4 mph). This study demonstrates that bidimensional multiscale entropy is able to quantify complexity changes of plantar pressure images after different walking speeds.

Impaired dermal microvascular reactivity and implications for diabetic wound formation and healing: an evidence review

Diabetic foot ulcers (DFUs) are among the most consequential and costly complications faced by patients with diabetes and the global healthcare system. Acknowledged risk factors for DFUs include diabetic peripheral neuropathy (DPN), peripheral arterial disease (PAD), microtrauma and foot deformities. Research on additional risk factors for DFUs has recently focused on dysregulated, autonomic vasomotor control in the skin of patients with DPN. In particular, impaired dermal microvascular reactivity (IDMR) with its attendant reduction in nutritive capillary blood flow has been identified as an emerging risk factor. This especially relates to refractory wounds noted in patients without overt PAD signs. In this paper, evidence will be reviewed supporting the evolving understanding of IDMR and its impact on DFU formation and healing. Advances in diagnostic instrumentation driving this research along with the most promising potential therapies aimed at improving microvascular function in the diabetic foot will be discussed in brief.

Laser Doppler Spectrum Analysis Based on Calculation of Cumulative Sums Detects Changes in Skin Capillary Blood Flow in Type 2 Diabetes Melitus

In this article, we introduce a new method of signal processing and data analysis for the digital laser Doppler flowmetry. Our approach is based on the calculation of cumulative sums over the registered Doppler power spectra. The introduced new parameter represents an integral estimation for the redistribution of moving red blood cells over the range of speed. The prototype of the device implementing the technique is developed and tested in preliminary clinical trials. The methodology was verified with the involvement of two age groups of healthy volunteers and in a group of patients with type 2 diabetes mellitus. The main practical result of the study is the development of a set of binary linear classifiers that allow the method to identify typical patterns of the microcirculation for the healthy volunteers and diabetic patients based on the presented diagnostic algorithm.

Effect of Different Local Vibration Frequencies on the Multiscale Regularity of Plantar Skin Blood Flow

Local vibration has shown promise in improving skin blood flow (SBF). However, there is no consensus on the selection of the best vibration frequency. An important reason may be that previous studies utilized time- and frequency-domain parameters to characterize vibration-induced SBF responses. These parameters are unable to characterize the structural features of the SBF response to local vibrations, thus contributing to the inconsistent findings seen in vibration research. The objective of this study was to provide evidence that nonlinear dynamics of SBF responses would be an important aspect for assessing the effect of local vibration on SBF. Local vibrations at 100 Hz, 35 Hz, and 0 Hz (sham vibration) with an amplitude of 1 mm were randomly applied to the right first metatarsal head of 12 healthy participants for 10 min. SBF at the same site was measured for 10 min before and after local vibration. The degree of regularity of SBF was quantified using a multiscale sample entropy algorithm. The results showed that 100 Hz vibration significantly increased multiscale regularity of SBF but 35 Hz and 0 Hz (sham vibration) did not. The significant increase of regularity of SBF after 100 Hz vibration was mainly attributed to increased regularity of SBF oscillations within the frequency interval at 0.0095–0.15 Hz. These findings support the use of multiscale regularity to assess effectiveness of local vibration on improving skin blood flow.

Using laser Doppler flowmetry with wavelet analysis to study skin blood flow regulations after cupping therapy

BACKGROUND

The purpose of this study was to use laser Doppler flowmetry (LDF) with wavelet analysis to investigate skin blood flow control mechanisms in response to various intensities of cupping therapy. To the best of our knowledge, this is the first study to assess skin blood flow control mechanism in response to cupping therapy using wavelet analysis of laser Doppler blood flow oscillations.

MATERIALS AND METHODS

Twelve healthy participants were recruited for this repeated-measures study. Three different intensities of cupping therapy were applied using 3 cup sizes at 35, 40, and 45 mm (in diameter) with 300 mm Hg negative pressure for 5 minutes. LDF was used to measure skin blood flow (SBF) on the triceps before and after cupping therapy. Wavelet analysis was used to analyze the blood flow oscillations (BFO) to assess blood flow control mechanisms.

RESULTS

The wavelet amplitudes of metabolic and cardiac controls after cupping therapy were higher than those before cupping therapy. For the metabolic control, the 45-mm cupping protocol (1.65 ± 0.09) was significantly higher than the 40-mm cupping protocol (1.40 ± 0.10, P < .05) and the 35-mm cupping protocol (1.35 ± 0.12, P < .05). No differences were showed in the cardiac control among the 35-mm (1.61 ± 0.20), 40-mm (1.64 ± 0.24), and 45-mm (1.27 ± 0.25) cupping protocols.

CONCLUSION

The metabolic and cardiac controls significantly contributed to the increase in SBF after cupping therapy. Different intensities of cupping therapy caused different responses within the metabolic control and not the cardiac control.

Microvascular Control Mechanism of the Plantar Foot in Response to Different Walking Speeds and Durations: Implication for the Prevention of Foot Ulcers

Physical activity has been recommended by the American Diabetes Association (ADA) as a preventive intervention of diabetes complications. However, there is no study investigating how microvascular control mechanism respond to different walking intensities in people with and without diabetes. The purpose of this study was to assess microvascular control mechanism of the plantar foot in response to various walking speeds and durations in 12 healthy people using spectral analysis of skin blood flow (SBF) oscillations. A 3×2 factorial design, including 3 speeds (3, 6, and 9 km/h) and 2 durations (10 and 20 minutes), was used in this study. Plantar SBF was measured using laser Doppler flowmetry over the first metatarsal head. Borg Rating of Perceived Exertion (RPE) scale and heart rate maximum were used to assess the walking intensity. Wavelet analysis was used to quantify regulations of metabolic (0.0095-0.02 Hz), neurogenic (0.02-0.05 Hz), myogenic (0.05-0.15 Hz), respiratory (0.15-0.4 Hz), and cardiac (0.4-2 Hz) controls. For 10-minute walking, walking at 9 km/h significantly increased the ratio of wavelet amplitudes of metabolic, neurogenic, myogenic, respiratory, and cardiac mechanisms compared with 3 km/h (P < .05). For 20-minute walking, walking at 6 km/h significantly increased the ratio of wavelet amplitudes of metabolic, myogenic, respiratory, and cardiac compared with 3 km/h (P < .05). RPE showed a significant interaction between the speed and duration factors (P < .01). This is the first study demonstrating that different walking speeds and durations caused different plantar microvascular regulations.

Effects of Preconditioning Local Vibrations on Subsequent Plantar Skin Blood Flow Response to Walking

Weight-bearing exercise such as walking may increase risk of foot ulcers in people with diabetes mellitus (DM) because of plantar ischemia due to repetitive, high plantar pressure. Applications of local vibrations on plantar tissues as a preconditioning intervention before walking may reduce plantar tissue ischemia during walking. The objective of this study was to explore whether preconditioning local vibrations reduce reactive hyperemia after walking. A double-blind, repeated-measures, and crossover design was tested in 10 healthy participants without DM. The protocol included 10-minute baseline, 10-minute local vibrations (100 Hz or sham), 10-minute walking, and 10-minute recovery periods. The order of local vibrations was randomly assigned. Skin blood flow (SBF) was measured over the first metatarsal head during baseline and recovery periods. SBF responses were characterized as peak SBF, total SBF, and recovery time of reactive hyperemia. SBF was expressed as a ratio of recovery to baseline SBF to quantify the changes. Peak SBF in the vibration protocol (6.98 ± 0.87) was significantly lower than the sham control (9.26 ± 1.34, P < .01). Total SBF in the vibration protocol ([33.32 ± 7.98] × 103) was significantly lower than the sham control ([48.09 ± 8.9] × 103, P < .05). The recovery time in the vibration protocol (166.08 ± 32.71 seconds) was not significantly different from the sham control (223.53 ± 38.85 seconds, P = .1). Local vibrations at 100 Hz could reduce walking-induced hyperemic response on the first metatarsal head. Our finding indicates that preconditioning local vibrations could be a potential preventive intervention for people at risk for foot ulcers.

Impact of local thermal stimulation on the correlation between oxygen saturation and speed-resolved blood perfusion

The physiologically important relationship between oxygen saturation and blood flow is not entirely understood, particularly with regard to the multiple velocity components of flow and temperature. While our previous studies used classic laser Doppler flowmetry combined with an enhanced perfusion probe to assess local blood flow following thermal stimulation, oxygen saturation signals were not assessed. Thus, the current study used multiscale entropy (MSE) and multiscale fuzzy entropy (MFE) to measure the complexity of oxygen saturation signals following thermal stimulation in healthy subjects. The results indicate that thermal stimulation increases oxygen saturation and affects the measured signal complexity in a temperature-dependent fashion. Furthermore, stimulus temperature not only affects the correlation between speed-resolved blood perfusion and oxygen saturation, but also the correlation between the complexity area indices (CAI) of the two signals. These results reflect the complexity of local regulation and adaptation processes in response to stimuli at different temperatures.

Effect of Exercise on Risk Factors of Diabetic Foot Ulcers: A Systematic Review and Meta-Analysis

The objectives of this study were to examine the effectiveness of different types of exercise on risk factors of diabetic foot ulcers, including glycated hemoglobin, peripheral arterial disease, and diabetic peripheral neuropathy, in people with type 2 diabetes mellitus. PubMed, Web of Science, Cochrane Library, Scopus, and CINAHL were searched from inception to January 2018 for relevant articles. Eligible studies were randomized controlled trials that examined effects of exercise on the selected risk factors. Twenty randomized controlled trials with 1357 participants were included in the meta-analyses. The differences in postintervention values of glycated hemoglobin and ankle brachial index between exercise and control groups were synthesized, yielding mean differences of -0.45% (P < 0.00001) and 0.03 (P = 0.002), respectively; the differences in within-group changes in glycated hemoglobin were synthesized, yielding mean differences of -0.19% (P = 0.1), -0.25% (P = 0.0006), and -0.64% (P = 0.006) for aerobic versus resistance, combined versus aerobic, and combined versus resistance exercise, respectively. Exercise has a significant effect on reducing glycated hemoglobin, whereas combined exercise is more effective compared with aerobic or resistance exercise alone. Exercise also improves ankle brachial index. However, evidence regarding the association between exercise and peripheral neuropathy and risks of diabetic foot ulcers in people with type 2 diabetes mellitus remains insufficient.

Exploring the relationship between the speed-resolved perfusion of blood flux and HRV following different thermal stimulations using MSE and MFE analyses

Our previous study employed the classic laser Doppler flux (LDF) to explore the complexity of local blood flow signals and their relationship with heart rate variability (HRV). However, microcirculation blood flow is composed of different velocity components. To investigate the complexity of local speed-resolved perfusion and HRV following stimulation with different temperatures in healthy subjects, multiscale entropy (MSE) and multiscale fuzzy entropy (MFE) were used to measure the complexity of local speed-resolved perfusion signals. MSE was also used to evaluate the complexity of HRV. The results indicated that thermal stimulation increased all components of local speed-resolved perfusion and that stimulation with different temperatures resulted in different changes in the complexity area index. However, the same stimulation had no effect on the MSE of HRV. Further research showed that 44°C thermal stimulation resulted in a weak correlation between the composite speed-resolved perfusion and the HRV complexity. The current study provides a new approach for studying the relationship between speed-resolved perfusion signals and cardiac function.

Optical probe pressure effects on cutaneous blood flow

The variation of blood flow characteristics caused by the probe pressure during noninvasive studies is of particular interest within the context of fundamental and applied research. It has been shown previously that the weak local pressure induces vasodilation, whereas the increased pressure is able to stop the blood flow in the compressed area, as well as to significantly change optical signals.The blood flow oscillations measured by laser Doppler flowmetry (LDF) characterize the functional state of the microvascular system and can be used for noninvasive diagnostics of its abnormality. This study was intended to identify the patterns of the relationship between the oscillating components of blood flow registered by the LDF method under different levels of pressure applied to an optical fiber probe.For this purpose, we have developed an original optical probe capable of regulating the applied pressure. The developed protocol included six sequential records of the blood perfusion at a pressure within the 0 to 200 mmHg range with unloading at the last stage.Using wavelet analyses, we traced the variation of energy of oscillations for these records in five frequency bands associated with different vascular tone regulation mechanisms. Six young volunteers of the same age (three males and three females) were included in this preliminary study and the protocol was repeated five times in each volunteer. Accordingly, 30 LDF records were available for the analyses. As expected, the LDF signal increases at weak pressure (30 mmHg) and decreases at increased pressure. The statistically stable amplification of endothelial associated blood flow oscillations under the 90 mmHg pressure allowed us to put forward a hypothesis that the endothelial activity increases. The possible causes of this phenomenon are discussed.

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.

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.

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.