Effect of functional electrostimulation on impaired skin vasodilator responses to local heating in spinal cord injury

Impact of tetraplegia vs. paraplegia on venoarteriolar, myogenic and maximal cutaneous vasodilation responses of the microvasculature: Implications for cardiovascular disease

Objective: Cardiovascular disease (CVD) is a leading cause of mortality in persons with SCI. While macrovascular remodeling and function after SCI is well documented, changes in the microvascular structure and function are comparably understudied, but importantly predict CVD risk. Specifically, the integrity of venoarteriolar (VAR), myogenic (MYO) and maximal vasodilation responses are largely unknown after SCI, especially in persons with tetraplegia (TP) at highest risk of CVD. This is the first to examine the differences in VAR (cuff inflation), MYO (limb dependency) and maximal vasodilation responses of the microvasculature between able bodied (AB) versus those with TP and paraplegia (PP).Design: Observational.Setting: Laboratory.Participants: Eight AB, 6 TP, and 8 PP persons.Interventions: One forearm and calf were treated topically with lidocaine 2.5%/prilocaine 2.5% while contralateral limb served as a control. Laser doppler flowmeters were applied over treated and control sites during limb dependency, cuff inflation and local skin heating (Tloc) up to 42°C.Outcome measures: Skin vascular resistance (SkVR) change with cuff inflation and limb dependency and maximal cutaneous vascular conductance (CVC) during local heating.Results: Change in SkVR was not significantly different between groups or extremity (upper vs. lower) during cuff inflation or limb dependency. However, CVC at Tloc 42°C was significantly different in the lower extremity (LE) of TP and PP (P = 0.007, 0.35) compared to AB.Conclusion: Increases in SkVR during cuff inflation (VAR) and limb dependency (VAR and MYO) are unaltered after SCI, however maximal vasodilation in the LE post-SCI is higher than AB persons.

Femoral Artery Blood Flow and Microcirculatory Perfusion During Acute, Low-Level Functional Electrical Stimulation in Spinal Cord Injury

OBJECTIVE

Functional electrical stimulation (FES) may help to reduce the risk of developing macrovascular and microvascular complications in people with spinal cord injury. Low-intensity FES has significant clinical potential because this can be applied continuously throughout the day. This study examines the acute effects of low-intensity FES using wearable clothing garment on vascular blood flow and oxygen consumption in people with spinal cord injury.

DESIGN

This was a cross-sectional observation study.

METHODS

Eight participants with a motor complete spinal cord injury received four 3-min unilateral FES to the gluteal and hamstring muscles. Skin and deep femoral artery blood flow and oxygen consumption were measured at baseline and during each bout of stimulation.

RESULTS

Femoral artery blood flow increased by 18.1% with the application of FES (P = 0.02). Moreover, femoral artery blood flow increased further during each subsequent block of FES (P = 0.004). Skin perfusion did not change during an individual block of stimulation (P = 0.66). Skin perfusion progressively increased with each subsequent bout (P < 0.001). There was no change in femoral or skin perfusion across time in the nonstimulated leg (all P > 0.05).

CONCLUSION

Low-intensity FES acutely increased blood flow during stimulation, with a progressive increase across subsequent FES bouts. These observations suggest that continuous, low-intensity FES may represent a practical and effective strategy to improve perfusion and reduce the risk of vascular complications.

Impact of passive heat stress on persons with spinal cord injury: Implications for Olympic spectators

Environmental heat stress can negatively impact health, work capacity, and athletic performance and potentially to lead to life-threatening consequences if not mitigated. With the upcoming Toyko Olympic games to be held during anticipated warm ambient temperatures (up to 29°C), and with spectators potentially spending long durations of time outdoors, certain populations of persons with impaired thermoregulatory capacity will be at higher risk of heat-related illness from passive heat stress. Persons with spinal cord injury (SCI) are one of these groups as a result of a decentralized sympathetic nervous system, which leaves them with impairment in convective and evaporative cooling via vasodilation and sweating, respectively. This review summarizes (1) thermoregulatory physiological responses of persons with SCI under passive heat stress: the effect of level and completeness of injury; (2) the impact of passive heat stress on quality of life (QOL), outdoor participation, behavioral thermoregulation, and cognition; (3) recommendations and education for clinicians providing health care for persons with SCI; and (4) suggestions of future directions for exploring the gaps in the literature on passive heat stress in persons with SCI. This article aims to equip consumers with SCI and health-care professionals with the most up-to-date knowledge on passive heat stress responses in persons with SCI, so that their attendance at the Olympic games can be done with maximal safety and enjoyment.

Acute heat stress reduces biomarkers of endothelial activation but not macro- or microvascular dysfunction in cervical spinal cord injury

Cardiovascular diseases (CVD) are highly prevalent in spinal cord injury (SCI), and peripheral vascular dysfunction might be a contributing factor. Recent evidence demonstrates that exposure to heat stress can improve vascular function and reduce the risk of CVD in uninjured populations. We therefore aimed to examine the extent of vascular dysfunction in SCI and the acute effects of passive heating. Fifteen participants with cervical SCI and 15 uninjured control (CON) participants underwent ultrasound assessments of vascular function and venous blood sampling for biomarkers of endothelial activation (i.e., CD62e⁺) and apoptosis (i.e., CD31⁺/42b^-) before and after a 60-min exposure to lower limb hot water immersion (40°C). In SCI, macrovascular endothelial function was reduced in the brachial artery [SCI: 4.8 (3.2)% vs. CON: 7.6 (3.4)%, P = 0.04] but not the femoral artery [SCI: 3.7 (2.6)% vs. CON: 4.0 (2.1)%, P = 0.70]. Microvascular function, via reactive hyperemia, was ~40% lower in SCI versus CON in both the femoral and brachial arteries ( P < 0.01). Circulating concentrations of CD62e⁺ were elevated in SCI versus CON [SCI: 152 (106) microparticles/µl vs. CON: 58 (24) microparticles/µl, P < 0.05]. In response to heating, macrovascular and microvascular function remained unchanged, whereas increases (+83%) and decreases (-93%) in antegrade and retrograde shear rates, respectively, were associated with heat-induced reductions of CD62e⁺ concentrations in SCI to levels similar to CON ( P = 0.05). These data highlight the potential of acute heating to provide a safe and practical strategy to improve vascular function in SCI. The chronic effects of controlled heating warrant long-term testing. NEW & NOTEWORTHY Individuals with cervical level spinal cord injury exhibit selectively lower flow-mediated dilation in the brachial but not femoral artery, whereas peak reactive hyperemia was lower in both arteries compared with uninjured controls. After 60 min of lower limb hot water immersion, femoral artery blood flow and shear patterns were acutely improved in both groups. Elevated biomarkers of endothelial activation in the spinal cord injury group decreased with heating, but these biomarkers remained unchanged in controls.

Cutaneous microvascular perfusion responses to insulin iontophoresis are differentially affected by insulin resistance after spinal cord injury

NEW FINDINGS

What is the central question of this study? What impact does insulin resistance have on cutaneous perfusion responses to insulin iontophoresis in vascular beds with markedly reduced or functionally ablated sympathetic nervous system vasomotor function resulting from spinal cord injury? What is the main finding and its importance? Persons with spinal cord injury have sublesional microvascular endothelial dysfunction, as indicated by a blunted cutaneous perfusion response to acetylcholine iontophoresis, and the presence of insulin resistance has a further confounding effect on endothelium-mediated changes to cutaneous perfusion in the lower extremities. Endothelium-mediated mechanisms that regulate skin blood flow might play an integral role in optimizing skin perfusion in vascular beds with sympathetic nervous system vasomotor impairment, such as in spinal cord injury (SCI). Insulin is a vasoactive hormone and second messenger of nitric oxide that facilitates endothelium-mediated dilatation. The effects of insulin resistance (IR) on sublesional cutaneous perfusion responses to insulin provocation have yet to be described in persons with SCI. Persons with SCI and an able-bodied (AB) cohort were divided into subgroups based upon fasting plasma insulin concentration cut-offs for IR (≥13.13 mIU ml^-1 ) or insulin sensitivity (IS; <13.13 mIU ml^-1 ), as follows: AB, IS (ABIS, n = 21); SCI, IS (SCIS, n = 21); AB, IR (ABIR, n = 9); and SCI, IR (SCIR, n = 11). Laser Doppler flowmetry characterized peak blood perfusion unit (BPU) responses (percentage change from baseline) to insulin, acetylcholine or placebo iontophoresis in the lower extremities; BPU responses were log₁₀ transformed to facilitate comparisons, and the net insulin response (NetIns) BPU response was calculated (insulin minus placebo BPU response). The NetIns was significantly greater in both IS groups compared with their corresponding IR group. The acetylcholine-mediated BPU responses in the SCI subgroups were significantly lower than those in the ABIS group. The proportional BPU responses of NetIns to acetylcholine in the IS cohorts (i.e. ABIS and SCIS) were significantly greater (P < 0.05) than that of each IR subgroup. The presence of IR has a confounding effect on sublesional microvascular endothelium-mediated cutaneous perfusion responses to provocation. Preservation of endothelial sensitivity to its agonists appears to be an important modifiable risk factor to optimize cutaneous perfusion in the lower extremities of persons with SCI.

Effects of vehicle microdialysis solutions on cutaneous vascular responses to local heating

Microdialysis is a minimally invasive technique often paired with laser Doppler flowmetry to examine cutaneous microvascular function, yet presents with several challenges, including incompatibility with perfusion of highly lipophilic compounds. The present study addresses this methodological concern, with an emphasis on the independent effects of commonly used vehicle dialysis solutions to improve solubility of pharmacological agents with otherwise low aqueous solubility. Four microdialysis fibers were placed in the ventral forearm of eight subjects (4 men, 4 women; 25 ± 1 yr) with sites randomized to serve as 1) control (lactated Ringer's), 2) Sodium carbonate-bicarbonate buffer administered at physiological pH [SCB-HCl; pH 7.4, achieved via addition of hydrochloric acid (HCl)], 3) 0.02% ethanol, and 4) 2% dimethyl sulfoxide (DMSO). After baseline (34°C), vehicle solutions were administered throughout a standardized local heating protocol to 42°C. Laser Doppler flowmetry provided an index of blood flow. Cutaneous vascular conductance was calculated and normalized to maximum (%CVC_max, sodium nitroprusside and 43°C local heat). The SCB-HCl solution increased baseline %CVC_max (control: 9.7 ± 0.8; SCB-HCl: 21.5 ± 3.5%CVC_max; P = 0.03), but no effects were observed during heating or maximal vasodilation. There were no differences with perfusion of ethanol or DMSO at any stage of the protocol ( P > 0.05). These data demonstrate the potential confounding effects of some vehicle dialysis solutions on cutaneous vascular function. Notably, this study provides evidence that 2% DMSO and 0.02% ethanol are acceptable vehicles with no confounding local vascular effects to a standardized local heating protocol at the concentrations presented. NEW & NOTEWORTHY This study examined the independent effects of common vehicle solutions on cutaneous vascular responses. A basic buffer (SCB-HCl) caused baseline vasodilation; 2% DMSO and 0.02% ethanol had no effects. This highlights the need for considering potential confounding effects of solubilizing solutions when combined with low aqueous soluble pharmacological agents. Importantly, DMSO and ethanol do not appear to influence cutaneous vascular function during baseline or local heating at the concentrations studied, allowing their use without confounding effects.

Changes in sympathetic neurovascular function following spinal cord injury

The effects of spinal cord injury (SCI) on sympathetic neurovascular transmission have generally been ignored. This review describes changes in sympathetic nerve-mediated activation of arterial vessels to which ongoing sympathetic activity has been reduced or silenced following spinal cord transection in rats. In all vessels studied in rats, SCI markedly enhanced their contractile responses to nerve activity. However, the mechanisms that augment neurovascular transmission differ between the rat tail artery and mesenteric artery. In tail artery, the enhancement of neurovascular transmission cannot be attributed to changes in sensitivity of the vascular muscle to α₁- or α₂-adrenoceptor agonists. Instead the contribution of L-type Ca²⁺ channels to activation of the smooth muscle by nerve-released noradrenaline is greatly increased following SCI. By contrast, mesenteric arteries from SCI rats had increased sensitivity to phenylephrine but not to methoxamine. While both phenylephrine and methoxamine are α₁-adrenoceptor agonists, only phenylephrine is a substrate for the neuronal noradrenaline transporter. Therefore the selective increase in sensitivity to phenylephrine suggests that the activity of the neuronal noradrenaline transporter is reduced. While present evidence suggests that sympathetic vasoconstrictor neurons do not contribute to the normal regulation of peripheral resistance below a complete SCI in humans, the available evidence does indicate that these experimental findings in animals are likely to apply after SCI in humans and contribute to autonomic dysreflexia.

Acute black tea consumption improves cutaneous vascular function in healthy middle-aged humans

BACKGROUND & AIMS

Dietary flavonoids, such as those present in black tea, are associated with reduced risk of cardiovascular disease (CVD), possibly through improving nitric oxide (NO) mediated vascular function. The aim of this study was to examine the effect of acute black tea ingestion on cutaneous microvascular function.

METHODS

Twenty healthy participants (58 ± 5 y, 9 men) attended two experimental trials (tea, placebo), 7-days apart in a randomised, controlled, double-blind, cross-over design. Participants ingested a single dose of 200 ml black tea or placebo, followed by assessment of forearm cutaneous perfusion using laser-Doppler flowmetry (LDF) using three distinct heating protocols, enabling us to distinguish between axon- and endothelium-dependent vasodilation: 1. rapid 42°C, 2. rapid 39°C and 3. gradual 42°C. On the contralateral arm, full-field laser perfusion imaging (FLPI) was used to assess forearm perfusion during gradual 42°C. Data were presented as cutaneous vascular conductance (CVC; flux/mean arterial pressure, MAP) and CVC expressed as a percentage of maximal CVC (%CVC_max).

RESULTS

Rapid local heating to 39°C or 42°C demonstrated no effect of tea for flux, CVC or %CVC_max (all P > 0.05). Gradual local heating to 42 °C, however, produced a higher skin blood flow following black tea ingestion for absolute CVC (P = 0.04) when measured by LDF, and higher absolute flux (P < 0.001) and CVC (P < 0.001) measured with FLPI. No effect of tea was found for %CVC_max when assessed by either LDF or FLPI.

CONCLUSIONS

Acute tea ingestion enhanced cutaneous vascular responses to gradual local heating to 42 °C in healthy, middle-aged participants, possibly through a mechanism related to activation of endothelium-derived chemical mediators, such as NO. These improvements may contribute to the cardiovascular health benefits of regular tea ingestion.

Passive heat therapy improves cutaneous microvascular function in sedentary humans via improved nitric oxide-dependent dilation

Passive heat therapy (repeated hot tub or sauna use) reduces cardiovascular risk, but its effects on the mechanisms underlying improvements in microvascular function have yet to be studied. We investigated the effects of heat therapy on microvascular function and whether improvements were related to changes in nitric oxide (NO) bioavailability using cutaneous microdialysis. Eighteen young, sedentary, otherwise healthy subjects participated in 8 wk of heat therapy (hot water immersion to maintain rectal temperature ≥38.5°C for 60 min/session; n = 9) or thermoneutral water immersion (sham, n = 9), and participated in experiments before and after the 8-wk intervention in which forearm cutaneous hyperemia to 39°C local heating was assessed at three microdialysis sites receiving 1) Lactated Ringer's (Control), 2) N(ω)-nitro-l-arginine (l-NNA; nonspecific NO synthase inhibitor), and 3) 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (Tempol), a superoxide dismutase mimetic. The arm used for microdialysis experiments remained out of the water at all times. Data are means ± SE cutaneous vascular conductance (CVC = laser Doppler flux/mean arterial pressure), presented as percent maximal CVC (% CVCmax). Heat therapy increased local heating plateau from 42 ± 6 to 53 ± 6% CVCmax (P < 0.001) and increased NO-dependent dilation (difference in plateau between Control and l-NNA sites) from 26 ± 6 to 38 ± 4% CVCmax (P < 0.01), while no changes were observed in the sham group. When data were pooled across all subjects at 0 wk, Tempol had no effect on the local heating response (P = 0.53 vs. Control). There were no changes at the Tempol site across interventions (P = 0.58). Passive heat therapy improves cutaneous microvascular function by improving NO-dependent dilation, which may have clinical implications.

Seven-day remote ischemic preconditioning improves local and systemic endothelial function and microcirculation in healthy humans

BACKGROUND

Ischemic preconditioning (IPC) protects tissue against ischemia-induced injury inside and outside ischemic areas. The purpose was to examine the hypothesis that daily IPC leads to improvement in endothelial function and skin microcirculation not only in the arm exposed to IPC but also in the contralateral arm.

METHODS

Thirteen healthy, young, normotensive male individuals (aged 22±2 years) were assigned to 7-day daily exposure of the arm to IPC (4×5 minutes). Assessment of brachial artery endothelial function (using flow-mediated dilation (FMD)) and forearm microcirculation (cutaneous vascular conductance (CVC) at baseline and during local heating) was performed before and after 7 days to examine the local (i.e., intervention arm) and remote (i.e., control arm) effect of IPC. We repeated the assessment tests 8 days after the intervention (Post+8).

RESULTS

FMD increased after repeated IPC (P = 0.03) and remained significantly elevated at Post+8 in the intervention (5.0±2.2%, 6.1±2.2%, and 6.6±2.3%) and contralateral arms (5.4±2.2%, 6.0±2.2%, and 7.5±2.2%). Forearm CVC also increased following repeated IPC (P = 0.006) and remained elevated at Post+8 in both arms (intervention: 0.12±0.03, 0.14±0.04, 0.16±0.04 mV/mm Hg; contralateral: 0.14±0.04, 0.015±0.04, 0.17±0.07). No interaction between IPC arm and time was evident for FMD and CVC (both P > 0.05). IPC intervention did not alter CVC responses to local heating (P > 0.05).

CONCLUSIONS

Daily exposure to IPC for 7 days leads to local and remote improvements in brachial artery FMD and resting skin microcirculation that remain after cessation of the intervention and beyond the late phase of protection. These findings may have clinical relevance for micro- and macrovascular improvements.

Expression of genes involved in fatty acid transport and insulin signaling is altered by physical inactivity and exercise training in human skeletal muscle

Physical deconditioning is associated with the development of chronic diseases, including type 2 diabetes and cardiovascular disease. Exercise training effectively counteracts these developments, but the underlying mechanisms are largely unknown. To gain more insight into these mechanisms, muscular gene expression levels were assessed after physical deconditioning and after exercise training of the lower limbs in humans by use of gene expression microarrays. To exclude systemic effects, we used human models for local physical inactivity (3 wk of unilateral limb suspension) and for local exercise training (6 wk of functional electrical stimulation exercise of the extremely deconditioned legs of individuals with a spinal cord injury). The most interesting subset of genes, those downregulated after deconditioning as well as upregulated after exercise training, contained 18 genes related to both the "insulin action" and "adipocytokine signaling" pathway. Of these genes, the three with strongest up/downregulation were the muscular fatty acid-binding protein-3 (FABP3), the fatty acid oxidizing enzyme hydroxyacyl-CoA dehydrogenase (HADH), and the mitochondrial fatty acid transporter solute carrier 25 family member A20 (SLC25A20). The expression levels of these genes were confirmed using RT-qPCR. The results of the present study indicate an important role for a decreased transport and metabolism of fatty acids, which provides a link between physical activity levels and insulin signaling.

Exercise training and the control of skin blood flow in older adults

The ability to control skin blood flow decreases with primary aging, making older adults less able to adequately thermoregulate and repair cutaneous wounds. Lifestyle factors such as physical activity, diet, and smoking might interact with the aging process to modulate "normal" age-associated changes in the cutaneous microcirculation. The main focus of this brief review is the effects of exercise training on the control of skin blood flow in older adults.

Blood vessel remodeling and physical inactivity in humans

Physical inactivity is associated with an increase in cardiovascular risk that cannot be fully explained by traditional or novel risk factors. Inactivity is also associated with changes in hemodynamic stimuli, which exert direct effects on the vasculature leading to remodeling and a proatherogenic phenotype. In this review, we synthesize and summarize in vivo evidence relating to the impact of local and systemic models of physical inactivity on conduit arteries, resistance vessels, and the microcirculation in humans. Taken together, the literature suggests that a rapid inward structural remodeling of vessels occurs in response to physical inactivity. The magnitude of this response is dependent on the "dose" of inactivity. Moreover, changes in vascular function are found at resistance and microvessel levels in humans. In conduit arteries, a strong interaction between vascular function and structure is present, which results in conflicting data regarding the impact of inactivity on conduit artery function. While much of the cardioprotective effect of exercise is related to the nitric oxide pathway, deconditioning may primarily be associated with activation of vasoconstrictor pathways. The effects of deconditioning on the vasculature are therefore not simply the opposite of those in response to exercise training. Given the importance of sedentary behavior, future studies should provide further insight into the impact of inactivity on the vasculature and other (novel) markers of vascular health. Moreover, studies should examine the role of (hemodynamic) stimuli that underlie the characteristic vascular adaptations during deconditioning. Our review concludes with some suggestions for future research directions.

Aging and aerobic fitness affect the contribution of noradrenergic sympathetic nerves to the rapid cutaneous vasodilator response to local heating

Sedentary aging results in a diminished rapid cutaneous vasodilator response to local heating. We investigated whether this diminished response was due to altered contributions of noradrenergic sympathetic nerves by assessing 1) the age-related decline and 2) the effect of aerobic fitness. Using laser-Doppler flowmetry, we measured skin blood flow (SkBF) in young (24 ± 1 yr) and older (64 ± 1 yr) endurance-trained and sedentary men ( n = 7 per group) at baseline and during 35 min of local skin heating to 42°C at 1) untreated forearm sites, 2) forearm sites treated with bretylium tosylate (BT), which prevents neurotransmitter release from noradrenergic sympathetic nerves, and 3) forearm sites treated with yohimbine + propranolol (YP), which antagonizes α- and β-adrenergic receptors. SkBF was converted to cutaneous vascular conductance (CVC = SkBF/mean arterial pressure) and normalized to maximal CVC (%CVCmax) achieved by skin heating to 44°C. Pharmacological agents were administered using microdialysis. In the young trained group, the rapid vasodilator response was reduced at BT and YP sites ( P < 0.05); by contrast, in the young sedentary and older trained groups, YP had no effect ( P > 0.05), but BT did ( P > 0.05). Neither BT nor YP affected the rapid vasodilator response in the older sedentary group ( P > 0.05). These data suggest that the age-related reduction in the rapid vasodilator response is due to an impairment of sympathetic-dependent mechanisms, which can be partly attenuated with habitual aerobic exercise. Rapid vasodilation involves noradrenergic neurotransmitters in young trained men and nonadrenergic sympathetic cotransmitters (e.g., neuropeptide Y) in young sedentary and older trained men, possibly as a compensatory mechanism. Finally, in older sedentary men, the rapid vasodilation appears not to involve the sympathetic system.

Thermal provocation to evaluate microvascular reactivity in human skin

With increased interest in predictive medicine, development of a relatively noninvasive technique that can improve prediction of major clinical outcomes has gained considerable attention. Current tests that are the target of critical evaluation, such as flow-mediated vasodilation of the brachial artery and pulse-wave velocity, are specific to the larger conduit vessels. However, evidence is mounting that functional changes in the microcirculation may be an early sign of globalized microvascular dysfunction. Thus development of a test of microvascular reactivity that could be used to evaluate cardiovascular risk or response to treatment is an exciting area of innovation. This mini-review is focused on tests of microvascular reactivity to thermal stimuli in the cutaneous circulation. The skin may prove to be an ideal site for evaluation of microvascular dysfunction due to its ease of access and growing evidence that changes in skin vascular reactivity may precede overt clinical signs of disease. Evaluation of the skin blood flow response to locally applied heat has already demonstrated prognostic utility, and the response to local cooling holds promise in patients in whom cutaneous disorders are present. Whether either of these tests can be used to predict cardiovascular morbidity or mortality in a clinical setting requires further evaluation.