Cathodal current-induced vasodilation to single application and the amplified response to repeated application in humans rely on aspirin-sensitive mechanisms

Current-Induced Vasodilation Specifically Detects, and Correlates With the Time Since, Last Aspirin Intake: An Interventional Study of 830 Patients

BACKGROUND

Galvanic current-induced vasodilation (CIV) is impaired in patients under low-dose aspirin (ASA; ≤ 500 mg/day), but potential covariates and the impact of the time since the last ASA intake are unknown.

OBJECTIVES

We used tissue viability imaging (TiVi) in patients at risk of cardiovascular disease and examined its association with self-reported treatments.

PATIENTS/METHODS

We recorded the age, gender, height, weight, smoking status, and use of 14 different drug categories in 822 patients either with known peripheral artery disease or at risk thereof. The difference between TiVi arbitrary units (TAUs) where stimulation was applied and an adjacent skin area was recorded, as well as the time since the last ASA intake. Step-by-step regression analysis was used to determine the factors that affect CIV amplitude.

RESULTS AND CONCLUSIONS

CIV was 28.2 ± 22.9 vs. 14.6 ± 18.0 TAUs (P < 0.001) in patients treated with ASA (n = 287) and not treated with ASA (n = 535), respectively. The main determinants of CIV amplitude, by order of importance, were: aspirin intake, diabetes mellitus, age, and male sex. In ASA-treated patients, the main determinants were diabetes mellitus, time since the last ASA intake, male gender, and age. Non-invasive determination of the physiological effects of low-dose ASA is feasible in routine clinical practice. It could be a clinical approach to provide objective evidence of ASA intake, and potentially could be used to test adherence to treatment in ASA-treated patients.

Effects of Ice Massage Prior to an Iontophoresis Treatment Using Dexamethasone Sodium Phosphate

CONTEXT

Low current intensity iontophoresis treatments have increased skin perfusion over 700% from baseline potentially altering drug clearance from or diffusion to the targeted area.

OBJECTIVE

To determine the effects of a preceding 10-minute ice massage on subcutaneous dexamethasone sodium phosphate (Dex-P) concentration and skin perfusion during and after a 4-mA iontophoresis treatment.

DESIGN

Controlled laboratory study.

SETTING

Research laboratory.

PATIENTS OR OTHER PARTICIPANTS

Twenty-four participants (male = 12, female = 12; age = 25.6 [4.5] y, height = 173.9 [8.51] cm, mass = 76.11 [16.84] kg).

INTERVENTION(S)

Participants were randomly assigned into 2 groups: (1) pretreatment 10-minute ice massage and (2) no pretreatment ice massage. Treatment consisted of an 80-mA·minute (4 mA, 20 min) Dex-P iontophoresis treatment. Microdialysis probes (3 mm deep in the forearm) were used to assess Dex-P, dexamethasone (Dex), and its metabolite (Dex-Met) concentrations. Skin perfusion was measured using laser Doppler flowmetry.

MAIN OUTCOME MEASURE(S)

Microdialysis samples were collected at baseline, at conclusion of treatment, and every 20 minutes posttreatment for 60 minutes. Samples were analyzed to determine Dex-Total (Dex-Total = Dex-P + Dex + Dex-Met). Skin perfusion was calculated as a percentage change from baseline. A mixed-design analysis of variance was used to determine Dex-Total and skin perfusion difference between groups overtime.

RESULTS

There was no difference between groups (P = .476), but [Dex-Total] significantly increased over the course of the iontophoresis and posttreatment time (P < .001). Dex-P was measured in 18 of 24 participants with a mean concentration of 0.67 (1.09) μg/mL. Skin perfusion was significantly greater in the no ice treatment group (P = .002). Peak skin perfusion reached 27.74% (47.49%) and 117.39% (103.45%) from baseline for the ice and no ice groups, respectively.

CONCLUSIONS

Ice massage prior to iontophoresis does not alter the tissue [Dex-Total] even with less skin perfusion.

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

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

Mechanisms of electrical vasoconstriction

BACKGROUND

Electrical vasoconstriction is a promising approach to control blood pressure or restrict bleeding in non-compressible wounds. We explore the neural and non-neural pathways of electrical vasoconstriction in-vivo.

METHODS

Charge-balanced, asymmetric pulses were delivered through a pair of metal disc electrodes. Vasoconstriction was assessed by measuring the diameter of rat saphenous vessels stimulated with low-voltage (20 V, 1 ms) and high-voltage (150 V, 10 μs) stimuli at 10 Hz for 5 min. Activation pathways were explored by topical application of a specific neural agonist (phenylephrine, alpha-1 receptor), a non-specific agonist (KCl) and neural inhibitors (phenoxybenzamine, 25 mg/ml; guanethidine, 1 mg/ml). Acute tissue damage was assessed with a membrane permeability (live-dead) fluorescent assay. The Joule heating in tissue was estimated using COMSOL Multiphysics modeling.

RESULTS

During stimulation, arteries constricted to 41 ± 8% and 37 ± 6% of their pre-stimulus diameter with low- and high-voltage stimuli, while veins constricted to 80 ± 18% and 40 ± 11%, respectively. In arteries, despite similar extent of constriction, the recovery time was very different: about 30 s for low-voltage and 10 min for high-voltage stimuli. Neural inhibitors significantly reduced low-voltage arterial constriction, but did not affect high-voltage arterial or venous constriction, indicating that high-voltage stimuli activate non-neural vasoconstriction pathways. Adrenergic pathways predominantly controlled low-voltage arterial but not venous constriction, which may involve a purinergic pathway. Viability staining confirmed that stimuli were below the electroporation threshold. Modeling indicates that heating of the blood vessels during stimulation (< 0.2 °C) is too low to cause vasoconstriction.

CONCLUSIONS

We demonstrate that low-voltage stimuli induce reversible vasoconstriction through neural pathways, while high-voltage stimuli activate non-neural pathways, likely in addition to neural stimulation. Different stimuli providing precise control over the extent of arterial and venous constriction as well as relaxation rate could be used to control bleeding, perfusion or blood pressure.

Evidence for a vasomotor cyclo-oxygenase dependent mechanism of sensitization at the cutaneous level

AIMS

Current-induced vasodilation (CIV) is an axon-reflex response observed during monopolar current application such as iontophoresis. Cyclo-oxygenase derivates (COD) participate in CIV and act as sensitizing agents at the anodal level. Mechanisms involved during cathodal current application (CCA) are partially unknown. In a randomized double-blind crossover trial, we tested in 16 healthy subjects (i) the influence of the inter-stimulation interval (I-I) by comparing CIV following all-at-once 10 s CCA against 2 × 5 s CCA with intervals ranging from15 s-16 min and (ii) the participation of COD in CIV using 1 g aspirin or placebo intake.

METHODS

Measurements were repeated 2 h and 14 days after treatment. Laser Doppler flowmetry assessed cutaneous blood flow, reported in multiples of baseline.

RESULTS

Before treatment, peak vasodilation 10 min after the last current application (CVCstim2 ) increased compared with baseline whatever the I-I. Increase in CVCstim2 from baseline was greater for the 4 min (9.4 (5.3, 10.9) times; median (1(st) percentile, 3(rd) percentile)) and higher I-Is compared with all-at-once delivery (3.0 (2.1, 4.3) times, P < 0.05). The response was similar after placebo but aspirin abolished this vasodilation (increase by 1.2 (1.1, 1.3) times for all-at-once delivery and by 1.5 (1.3, 1.7) ± 0.3 times for 4 min interval, 2 h after aspirin intake) that recovered after 14 days.

CONCLUSIONS

This confirms the participation of COD in CIV with CCA and their sensitizing action. This model can represent an attractive way to study the axon-reflex and sensitizing function of COD in humans.

The time course of dexamethasone delivery using iontophoresis through human skin, measured via microdialysis

STUDY DESIGN

Controlled laboratory study.

OBJECTIVE

To determine the time course of dexamethasone sodium phosphate (Dex-P) during iontophoresis to underlying tissues using microdialysis.

BACKGROUND

In human participants, real-time information of Dex-P transdermal delivery during iontophoresis is unknown.

METHODS

Sixty-four healthy male participants (mean ± SD age, 24.2 ± 3.3 years; height, 181.8 ± 26.1 cm; mass, 82.4 ± 11.8 kg; subcutaneous fat thickness, 0.61 ± 0.19 cm) were randomly assigned into 1 of 6 groups: (1) 1-mA current, 1-mm probe depth; (2) 1-mA current, 4-mm probe depth; (3) 2-mA current, 1-mm probe depth; (4) 2-mA current, 4-mm probe depth; (5) in vivo retrodialysis; and (6) skin perfusion flowmetry. Microdialysis probes were used to assess the combined recovery (Dex-total) of Dex-P, dexamethasone, and its metabolite.

RESULTS

There was no difference in Dex-total between current intensities (P = .99), but a greater amount of Dex-total was recovered superficially at 1 mm compared to the 4-mm depth (P<.0001). Peak concentration mean ± SD values for the 1- and 2-mA currents at 1 mm were 10.8 ± 8.1 and 7.7 ± 5.5 µg/mL, and at 4 mm were 2.0 ± 0.8 and 1.3 ± 0.9 µg/mL, respectively. Peak skin perfusion was 741.4% ± 408.7% and 711.6% ± 260.8% at baseline for 1- and 2-mA intensities, respectively. Skin perfusion returned to baseline levels earlier during 1-mA intensity at a 110 mA · min dose within the treatment, compared to 2 mA at 60 minutes posttreatment.

CONCLUSION

Transdermal delivery of Dex-P during iontophoresis was successfully measured in vivo through human skin. Measurable concentrations of Dex-total were found regardless of current intensity. Although current-induced vasodilation occurred, it did not significantly affect the tissue accumulation of Dex-total.

Cutaneous vasodilator and vasoconstrictor mechanisms in temperature regulation

In this review, we focus on significant developments in our understanding of the mechanisms that control the cutaneous vasculature in humans, with emphasis on the literature of the last half-century. To provide a background for subsequent sections, we review methods of measurement and techniques of importance in elucidating control mechanisms for studying skin blood flow. In addition, the anatomy of the skin relevant to its thermoregulatory function is outlined. The mechanisms by which sympathetic nerves mediate cutaneous active vasodilation during whole body heating and cutaneous vasoconstriction during whole body cooling are reviewed, including discussions of mechanisms involving cotransmission, NO, and other effectors. Current concepts for the mechanisms that effect local cutaneous vascular responses to local skin warming and cooling are examined, including the roles of temperature sensitive afferent neurons as well as NO and other mediators. Factors that can modulate control mechanisms of the cutaneous vasculature, such as gender, aging, and clinical conditions, are discussed, as are nonthermoregulatory reflex modifiers of thermoregulatory cutaneous vascular responses.

Reducing transcranial direct current stimulation-induced erythema with skin pretreatment: considerations for sham-controlled clinical trials

OBJECTIVES

Transcranial direct current stimulation (tDCS)-induced erythema (skin reddening) has been described as an adverse effect that can harm blinding integrity in sham-controlled designs. To tackle this issue, we investigated whether the use of topical pretreatments could decrease erythema and other adverse effects associated with tDCS.

MATERIALS AND METHODS

Thirty healthy volunteers were recruited, and four interventions were applied 30 min prior to tDCS in a Latin square design: placebo, ketoprofen 2%, hydroxyzine 1%, and lidocaine 5%. TDCS was applied for 30 min (2 mA, anode and cathode over F3 and F4, respectively) in two active sessions with a minimum 1-week interval. The Draize erythema scoring system scale was used to assess erythema intensity; a tDCS questionnaire was used to assess other adverse effects (e.g., tingling, itching, burning sensation, and pain).

RESULTS

We found that ketoprofen (but not hydroxyzine or lidocaine) significantly attenuated tDCS-induced erythema regarding intensity and duration, with a medium effect compared with placebo. Erythema was overall mild, short-lived (lasting 18-24 min after tDCS ending), and more intense under the anode. Subjects with darker skin color also tended to present less intense tDCS-induced erythema. The prevalence of other adverse effects was low and did not differ between dermatological groups.

CONCLUSIONS

Ketoprofen 2% topical pretreatment might be an interesting strategy to reduce tDCS-induced erythema and might be useful for blinding improvement in further sham-controlled tDCS trials.

High frequency electrical stimulation concurrently induces central sensitization and ipsilateral inhibitory pain modulation

BACKGROUND

In healthy humans, analgesia to blunt pressure develops in the ipsilateral forehead during various forms of limb pain. The aim of the current study was to determine whether this analgesic response is induced by ultraviolet B radiation (UVB), which evokes signs of peripheral sensitization, or by high-frequency electrical stimulation (HFS), which triggers signs of central sensitization.

METHODS

Before and after HFS and UVB conditioning, sensitivity to heat and to blunt and sharp stimuli was assessed at and adjacent to the treated site in the forearm. In addition, sensitivity to blunt pressure was measured bilaterally in the forehead. The effect of ipsilateral versus contralateral temple cooling on electrically evoked pain in the forearm was then examined, to determine whether HFS or UVB conditioning altered inhibitory pain modulation.

RESULTS

UVB conditioning triggered signs of peripheral sensitization, whereas HFS conditioning triggered signs of central sensitization. Importantly, ipsilateral forehead analgesia developed after HFS but not UVB conditioning. In addition, decreases in electrically evoked pain at the HFS-treated site were greater during ipsilateral than contralateral temple cooling, whereas decreases at the UVB-treated site were similar during both procedures.

CONCLUSIONS

HFS conditioning induced signs of central sensitization in the forearm and analgesia both in the ipsilateral forehead and the HFS-treated site. This ipsilateral analgesia was not due to peripheral sensitization or other non-specific effects, as it failed to develop after UVB conditioning. Thus, the supra-spinal mechanisms that evoke central sensitization might also trigger a hemilateral inhibitory pain modulation process. This inhibitory process could sharpen the boundaries of central sensitization or limit its spread.

Platelet inhibition by low-dose aspirin but not by clopidogrel reduces the axon-reflex current-induced vasodilation in humans

We previously showed a prolonged inhibition of current-induced vasodilation (CIV) after a single oral high dose of aspirin. In this study, we tested the hypothesis of platelet involvement in CIV. Nine healthy volunteers took 75 mg aspirin/day, 98 mg of clopidogrel bisulfate/day, or placebo for 4 days. CIV was induced by two consecutive 1-min anodal current applications (0.08 mA/cm(2)) through deionized water with a 10-min interval. CIV was measured with laser Doppler flowmetry and expressed as a percentage of baseline cutaneous vascular conductance: %C(b). In a second experiment in 10 volunteers, aspirin and placebo were given as in experiment 1, but a 26-h delay from the last aspirin intake elapsed before ACh iontophoresis and postocclusive hyperemia were studied in parallel to CIV. In experiment 1, the means +/- SE amplitude of CIV was 822 +/- 314, 313 +/- 144, and 746 +/- 397%C(b) with placebo, aspirin (P < 0.05 from placebo and clopidogrel), and clopidogrel (NS from placebo), respectively. In experiment 2, CIV impairment with aspirin was confirmed: CIV amplitudes were 300 +/- 99, and 916 +/- 528%C(b) under aspirin and placebo, respectively (P < 0.05), whereas vasodilation to ACh iontophoresis (322 +/- 74 and 365 +/- 104%C(b)) and peak postocclusive hyperemia (491 +/- 137 and 661 +/- 248%C(b)) were not different between aspirin and placebo, respectively. Low-dose aspirin, even 26 h after oral administration, impairs CIV, while ACh-mediated vasodilation and postocclusive hyperemia are preserved. If platelets are involved in the neurovascular mechanism triggered by galvanic current application in humans, it is likely to occur through the cyclooxygenase but not the ADP pathway.

WISE 2005: chronic bed rest impairs microcirculatory endothelium in women

Sedentary behavior has deleterious effects on the cardiovascular system, including reduced endothelial functions. A 2-mo bed rest study in healthy women [women international space simulation for exploration (WISE) 2005 program] presented a unique opportunity to analyze the specific effects of prolonged inactivity without other vascular risk factors on the endothelium. We investigated endothelial properties before and after 56 days of bed rest in 8 subjects who performed no exercise (control group: No-EX) and in 8 subjects who regularly performed treadmill exercise in a lower body negative pressure chamber as well as resistance exercise (countermeasure group, EX). A functional evaluation of the microcirculation in the skin was assessed with laser Doppler. We studied endothelium-dependent and -independent vasodilation using iontophoresis of acetylcholine and sodium nitroprusside, respectively. We also measured circulating endothelial cells (CECs), an index of endothelial damage. In the No-EX group, endothelium-dependent vasodilation was significantly reduced (35.4 +/- 4.8% vs. 24.1 +/- 3.8%, P < 0.05) by bed rest with a significant increase in the number of CECs (3.6 +/- 1.4 vs. 10.6 +/- 2.7 ml(-1), P < 0.05). In the EX group, endothelium-dependent vasodilation and number of CECs were preserved. Our study shows that in humans prolonged bed rest causes impairment of endothelium-dependent function at the microcirculatory level, along with an increase in circulating endothelial cells. Microcirculatory endothelial dysfunction might participate in cardiovascular deconditioning, as well as in several bed rest-induced pathologies. We therefore conclude that the endothelium should be a target for countermeasures during periods of prolonged deconditioning.

Repeated cycles of electrical stimulation decrease vasoconstriction and axon-reflex vasodilation to noradrenaline in the human forearm

AIM

To investigate whether desensitization to the vasomotor effects of noradrenaline is a specific effect of electrical stimulation.

METHODS

Three sites on the forearm of 10 healthy volunteers were stimulated with 0.2 mA direct current for 2 min twice daily for 10 days. Noradrenaline and histamine were then displaced from ring-shaped iontophoresis chambers into two of the pretreated sites and two untreated sites on the contralateral forearm. Axon-reflex vasodilation was measured from the centre of the ring described by the iontophoresis chamber with a laser Doppler flowmeter. One or two days later, noradrenaline and vasopressin were introduced into pretreated and untreated sites by iontophoresis, and vasoconstriction at sites of administration was measured in the heated forearm.

RESULTS

The pretreatment blocked vasoconstriction to noradrenaline [median increase in flow 1%, interquartile range (IR) -41 to 52%; median decrease at the untreated site 53%, IR. -70 to -10%; P < 0.05], but did not block vasoconstriction to vasopressin (median decrease 42% at the untreated site and 45% at the pretreated site). Axon-reflex vasodilation to noradrenaline was diminished at the pretreated site (median increase in flow 33%, IR 2-321%; untreated site 247%, IR 31-1087%; P < 0.05). However, axon-reflex vasodilation to histamine did not differ significantly between the pretreated site (median increase 1085%) and the untreated site (median increase 1345%).

CONCLUSIONS

The conditioning pretreatment appears to evoke a specific decrease in responsiveness to noradrenaline. Repeated cycles of electrical stimulation may downregulate neural and vascular responses to noradrenaline by repetitively activating cutaneous sympathetic nerve fibres.