Elsevier

Microvascular Research

Volume 113, September 2017, Pages 29-39
Microvascular Research

Assessing cutaneous microvascular function with iontophoresis: Avoiding non-specific vasodilation

https://doi.org/10.1016/j.mvr.2017.04.006Get rights and content

Abstract

Aim

Iontophoresis of vasoactive agents is commonly used to assess cutaneous microvascular reactivity. However, it is known that iontophoresis can be limited by confounding non-specific vasodilatory effects. Despite this, there is still no standardization of protocols or data expression. Therefore, this study evaluated commonly used protocols of iontophoresis by assessing each for evidence of non-specific vasodilatory effects and examined the reproducibility of those protocols that are free of non-specific responses.

Methods

Twelve healthy participants were administered doses of acetylcholine (ACh) 1–2% and sodium nitroprusside (SNP) 1%, diluted in sodium chloride 0.9% or deionized water, and insulin 100 U/mL in a sterile diluent using iontophoresis coupled with laser speckle contrast imaging (LSCI). Increases in blood flux at a control electrode, containing the diluent only, indicated a non-specific response. Reproducibility of iontophoresis protocols that were free of non-specific vasodilatory effects were subsequently compared to that of post-occlusive reactive hyperemia (PORH), used as a standard, in 20 healthy participants.

Results

Iontophoresis of ACh or SNP in sodium choloride (0.02 mA for 200 and 400 s, respectively) and ACh in deionized water (0.1 mA for 30 s) mediated the least non-specific vasodilatory effects. Microvascular responses to insulin were mediated mainly by non-specific effects. Compared to PORH, the intraday and interday reproducibility for iontophoresis of ACh and SNP (0.02 mA for 200 and 400 s, respectively) with LSCI was weaker, but still deemed good to excellent when data was expressed, in perfusion units or cutaneous vascular conductance, as the absolute peak blood flux response to the vascular reactivity test or as the change in blood flux between peak and baseline values.

Conclusion

This study provides updated recommendations for assessing cutaneous microvascular function with iontophoresis.

Introduction

Cardiovascular disease (CVD) remains the single leading cause of death representing an estimated 31% of the global mortality rate (World Health Organisation. Cardiovascular diseases fact sheet, 2016). In recent decades it has become widely accepted that vascular dysfunction is a primary mechanism in the pathogenesis of CVD, occurring long before atherosclerotic remodeling of the vascular network or even the onset of obesity, metabolic syndrome or diabetes (Loader et al., 2015). Indeed, vascular dysfunction may also contribute to the initial development and adverse progression of such cardiometabolic diseases (Levy et al., 2008). Interestingly, emerging evidence suggests that coronary microvascular disease may explain the occurrence of myocardial ischemia, heart failure and CVD mortality following myocardial infarction without apparent coronary macrovascular disease (Jzerman et al., 2003); highlighting the microcirculation, which represents most of the arterial vascular network and exerts dominant control over local blood flow to the nutritive network, as an important concentration in the study of vascular function. Increased focus in the field of microvascular research warrants the need to comprehensively evaluate the reliability of existing methods that are used to assess microvascular function in order to further determine their potential clinical and prognostic applications.

Microvascular function is often assessed at the cutaneous microcirculation due, mostly, to its accessibility and its potential role as a surrogate marker of systemic microvascular function (Humeau-Heurtier et al., 2013); using common tests of vascular reactivity that include iontophoresis and post-occlusive reactive hyperemia (PORH) coupled with laser-based technologies, such as laser speckle contrast imaging (LSCI), laser Doppler flowmetry and laser Doppler [scanner] imaging (Roustit and Cracowski, 2013). Iontophoresis involves the delivery of a vasoactive agent to the cutaneous microcirculation using a low-intensity electrical current and, in microvascular research, is typically used to administer acetylcholine or insulin to assess endothelial function, or sodium nitroprusside that examines vascular smooth muscle function (Tesselaar and Sjoberg, 2011). However, it is known that iontophoresis may induce non-specific vasodilatory effects that are influenced by variations in the type of diluent used for each vasoactive agent, administration of a higher intensity current or a higher total iontophoretic current density; or by the method of electrical current delivery (e.g. continuous or multiple pulses) (Droog et al., 2004). Ultimately, non-specific vasodilatory effects confound the microvascular response to the vasoactive agent being studied, limiting the technique and the overall interpretation of the microcirculatory data, as well as any subsequent conclusions. Despite this, there are still no standardized protocols for iontophoresis; and, consequently, there is an array of methods for iontophoresis being utilized in the literature with no consensus as to which are free of non-specific vasodilatory effects.

Therefore, the primary objective of this present study was to examine the reliability of several published protocols for iontophoresis of acetylcholine, sodium nitroprusside and insulin by evaluating each for evidence of non-specific vasodilatory effects. The second objective of this study was to compare the reproducibility of those protocols that were found to be free of non-specific vasodilatory effects to the excellent reproducibility of PORH (Roustit et al., 2010), as observed when performed in conjunction with LSCI.

Section snippets

Materials and methods

This study was comprised of two separate protocols. As explained in detail below, Protocol A evaluated the reliability of several published protocols of iontophoresis by testing each method for evidence of non-specific vasodilatory effects. Protocol B was then conducted to compare the intraday and interday (morning and afternoon) reproducibility of iontophoresis protocols that were found to be free of non-specific vasodilatory effects in Protocol A to the excellent reproducibility of PORH (

Acetylcholine

In each acetylcholine protocol there was a rapid significant increase in cutaneous microvascular blood flux from baseline values within two minutes from beginning the administration of the anodal electrical current (Fig. 1). When each protocol was performed at the control electrode containing the diluent only (i.e. sodium chloride 0.9% or deionized water), the change in blood flux was not similar to the response to iontophoresis at the electrode containing both the vasoactive agent and diluent.

Discussion

Currently, there are no standardized methods for iontophoresis in the literature and, consequently, researchers are using an array of protocols that may be at risk of inducing non-specific vasodilatory effects; confounding the overall interpretation of the microvascular data and any subsequent conclusions. Considering this, this study aimed to provide updated recommendations for using iontophoresis to assess cutaneous microvascular function; assessing the reliability of several published

Funding

JL is supported by the National Health and Medical Research Council of Australia 1114350 Dora Lush Biomedical Research Postgraduate Scholarship and an Australian Government Research Training Program Scholarship. SS is supported by the National Health and Medical Research Council of Australia 1041796.

Conflicts of interest

The authors have no conflicts to disclose.

Acknowledgments

JL, GW, CL were responsible for the concept and design of the study. JL performed data acquisition, analysed the data and interpreted the data. JL performed drafting of the manuscript. JL, GW, MR, FT, SS and CL provided administrative, technical or material support. All authors critically revised the manuscript for important intellectual content and approved the final version of the manuscript.

References (29)

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