Joakim Henricson 1 Doctoral Stud., Gert Nilsson 2,3 Prof., Folke Sjöberg 1 Prof.

1. Microvascular Response to Iontophoretically Applied Acetylcholine Investigated by Tissue Viability Imaging Joakim Henricson 1 Doctoral Stud., Gert Nilsson 2,3 Prof., Folke Sjöberg 1 Prof. 1 Department of Biomedicine and Surgery, Linköping University Hospital, Linköping, Sweden 2Department of Biomedical Engineering, Linköping University Hospital, Linköping, Sweden 3 WheelsBridge AB, Lövbergsvägen 13, S-589 37 Linköping, Sweden Introduction Increasing efforts are directed towards the development of vascular models in which the vessels and vascular effects are investigated in a tissue environment incorporating the influences of not only the vessels themselves – as in the case for the in vitro models – but also comprising nerve endings, endothelial cells mechanical and humoral factors. Acetylcholine delivered to skin tissue by use of a small current (iontophoresis) and blood flow assessment using laser Doppler perfusion methodology offers such a model and has been used extensively for more than ten years in a multitude of experimental settings. We propose a new technique called Tissue Viability Imaging (TiVi), based on the method of polarization spectroscopy of blood in superficial skin tissue, with the ability to “see through” the surface of the skin and collect spectroscopic information about microvascular red blood cell concentration. Aim To investigate the microvascular response to iontophoretically applied acetylcholine by using TiVi technology and if possible present these results as dose response curves. Methods Eight healthy volunteers gave informed consent to participate (male 4, mean age 23 years). Electrodes were attached to the volar side of subject’s forearms. Acetylcholine dissolved in physiological saline to a concentration of 10 mg/ml was anodally iontophorised to evoke vascular response. TiVi values were collected at a rate of one image every 5 second at a distance of approximately 10 cm. Data was analysed using TiVi600 software by WheelsBridge AB, Linköping, Sweden and GraphPad Prism 4 by GraphPad Software inc. Figure 1. Iontophoresis of acetylcholine. Tissue Viability Imaging system in use. Theory Iontophoresis is the facilitated transport of charged substances over the skin by the use of a small current (Figure 1). TiVi technology uses linearly polarized light in the visible region that is partly reflected by the skin surface and partly diffusely scattered in the dermal tissue matrix. The directly reflected light retains its original linear polarization state and is blocked by the orthogonally placed filter over the detector and only linearly polarized light backscattered from red blood cells (RBCs) and other tissue is used for analysis. Evaluation of the RBC concentration and the associated tissue viability is possible since the light absorption of the static tissue components in dermis is smaller and less dependent on wavelength than the light absorption in RBCs (Figure 2). Figure 2. Schematic of TiVi (RP = Randomly Polarized, LP = Linearly Polarized). Results Iontophoretic administration of the endothelium dependent and vasodilating drug acetylcholine in combination with TiVi technology has shown promising results. The effect of the drug can be measured by TiVi and presented as dose response curves (Figure 3). Effective dose 50% was 2.2 mC (95% Ci 1.8-2.7 mC). These results open up for new possibilities in pharmacological in vivo studies of the cutaneous microvasculature and comparison of obtained results to corresponding in vitro studies. Figure 3. Dose dependent vasodilatory effect of iontophoretically applied acetylcholine measured by TiVi (n=8). Correspondence: Joakim Henricson, joahe@ibk.liu.se