A Physiological Validation of Spectral Analysis for Skin Flowmotion

Abstract

In a series of studies, our laboratory has utilized microneurography coupled with laser Doppler flowmetry to examine sympathetic neural control of the cutaneous microvasculature during thermoregulatory stimuli. However, these techniques have limited clinical utility. Spectral analysis of the low frequency periodic oscillations in laser Doppler flowmetry‐derived red blood cell flux (i.e., skin flowmotion) is a noninvasive technique that has been used to quantify cutaneous microvascular control mechanisms (e.g., endothelial, neurogenic, myogenic). To date, conclusions regarding the mechanistic regulation of cutaneous microvascular function derived from spectral analysis have been primarily limited to basal conditions. Thus, the aim here was to examine the validity of spectral analysis to quantify sympathetic neural control of skin blood flow during a dynamic physiological stimulus (local skin heating to 42°C). We hypothesized that pharmacological blockade of sensory nerves would reduce the spectral power of the neurogenic frequency band during local heating. Eleven young healthy adults (7 women; 22±3 yrs, 25±2 kg/m2) participated. Laser Doppler flux was measured continuously during baseline (33°C) and a standardized local heating protocol (42°C) at a non‐treated control site (con) and a site treated with topical lidocaine (lido; 2.5% prilocaine+2.5% lidocaine) to pharmacologically block sensory nerves. Topical lido was applied for a minimum of 45 minutes prior to testing. Cutaneous vascular conductance was calculated and expressed as a percentage of maximum (local heat of 43°C). A raw Fourier transform was used to calculate spectral power around the 0.0095–0.02 Hz, 0.02–0.05 Hz, and 0.05–0.15 Hz frequency intervals, considered to correspond to endothelial, neurogenic, and myogenic activity, respectively. The initial peak of the local heating response was reduced by lido, (con: 62±2% vs. lido: 25±5%, p<0.01), validating adequate sensory nerve blockade. However, there was no difference between sites during baseline (con: 12±1% vs. lido: 9±2%, p=0.07) or the local heating‐induced plateau (con: 95±3% vs. lido: 87±7%, p=0.20). There was no effect of lido on spectral power of any of the frequency intervals at baseline or during local heating. Total spectral power (i.e., the sum of the spectral power for all frequency bands) was significantly related to conductance in both sites at baseline (con: R2=0.69, p<0.01; lido: R2=0.72, p<0.01) and during local heating (con: R2=0.22, p<0.02; lido: R2=0.71, p<0.01). In conclusion, the physiological increase in cutaneous vascular conductance during local heating was mirrored by an increase in total spectral power. Although blockade of sensory nerves reduced the increase in conductance during the initial peak of the local heating response, it did not affect the neurogenic frequency interval derived from spectral analysis. These preliminary data suggest that skin flowmotion may not be a valid approach to evaluate neural control of cutaneous microvascular function during dynamic conditions.Support or Funding InformationAmerican Heart Association Grant #18UFEL33900164; College of Health and Human Development Summer 2019 Undergraduate Research Award; Noll Endowment for Undergraduate Research