Abstract
Laser Doppler flowmetry (LDF) and reflection photoplethysmography (PPG) are standard technologies to access microcirculatory function in vivo. However, different light frequencies mean different interaction with tissues, such that LDF and PPG flowmotion curves might have distinct meanings, particularly during adaptative (homeostatic) processes. Therefore, we analyzed LDF and PPG perfusion signals obtained in response to opposite challenges. Young healthy volunteers, both sexes, were assigned to Group 1 (n = 29), submitted to a normalized Swedish massage procedure in one lower limb, increasing perfusion, or Group 2 (n = 14), submitted to a hyperoxia challenge test, decreasing perfusion. LDF (Periflux 5000) and PPG (PLUX-Biosignals) green light sensors applied distally on both lower limbs recorded perfusion changes for each experimental protocol. Both techniques detected the perfusion increase with massage, and the perfusion decrease with hyperoxia, in both limbs. Further analysis with the wavelet transform (WT) revealed better depth-related discriminative ability for PPG (more superficial, less blood sampling) compared with LDF in both challenges. Spectral amplitude profiles consistently demonstrated better sensitivity for LDF, especially regarding the lowest frequency components. Strong correlations between components were not found. Therefore, LDF and PPG flowmotion curves are not equivalent, a relevant finding to better study microcirculatory physiology.
Highlights
Laser Doppler flowmetry (LDF) and reflection photoplethysmography (PPG) are standard technologies to access microcirculatory function in vivo
Some perfusion differences between limbs are immediately noted in Phase I, with LDF as with PPG, no matter the group
Hyperoxia evokes a significant decrease of perfusion in Phase II in both limbs, detected with both technologies (LDF p < 0.001 on right and left feet; PPG p < 0.05 on right and left feet)
Summary
Laser Doppler flowmetry (LDF) and reflection photoplethysmography (PPG) are standard technologies to access microcirculatory function in vivo. Better sensors and more effective wireless communications have renewed its interest such that PPG is currently no longer only an alternative to electrocardiography (ECG) to measure heart rate, but rather an innovative breakthrough in the wearable health monitoring technologies era, with multiple applications in sight[7,8,9,10]. These two technologies, LDF and PPG, are based in similar biophysics but use different laser frequencies, such that the respective perfusion related information is obtained at different depths, resulting in different discrimination capacities. The second derivative wave of the original PPG signal (accelerometer data) was referred to contain important health-related information[8,25]
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