The aim of this study was to validate a non-invasive optical probe for simultaneous blood flow measurement at different vascular depths combining three photoplethysmography (PPG) channels and laser Doppler flowmeter (LDF). Wavelengths of the PPG were near-infrared 810 nm with source-to-detector separation of 10 and 25 mm, and green 560 nm with source-to-detector separation of 4 mm. The probe is intended for clinical studies of pressure ulcer aetiology. The probe was placed over the trapezius muscle, and depths from the skin to the trapezius muscle were measured using ultrasound and varied between 3.8 and 23 mm in the 11 subjects included. A provocation procedure inducing a local enhancement of blood flow in the trapezius muscle was used. Blood flows at rest and post-exercise were compared. It can be concluded that this probe is useful as a tool for discriminating between blood flows at different vascular tissue depths. The vascular depths reached for the different channels in this study were at least 23 mm for the near-infrared PPG channel (source-to-detector separation 25 mm), 10-15 mm for the near-infrared PPG channel (separation 10 mm), and shallower than 4 mm for both the green PPG channel (separation 4 mm) and LDF.
The aim of this study was to evaluate an integrated probe using LDF and multiple PPG, for the long-term aspects of skin temperature and blood flow variations at different tissue depths, and especially to investigate whether the presence of the probe affects the temperature. Measurements of temperature and blood flow were performed over 60 min on the lower back of ten subjects, lying on a mattress. The surface temperature of the skin was also measured before and after the 60 min period, and repeated with three probe configurations with the probe switched on, turned off and in the absence of a probe. A general increase in the blood flow was found to occur during the 60 min interval at all depths reached by this probe, but with variations over time. No difference was found in temperatures recorded for the different probe configurations. According to our measurements, the presence of the probe does not affect the skin surface temperature at 60 min. Our investigation shows that skin temperature can be expected to increase and approach the body core temperature by just lying in supine position on the mattress. The increase in temperature and blood flow that is known to occur, possibly attributable to pressure-induced vasodilatation, must be taken into consideration when performing these kinds of measurements.
Abstract-The aim of this study is to assess the optical cross interference in a system including laser Doppler flowmetry (LDF) and photoplethysmography (PPG) with regard to the illuminating power of PPG-LEDs and distance between the light detector/s and light source/s.Reduced or missing blood perfusion can lead to pressure ulcers. Monitoring changes in blood flow in areas prone to pressure ulcer development would be a valuable tool for prevention of pressure ulcer development.The probe, with one to two LDF-channel/s and two PPGchannels (PPG G/ 560 nm and PPG IR /810 nm), covers 10 cm x 10 cm. Influence from PPG-LEDs to the LDF-system and influence from the LDF-laser to the PPG-system was investigated. Three different light intensities were used for the PPG-LEDs.Recordings were repeated using two different placements of the LDF-fibre, changing the distance between light source/s and light detector/s of the reciprocal technique.The LDF did not show any influence from light from the PPG.PPG G is more affected by laser light than PPG IR . Laser light influenced PPG G , most at lowest intensity of the PPGLEDs. The influence of the laser light to the PPG-channels is less in the outer position of the LDF-fibre.Interference can be totally avoided by switching, only measuring by one technique at a time. Rapid flow changes are then not possible to monitor fully. When rapid blood flow variations at different vascular depths are of interest to monitor, placement of the LDF-fibre in the outer position and use of a higher light intensity of the PPG-LEDs might be an alternative. However, interference still can be present, and further, the measurement volume of LDF will be different from that covered by PPG-channels.
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