Assessment of the optical interference in a PPG-LDF system used for estimation of tissue blood flow

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 (PPGG/560 nm and PPGIR/810 nm), covers 10 cm × 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. PPGG is more affected by laser light than PPGIR. Laser light influenced PPGG, 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. © 2011 Springer-Verlag.