Use of wavelets to accurately determine parameters of laser Doppler reactive hyperemia

Laser Doppler flowmetry (LDF) is a noninvasive method to monitor skin perfusion and is very useful in studying reactive hyperemia signals. For this latter case, the determination of peak flow (pLDF) and time to peak flow (tpLDF) is of great interest in discriminating between subjects with peripheral arterial obliterative diseases and those who are healthy. However, the myogenic mechanism provokes marked oscillations on all LDF signals. Therefore, an accurate detection of the parameters is very difficult. The present study shows that wavelets are a powerful tool to overcome this drawback. Six different processes using several wavelets are tested on 44 signals recorded on 11 healthy volunteers. The results prove that thresholding coefficients of a multilevel wavelet decomposition does not allow a valuable computation of the parameters but that the reconstruction of the approximation branch is an efficient method to accurately determine pLDF and tpLDF. Using this latter method, mean results for a 3-min occlusion give pLDF = 46.80 a.u. and tpLDF = 17. 08 s. For a 2-min occlusion, pLDF and tpLDF are 39.19 a.u. and 11.63 s, respectively. For a 1-min occlusion, the results give pLDF = 36. 01 a.u., tpLDF = 8.48 s. Eventually, for a 30-s occlusion they give pLDF = 33.86 a.u. and tpLDF = 5.60 s. These results can now be compared with those obtained on pathological subjects.