Multifractal spectra of laser Doppler flowmetry signals in healthy and sleep apnea syndrome subjects Benjamin BUARD *1,2 , Wojciech TRZEPIZUR 3,4 , Guillaume MAHE 3 , François CHAPEAU-BLONDEAU 2 , David ROUSSEAU 2 , Frédéric GAGNADOUX 4 , Pierre ABRAHAM 3 , Anne HUMEAU 1,2 1 Groupe ESAIP, 18 rue du 8 mai 1945, BP 80022, 49180 Saint Barthélémy d'Anjou cedex, France. 2 Laboratoire d'Ingénierie des Systèmes Automatisés (LISA), Université d'Angers, 62 avenue Notre Dame du Lac, 49000 Angers, France. 3 Laboratoire de Physiologie et d’Explorations Vasculaires, UMR CNRS 6214-INSERM 771, Centre Hospitalier Universitaire d’Angers, 49033 cedex 01 Angers, France. 4 Département de Pneumologie, Centre Hospitalier Universitaire d'Angers, 49033 Angers cedex 01, France. ABSTRACT Laser Doppler flowmetry (LDF) signals give a peripheral view of the cardiovascular system. To better understand the possible modifications brought by sleep apnea syndrome (SAS) in LDF signals, we herein propose to analyze the complexity of such signals in obstructive SAS subjects, and to compare the results with those obtained in healthy subjects. SAS is a pathology that leads to a drop in the parasympathetic tone associated with an increase in the sympathetic tone in awakens SAS patients. Nine men with obstructive SAS and nine healthy men participated awaken in our study and LDF signals were recorded in the forearm. In our work, complexity of LDF signals is analyzed through the computation and analysis of their multifractal spectra. The multifractal spectra are estimated by first estimating the discrete partition function of the signals, then by determining their Renyi exponents with a linear regression, and finally by computing their Legendre transform. The results show that, at rest, obstructive SAS has no or little impact on the multifractal spectra of LDF signals recorded in the forearm. This study shows that the physiological modifications brought by obstructive SAS do not modify the complexity of LDF signals when recorded in the forearm. Keywords: Laser Doppler flowmetry, multifractal spectrum, sleep apnea syndrome, partition function, biomedical engineering 1. INTRODUCTION The peripheral cardiovascular system can be studied non invasively with laser Doppler flowmetry (LDF). The LDF technique relies on the Doppler effect generated by the interactions between photons from a laser light and moving blood cells of the microcirculation, mainly red blood cells [1-5]. It is commonly used in clinical research and gives a peripheral view of the cardiovascular system. LDF can be used in many purviews and on many organs, such as skin, brain, liver and intestines [6,7]. Both concentration and velocity of the moving scatterers affect the LDF perfusion estimate [4]. We herein propose to study the complexity of LDF signals recorded in the forearm of healthy and obstructive sleep apnea syndrome (SAS) subjects. It has been shown that LDF signals recorded in the forearm of young healthy subjects are weakly multifractal [8,9], and that aging can lead to a reduced multifractality [10]. However, other studies in this difficult field of interest are needed, first to progress in the characterization of the possible multifractal properties of LDF signals, and then to analyze how these potential multifractal characteristics evolve with pathologies. Two types of sleep apnea exist: central and obstructive SAS. Central SAS is a central nervous system disorder that occurs when the brain signal telling the body to breathe is delayed. But the most common kind of sleep apnea is the obstructive SAS. Obstructive SAS is a prevalent pathology, characterized by partial or total upper airway obstruction *bbuard@esaip.org ; phone + 33 (0)2 41 96 65 10; fax +33 (0)2 41 96 65 11 Therapeutic Laser Applications and Laser-Tissue Interactions IV, edited by Ronald Sroka, Lothar D. Lilge, Proc. of SPIE-OSA Biomedical Optics, SPIE Vol. 7373, 73730P · © 2009 SPIE-OSA CCC code: 1605-7422/09/$18 · doi: 10.1117/12.831901 SPIE-OSA Vol. 7373 73730P-1