Race-specific differences in the level of glycated hemoglobin are well known.However, these differences were detected by invasive measurement of mean oxygenation, and their understanding remains far from complete. Given that oxygen is delivered to the cells by hemoglobin through the cardiovascular system, a possible approach is to investigate the phase coherence between blood flow and oxygen transportation. Here we introduce a noninvasive optical method based on simultaneous recordings using NIRS, white light spectroscopy and LDF, combined with wavelet-based phase coherence analysis. Signals were recorded simultaneously for individuals in two groups of healthy subjects, 16 from Sub-Saharan Africa (BA group) and 16 Europeans (CA group). It was found that the power of myogenic oscillations in oxygenated and de-oxygenated hemoglobin is higher in the BA group, but that the phase coherence between blood flow and oxygen saturation, or blood flow and hemoglobin concentrations is higher in the CA group K E Y W O R D S blood flow, ethnic, laser Doppler flowmetry, near-infrared spectroscopy, oxygenation, physiological oscillations, wavelet phase coherence, white light spectroscopy

Non-invasive optical techniques in biomedicine have made notable advances in recent decades (Peng et al 2008, Vo-Dinh 2014, Tuchin 2016. One example is laser Doppler flowmetry (LDF). It has been shown to give results comparable to those from other methods of evaluating skin microvascular blood flow, and it possesses the particular advantage of continuous detection of microvascular blood flow in a volume of tissue, as opposed to axial flow in a single vessel (Nitzan et al 1988).LDF provides a simple and non-invasive approach for assessing the dynamical properties of the skin microcirculation, and it can be applied in both the healthy and pathological states (Stefanovska et al 1999). In combination with appropriate time-series analysis, it can yield valuable insights into the dynamics of microvascular blood flow. Its working principle depends on the Doppler shift in the frequency of light reflected from moving red blood corpuscles (erythrocytes). So it relies on the passage of incident light through the skin, twice. Some knowledge of the skin's optical properties is therefore required.As illustrated in figure 1, human skin (Kanitakis 2002) is made up of several layers, of which the melanin chromophores responsible for skin pigmentation reside in the epidermal layer (Costin and Hearing 2007).

We show how a non-autonomous dynamics approach using time-resolved analyses of power spectra and phase coherence can help in the noninvasive diagnosis of malaria. The work is based on studying oscillations in blood flow and the variability of the heart and respiratory frequencies. The model used assumes that the heart and respiration are two oscillatory pumps with variable frequencies and that the vascular resistance also changes in an oscillatory manner. Red blood cells circulating through the system deliver oxygen to each cell. Malaria changes the red blood cells so that this delivery is compromised. The oscillatory properties of both pumps are also affected. We quantify the latter and compare three groups of subjects: febrile malaria patients (37); non-febrile malaria patients (10); and healthy controls (51). For each subject, time series of skin blood flow, respiratory effort, cardiac activity (ECG) and skin temperature were recorded simultaneously over an interval of 30 minutes. The oscillatory components within the range 0.005 − 2 Hz were analysed and their degree of coordination throughout the cardiovascular system was assessed by wavelet phase coherence analysis. It is shown that malaria, either febrile or non-febrile, substantially reduces the coordination.