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Increasingly we are monitoring the distribution of oxygen through the microcirculation using optical techniques such as optical reflectance spectroscopy (ORS) and near-infrared spectroscopy. Mean blood oxygen saturation ([S.sub.mb][O.sub.2]) and tissue oxygenation index measured by these two techniques, respectively, evoke a concept of the measurement of oxygen delivery to tissue. This study aims to establish whether [S.sub.mb][O.sub.2] is an appropriate indicator of tissue oxygenation. Spontaneous fluctuations in [S.sub.mb][O.sub.2] observed as changes in concentration of oxyhemoglobin ([Hb[O.sub.2]]) and deoxyhemoglobin ([Hb]) were measured by ORS in the skin microcirculation of 30 healthy subjects (15 men, age 21-42 yr). Fourier analysis identified two distinctly different spontaneous falls in Sr, uO2. The first type of swing, thought to be induced by fluctuations in arterial blood volume, resulted from the effects of respiration, endothelial, sympathetic, and myogenic activity. There was no apparent change in [Hb]. In contrast, a second type of swing resulted from a fall in [Hb[O.sub.2]] accompanied by a rise in [Hb] and was only induced by endothelial and sympathetic activity. Thus the same fall in [S.sub.mb][O.sub.2] can be induced by two distinct responses. A 'type I' swing does not suggest an inadequacy in oxygen delivery whereas a 'type II' swing may indicate a change in oxygen delivery from blood to tissue. [S.sub.mb][O.sub.2] alone cannot therefore be accepted as a definitive marker of tissue oxygenation. optical reflectance spectroscopy; microcirculation