ObjectiveTo review optical spectroscopic techniques for assessment of the determinants of tissue oxygenation and to evaluate the notion that the disturbances in oxygen pathways in sepsis can be accounted for by enhanced functional shunting of parts of the microcirculation.Data ResourcesExperimental data from previous research and the literature were analyzed.Study SelectionThe data selected pertained to a) whether cellular distress in sepsis is caused by tissue hypoxia or disturbed metabolic pathways, b) optical spectroscopic techniques used to study microcirculatory oxygenation, and c) possible mechanisms underlying shunting of the microcirculation in hypoxemia and sepsis.Study SynthesisDespite resuscitation of oxygen-derived variables, signs of regional tissue hypoxia persist in sepsis. The mechanisms underlying this condition are expected to be associated with oxygen pathways in the microcirculation. Optical spectroscopic techniques are providing new insights into these mechanisms. These include absorption spectroscopy for hemoglobin saturation of erythrocytes, reduced nicotinamide adenine dinucleotide fluorescence for tissue mitochondrial bioenergetics, and palladium-porphyrin phosphorescence for microvascular PO2. Reduced nicotinamide adenine dinucleotide videofluorescence studies have shown the heterogeneous nature of hypoxia. Measurement of gut microvascular PO2in pigs has shown the development of a PO2gap between microvascular PO2and venous PO2during hemorrhage and endotoxemia, with a larger gap occurring in sepsis than in hemorrhage. It is hypothesized that this difference is caused by the enhanced shunting of the microcirculation present in sepsis.ConclusionsMicrocirculatory distress may form one of the earliest stages in the progress of sepsis to multiple organ failure, and shunting of the microcirculation may be an important contributing factor to this development. To evaluate the severity of microcirculatory distress and the effectiveness of resuscitation strategies, new clinical technologies aimed at the microcirculation will need to be developed. It is anticipated that optical spectroscopy will play a major role in the development of such tools. (Crit Care Med 1999; 27:1369-1377)