Mitochondrial dysfunction is an important element in the pathophysiology of sepsis. However, a valid non-invasive method to measure mitochondrial function is not yet available. In animal models, the PpIX-TSLT technique has shown to be a feasible technique to measure alterations in mitochondrial oxygen tension during endotoxemia. This has never been studied in humans during endotoxemia. Furthermore, the exposure to cold and especially the breathing technique may influence oxygen tension in the mitochondria as well, as blood gas parameters (e.g. pCO2, acid base balance etc.) fluctuate to a large extent during the practicing of this technique.