Mark Wefers Bettink

Mind the mitochondria! 2 31 measurement of oxygen consumption can start. Most common techniques measures oxygen consumption using Clark electrodes (75), like the high resolution respirometer (76). Further evaluation of the mitochondrial respiratory chain is possible by measuring individual complex activity or concentration of the complexes in isolated mitochondria (77). The location of the muscle biopsy maybe important, since a difference was found between leg muscle and diaphragm (48). Measurement of mitochondrial function is possible in isolated platelets or peripheral blood mononuclear cells (78,79). Of essence in measuring mitochondrial function is the choice of buffer in which the cells are measured, since exposing healthy cells to plasma obtained from patients with sepsis lead to mitochondrial dysfunction (80). In platelets early increase of mitochondrial oxygen metabolism has been linked to an increase in mortality (51,81). Since the location of biopsy and the chosen buffer clearly influence the measured mitochondrial function, interpretation of the results found by respirometers might be difficult. Furthermore, measuring mitochondrial function with ex vivo techniques requires special laboratory equipment (82) and trained staff. Both are not available in our general hospitals, which limits this type of monitoring to a research setting in academic hospitals. Measurement of aspects of mitochondrial function in vivo is also possible with Nuclear Magnetic Resonance (NMR)-technique and NADH fluorometry (83,84). With the NMR- technique, phosphorus (P) and its transport in the cell can be measured. phosphorus is used by the cell as an energy source (ATP), and thus metabolic flux can be measured. Since a metabolic flux measurement requires use of ATP, an active muscle is required during the NMR measurement. NADH fluoroscopy measures the NADH fluorescence at 450 nm (blue light). NADH shows fluorescence after photoexcitation with ultraviolet light, but NAD + does not show such fluorescence. Measuring the ratio between the reduced and oxidized form gives a relative redox potential. While this technique works well in a laboratory setting, e.g. in fluorescence microscopy on single cells, it is very difficult to achieve reliable measurements in vivo . Both NMR and NADH fluoroscopy measure metabolic state, but the inability to perform bedside monitoring and its costs makes NMR not suitable for standard clinical use in sepsis. NADH fluoroscopy is available for bedside monitoring and changes in metabolic state are possible in a clinical setting (85). However, standard monitoring of mitochondrial function by NADH fluoroscopy is not yet an option due to sensitivity to artifacts and interpretation issues.