Mark Wefers Bettink

Verification of calibration in man and comparison with vascular occlusion tests in healthy volunteers with COMET 4 63 1. Introduction Mitochondria are small intracellular organelles that generate energy for the cells in the form of adenosine triphosphate (ATP). Oxygen is of critical importance for efficient ATP generation through the process of oxidative phosphorylation, also called mitochondrial respiration. This function makes mitochondria the primary consumers of oxygen in the body and therefore the most desired location for measuring oxygen availability and consumption. An optical noninvasive technique has been developed for measuring mitochondrial oxygen tension (mitoPO 2 ) . MitoPO 2 is determined with the protoporphyrin IX-Triplet State Lifetime Technique (PpIX-TSLT) bymeasuring the oxygen-dependent delayedfluorescence lifetime of 5-aminolevulinic acid (ALA)-induced PpIX [1–3]. This measurement technique is incorporated in a medical device called the Cellular Oxygen METabolism monitor (COMET)[4]. Previous studies that used the protoporphyrin IX lifetime technique for cutaneous mitoPO 2 measurements in humans reported some remarkable results. Most importantly, relatively high average mitoPO 2 values of around 44 mmHg (5.9 kPa) [5] and 66 mmHg (8.8 kPa) [6] have been reported. In contrast, most textbooks mention normal values of mitochondrial oxygen tension as low as 7.5 mmHg (1 kPa) or less [7]. The calibration constants used in the COMET have been determined in animal studies [8]. A direct calibration in man has been lacking to preclude the high PO 2 values being a result of improper calibration. The first aim of this study was therefore to verify the calibration of COMET in human skin. No other clinical device is able to measure oxygenation at the mitochondrial level at the bedside. A direct comparison with other measurement techniques is thus unreliable because every tissue compartment, from intravascular to intracellular, has a different oxygen tension, leading to oxygen gradients. Due to the lack of a gold standard we aimed at using the same approach as used for in vivo calibration in animals, i.e. to use a combination of blocking oxygen supply by microvascular occlusion and blocking mitochondrial respiration by cyanide cream [8]. This provides a two-point calibration with a minimal mitoPO 2 value during microvascular occlusion and a known mitoPO 2 value after blockage of mitochondrial oxygen consumption. Next tomitoPO 2 measurements, the COMET system can be used to assess the parameters mitoVO 2 (a measure for oxygen consumption) and mitoDO 2 (as a measure for oxygen