Mark Wefers Bettink

Monitoring of mitochondrial oxygen tension in the operating theatre 8 149 is synthesized in the mitochondria, and administration of 5-aminolevulinic acid (ALA) substantially enhances the PpIX concentration. PpIX possesses a triplet state that reacts strongly with oxygen, making its lifetime oxygen-dependent. Population of the first excited triplet state occurs upon photo-excitation with a pulse of light, and causes the emission of red delayed fluorescence. The delayed fluorescence lifetime is related to mitoPO 2 according to the Stern-Volmer equation: in which τ is the measured delayed fluorescence lifetime, k q is the quenching constant and τ 0 is the lifetime at zero oxygen. The Stern- Volmer equation is valid for a homogenous oxygen distribution and after excitation with a pulse of light of which the lifetime is much shorter than τ . In case of a nonhomogenous oxygen distribution inside the measurement volume, a reliable estimation of the average PO 2 can be made by the rectangular distribution method (RDM) 10,11 . The signal/noise ratio (SNR) of resulting traces was calculated and defined as the ratio of maximum signal amplitude to the peak-to-peak noise. COMET ® evaluates signal quality, which is calculated from the SNR value; an increase of one in SNR is approximately 1% in signal quality up till a SNR of 50. Lifetime analysis operates stably at moderate signal quality ( > 20%) 5 . Therefore, only delayed fluorescence signals with a signal quality > 20% were analysed and included in the present dataset. Procedures (Measurement) Induction of anesthesia was performed based on the attending anesthesiologist’s preference. Intraoperative monitoring, ECG based heart rate, invasive blood pressure, body temperature, skin temperature and oxygen saturation, inspired oxygen fraction (FiO 2 ), anesthesia infusion rate and vasopressor pump setting were part of the standard monitoring and were stored in the electronic patient data management system. All patients were kept normothermic with use of a warm air blanket. Normovolemia was pursued with intravenous crystalloids based on the pulse pressure variation index (threshold above 13).