Mark Wefers Bettink

Chapter 9 176 In this study we found baseline mitoPO 2 values in the range of 51-60 mmHg. In a previous study in healthy volunteers we reported mean mitoPO 2 to be 44 mmHg, and in a very recently published study in critical care patients meanmitoPO 2 was reported to be around 60 mmHg (42). Such relatively high values match well with other oxygen measurements in skin (43). The differences between the studies could well be attributed to factors like skin temperature, filling status of the patient, and use of sedation/anesthesia, since such factors are known to influence skin perfusion. Clinical data until now are scarce and normal values for mitoPO 2 remain to be determined, as well as the influence of patient factors (such as age) and clinical circumstances. Although we do think mitochondrial oxygen tension is in general higher than anticipated (12), the reader should realize that mitoPO 2 in other organs and tissues is likely to differ. Differences in tissue oxygen levels exist between organs, tissues and tissue compartments (43) and metabolic activity (for example muscle contraction) is also of influence. To date, clinicians are in the dark about the effect of the altered neonatal (patho) physiology during major high-risk surgery on cellular oxygenation. In the past the focus was to optimize macrohemodynamics although the microcirculation has been increasingly recognized as an import variable in the critically ill neonate (44). To measure tissue oxygenation, a modality based on the principle of near infrared spectroscopy (NIRS) became popular. The optode of the NIRS emits near-infrared light, which easily penetrates biological tissue at a depth of approximately 2 to 3 cm.(45)(46) It measures the oxygenation of a combination of 75%venous, 20%arterial and 5%capillary blood, but does not provide information about the oxygen concentration at cellular level. Unfortunately, the clinical use of additional monitoring with NIRS have not been established yet.(47) The COMET allows us to look at oxygen availability at a cellular level. The neonatal skin is an ideal target organ for COMET measurements. It is the biggest organ in neonates and has a relative bigger surface and is more vascularized compared to adults. Skin blood circulation is very sensitive to changes in vascular resistance and blood pressure (48), potentially making the skin a good indicator for the (general) cardiopulmonary status of the neonate. Compared to interstitial measurements with for example oxygen electrodes COMET has some distinct advantages, such as no need for calibration, non-destructiveness (no need for needle placement), well-defined measurement compartment and very fast response time (no need for signal integration over longer periods of time). A disadvantage of the COMET technique is the necessary priming with ALA. Although previous studies in adults and this study in neonates, show that with some precaution’s application of ALA to the