Measuring Mitochondrial Oxygen Tension: From Basic Principles to Application in Humans

The protoporphyrin IX-triplet state lifetime technique (PpIX-TSLT) has been recently introduced as the first method to measure mitochondrial oxygen tension (mitoPo(2)) in living cells and tissues. The current implementation of the technique is based on oxygen-dependent quenching of the delayed fluorescence lifetime of 5-aminolevulinic-acid-enhanced mitochondrial PpIX. It represents a significant step forward in our ability to comprehensively measure tissue oxygenation. PpIX-TSLT is feasible for application in humans and recently we have been able to measure for the first time mitoPo(2) in humans. MitoPo(2) in intact tissues reflects the balance between oxygen supply and demand at the cellular level. Administration of aminolevulinic acid induces measurable mitochondrial levels of PpIX. PpIX acts as a mitochondrially located oxygen-sensitive dye by emitting a red delayed fluorescence after excitation with a pulse of green light. The lifetime of the delayed fluorescence is inversely related to Po-2 by the Stern-Volmer equation. In vivo measurements of mitoPo(2) in liver, heart, and skin of rats have revealed surprisingly high values of typically several tens of mm Hg. Clinical measurements of mitoPo(2) are possible as demonstrated by cutaneous measurements in healthy volunteers. Applications of PpIX-TSLT in anesthesiology and intensive care medicine might, e. g., be monitoring mitoPo(2) as a resuscitation end point, targeting oxygen homeostasis in the critically ill, and assessing mitochondrial function at the bedside. PpIX-TSLT likely also has applications in other fields also, e. g., providing an oxygen-related feedback signal in photodynamic therapy of malignant tumors

Microcirculatory and mitochondrial hypoxia in sepsis, shock, and resuscitation

After shock, persistent oxygen extraction deficit despite the apparent adequate recovery of systemic hemodynamic and oxygen-derived variables has been a source of uncertainty and controversy. Dysfunction of oxygen transport pathways during intensive care underlies the sequelae that lead to organ failure, and the limitations of techniques used to measure tissue oxygenation in vivo have contributed to the lack of progress in this area. Novel techniques have provided detailed quantitative insight into the determinants of microcirculatory and mitochondrial oxygenation. These techniques, which are based on the oxygen-dependent quenching of phosphorescence or delayed luminescence are briefly reviewed. The application of these techniques to animal models of shock and resuscitation revealed the heterogeneous nature of oxygen distributions and the alterations in oxygen distribution in the microcirculation and in mitochondria. These studies identified functional shunting in the microcirculation as an underlying cause of oxygen extraction deficit observed in states of shock and resuscitation. The translation of these concepts to the bedside has been enabled by our development and clinical introduction of hand-held microscopy. This tool facilitates the direct observation of the microcirculation and its alterations at the bedside under the conditions of shock and resuscitation. Studies identified loss of coherence between the macrocirculation and the microcirculation, in which resuscitation successfully restored systemic circulation but did not alleviate microcirculatory perfusion alterations. Various mechanisms responsible for these alterations underlie the loss of hemodynamic coherence during unsuccessful resuscitation procedures. Therapeutic resolution of persistent heterogeneous microcirculatory alterations is expected to improve outcomes in critically ill patients