Mitochondrial oxygen tension can be measured in vivo by means of oxygen-dependent quenching of delayed fluorescence of protoporphyrin IX (PpIX). Here we demonstrate that mitochondrial ${\mathrm{PO}}_2$ ($\text{mito}{\mathrm{PO}}_2$) can be measured in the skin of a rat after topical application of the PpIX precursor 5-aminolevulinic acid (ALA). Calibration of $\text{mito}{\mathrm{PO}}_2$ measurements was done by comparison with simultaneous measurements of the cutaneous microvascular ${\mathrm{PO}}_2$ This was done under three different conditions: in normal skin tissue, in nonrespiration skin tissue due to the application of cyanide, and in anoxic skin tissue after the ventilation with 100% nitrogen. The results of this study show that it is feasible to measure the $\text{mito}{\mathrm{PO}}_2$ after the topical application of ALA cream by means of the PpIX–triplet state lifetime technique.

Introduction: Although mitochondrial dysfunction is proposed to be involved in the pathophysiology of sepsis, conflicting results are reported. Variation in methods used to assess mitochondrial function might contribute to this controversy. A non-invasive method for monitoring mitochondrial function might help overcome this limitation. Therefore, this study explores the possibility of in vivo monitoring of mitochondrial oxygen tension (mitoPO₂) and local mitochondrial oxygen consumptionin in an endotoxin-induced septic animal model. Methods: Animals (rats n = 28) were assigned to a control group (no treatment), or to receive lipopolysaccharide without fluid resuscitation (LPS-NR) or lipopolysaccharide plus fluid resuscitation (LPS-FR). Sepsis was induced by intravenous LPS injection (1.6 mg/kg during 10 min), fluid resuscitation was performed by continuous infusion of a colloid solution, 7 ml kg⁻¹ h⁻¹ and a 2-ml bolus of the same colloid solution. MitoPO₂ and ODR were measured by means of the protoporphyrin IX-triplet state lifetime technique (PpIX-TSLT). Kinetic aspects of the drop in mitoPO₂ were recorded during 60s of skin compression. ODR was derived from the slope of the mitoPO₂ oxygen disappearance curve. Measurements were made before and 3 h after induction of sepsis. Results: At baseline (t0) all rats were hemodynamically stable. After LPS induction (t1), significant (p < 0.05) hemodynamic changes were observed in both LPS groups. At t0, mitoPO₂ and ODR were 59 ± 1 mmHg, 64 ± 3 mmHg, 68 ± 4 mmHg and 5.0 ± 0.3 mmHg s⁻¹, 5.3 ± 0.5 mmHg s⁻¹, 5.7 ± 0.5 mmHg s⁻¹ in the control, LPS-FR and LPS-NR groups, respectively; at t1 these values were 58 ± 5 mmHg, 50 ± 2.3 mmHg, 30 ± 3.3 mmHg and 4.5 ± 0.5 mmHg s⁻¹, 3.3 ± 0.3 mmHg s⁻¹, 1.8 ± 0.3 mmHg s⁻¹, respectively. At t1, only mitoPO₂ showed a significant difference between the controls and LPS-NR. In contrast, at t1 both LPS groups showed a significantly lower ODR compared to controls. Conclusion: These data show the feasibility to monitor alterations in mitochondrial oxygen consumption in vivo by PpIX-TSLT in a septic rat model. These results may contribute to the development of a clinical device to monitor mitochondrial function in the critically ill.

The protoporphyrin IX-triplet state lifetime technique (PpIX-TSLT) is proposed as a potential clinical non-invasive tool to monitor mitochondrial function. This technique has been evaluated in several animal studies. Mitochondrial respirometry allows measurement in vivo of mitochondrial oxygen tension (mitoPO₂) and mitochondrial oxygen consumption (mitoVO₂) in skin. This study describes the first use of a clinical prototype in skin of humans. The clinical prototype was tested in 30 healthy volunteers. A self-adhesive patch containing 2 mg 5-aminolevulinic acid (ALA) was applied on the skin of the anterior chest wall (sternal) for induction of mitochondrial protoporphyrin IX and was protected from light for 5 h. MitoPO₂ was measured by means of oxygen-dependent delayed fluorescence of protoporphyrin IX. MitoVO₂ was determined by dynamic mitoPO₂ measurements on the primed skin, while locally blocking oxygen supply by applying local pressure with the measurement probe. MitoPO₂ was recorded before and during a 60-s period of compression of the microcirculation, at an interval of 1 Hz. Oxygen consumption (i.e. the local oxygen disappearance rate) was calculated from the decay of the mitoPO₂ slope. Oxygen-dependent delayed fluorescence measurements were successfully performed in the skin of 27 volunteers. The average value (± SD) of mitoPO₂ was 44 ± 17 mmHg and mean mitoVO₂ values were 5.8 ± 2.3 and 6.1 ± 1.6 mmHg s^-1 at a skin temperature of 34°C and 40°C, respectively. No major discomfort during measurement and no long-term dermatological abnormalities were reported in a survey performed 1 month after measurements. These results show that the clinical prototype allows measurement of mitochondrial oxygenation and oxygen consumption in humans. The development of this clinically applicable device offers opportunities for further evaluation of the technique in humans and the start of first clinical studies.

Mitochondrial dysfunction has been linked to disease progression in COVID-19 patients. This observational pilot study aimed to assess mitochondrial function in COVID-19 patients at intensive care unit (ICU) admission (T1), seven days thereafter (T2), and in healthy controls and a general anesthesia group. Measurements consisted of in vivo mitochondrial oxygenation and oxygen consumption, in vitro assessment of mitochondrial respiration in platelet-rich plasma (PRP) and peripheral blood mononuclear cells (PBMCs), and the ex vivo quantity of circulating cell-free mitochondrial DNA (mtDNA). The median mitoVO₂ of COVID-19 patients on T1 and T2 was similar and tended to be lower than the mitoVO₂ in the healthy controls, whilst the mitoVO₂ in the general anesthesia group was significantly lower than that of all other groups. Basal platelet (PLT) respiration did not differ substantially between the measurements. PBMC basal respiration was increased by approximately 80% in the T1 group when contrasted to T2 and the healthy controls. Cell-free mtDNA was eight times higher in the COVID-T1 samples when compared to the healthy controls samples. In the COVID-T2 samples, mtDNA was twofold lower when compared to the COVID-T1 samples. mtDNA levels were increased in COVID-19 patients but were not associated with decreased mitochondrial O₂ consumption in vivo in the skin, and ex vivo in PLT or PBMC. This suggests the presence of increased metabolism and mitochondrial damage.

Introduction: The newly introduced Cellular Oxygen METabolism (COMET^®) monitor enables the measurement of mitochondrial oxygen tension (mitoPO₂) using the protoporphyrin IX triplet state lifetime technique (PpIX-TSLT). This study aims to investigate the feasibility and applicability of the COMET^® measurements in the operating theatre and study the behavior of the new parameter mitoPO2 during stable operating conditions. Methods: In this observational study mitochondrial oxygenation was measured in 20 patients during neurosurgical procedures using the COMET^® device. Tissue oxygenation and local blood flow were measured by the Oxygen to See (O2C). Primary outcomes included mitoPO₂, skin temperature, mean arterial blood pressure, local blood flow and tissue oxygenation. Results: All patients remained hemodynamically stable during surgery. Mean baseline mitoPO₂ was 60 ± 19 mmHg (mean ± SD) and mean mitoPO₂ remained between 40–60 mmHg during surgery, but tended to decrease over time in line with increasing skin temperature. Conclusion: This study presents the feasibility of mitochondrial oxygenation measurements as measured by the COMET^® monitor in the operating theatre and shows the parameter mitoPO₂ to behave in a stable and predictable way in the absence of notable hemodynamic alterations. The results provide a solid base for further research into the added value of mitochondrial oxygenation measurements in the perioperative trajectory.