Protoporphyrin IX-triplet State Lifetime Technique Research Articles

Body composition, mitochondrial oxygen metabolism and metabolome of patients with obesity before and after bariatric surgery (COMMITMENT): protocol for a monocentric prospective cohort study

IntroductionObesity, defined as a body mass index ≥30 kg/m2, is one of the most prevalent health conditions worldwide. It is part of the metabolic syndrome, which encompasses arterial hypertension, dyslipoproteinaemia and diabetes. Obesity is viewed as a systemic disease with pathophysiological mechanisms on the molecular level. Dysfunction of the mitochondrion and systemic low-grade inflammation are among the proposed causes for the metabolic changes. In severe cases of obesity, laparoscopic sleeve gastrectomy, a bariatric operation, can achieve the desired weight loss and has been associated with clinical outcome improvement. Hitherto, the influence of patients’ body composition on mitochondrial function and concomitant metabolic changes has not been fully understood. This study aims to quantify the patient’s body composition before and after laparoscopic sleeve gastrectomy and to correlate these findings with changes in mitochondrial oxygen metabolism, metabolome and immune status.Methods and analysisIn this prospective monocentric cohort study, patients undergoing laparoscopic sleeve gastrectomy (n=30) at Jena University Hospital (Germany) will be assessed before surgery and at four time points during a 1-year follow-up. Body composition will be measured by bioimpedance analysis. Non-invasive assessment of mitochondrial oxygen metabolism using protoporphyrin IX-triplet state lifetime technique (PPIX-TSLT) and blood sampling for, among other, metabolomic and immunological analysis, will be performed. The primary outcome is the difference in relative fat mass between the preoperative time point and 6 months postoperatively. Further outcomes comprise longitudinal changes of PPIX-TSLT and metabolic and immunological variables. Outcomes will be assessed using paired t-tests, Wilcoxon signed-rank tests and regression analyses.Ethics and disseminationThe study was approved by the Ethics Committee of Friedrich Schiller University Jena (2018-1192-BO). Written informed consent will be obtained from all patients prior to enrolment in the study. The results will be published in peer-reviewed journals and presented at appropriate conferences.Trial registration numberDRKS00015891.

MITOCHONDRIAL OXYGENATION DURING CARDIOPULMONARY BYPASS: A PILOT STUDY

Introduction Cardiopulmonary bypass (CPB) is used during cardiac-surgery and provides oxygenated blood to the tissues. Ensuring adequate oxygenation is essential for the preservation of organ function during CPB. Both hypoxia and hyperoxia result in undesired outcomes and a narrow window for optimal oxygenation exists. Current perioperative monitoring techniques are not always sufficient to monitor adequate oxygenation, especially in high-risk patients. Near-infrared spectroscopy (NIRS) tissue oxygen measurement (StO2) is widely used to monitor tissue oxygenation but responds slowly to change. However, the novel non-invasive COMET-monitor measures the mitochondrial oxygen tension (mitoPO2) in the epidermis using the enhanced mitochondrial Protoporphyrin IX Triplet State Lifetime Technique (PpIX-TSLT). The mitoPO2 reflects the local balance between oxygen supply and consumption and provides an insight into the microcirculatory system [1]. This pilot study investigates the correlation between mitoPO2 versus StO2 and varying hemodynamic conditions during CPB and the development of acute kidney injury (AKI). Methods This single center observational study examined 41 cardiac-surgery patients requiring CPB. Six-hours pre-operatively, patients received a 5-aminolevulinc acid plaster on the upper arm to facilitate the mitoPO2 measurements. After induction of anesthesia, both the COMET and INVOS measurements were executed in 5-minute intervals throughout the procedure. To compare mitoPO2 with StO2, NIRS probes were placed near the COMET probe. Post-operatively, both measurements were stopped simultaneously, and the patients were observed until discharge for the development of AKI. Results NIRS data was available for 31 of the 41 patients. The median mitoPO2 at the start of surgery was 63.5 [40.0 – 74.8] mmHg and significantly decreased (p Discussion In contrast to StO2, the mitoPO2 decreased significantly during CPB time. A hemodilution study in pigs also demonstrated that mitoPO¬2 is more likely to change than StO2, which reacted slowly to change. The mitoPO2 also significantly decreased during non-pulsatile flow and increased once pulsatile flow returned, whilst the StO2 did not. This highlights the sensitivity of mitoPO2 to detect circulatory and microvascular changes early through the analysis of oxygen delivery on a cellular level. Lastly, the association found between the AKI and the non-AKI group in time spent below a mitoPO2 of 20 mmHg must be further examined in a well powered study. However, it highlights the potential of this technique to be an early predictor of ischemia.

In Vivo Assessment of Mitochondrial Oxygen Consumption.

The Protoporphyrin IX-Triplet State Lifetime Technique (PpIX-TSLT) has been proposed by us as a potential clinical noninvasive tool for monitoring mitochondrial function. We have been working on the development of mitochondrial respirometry for monitoring mitochondrial oxygen tension (mitoPO2) and mitochondrial oxygen consumption (mitoVO2) in skin. In this work, we describe the principles of the method in small experimental animals.

A Pilot Study on the Association of Mitochondrial Oxygen Metabolism and Gas Exchange During Cardiopulmonary Exercise Testing: Is There a Mitochondrial Threshold?

Background: Mitochondria are the key players in aerobic energy generation via oxidative phosphorylation. Consequently, mitochondrial function has implications on physical performance in health and disease ranging from high performance sports to critical illness. The protoporphyrin IX-triplet state lifetime technique (PpIX-TSLT) allows in vivo measurements of mitochondrial oxygen tension (mitoPO2). Hitherto, few data exist on the relation of mitochondrial oxygen metabolism and ergospirometry-derived variables during physical performance. This study investigates the association of mitochondrial oxygen metabolism with gas exchange and blood gas analysis variables assessed during cardiopulmonary exercise testing (CPET) in aerobic and anaerobic metabolic phases.Methods: Seventeen volunteers underwent an exhaustive CPET (graded multistage protocol, 50 W/5 min increase), of which 14 were included in the analysis. At baseline and for every load level PpIX-TSLT-derived mitoPO2 measurements were performed every 10 s with 1 intermediate dynamic measurement to obtain mitochondrial oxygen consumption and delivery (mitoO2, mitoO2). In addition, variables of gas exchange and capillary blood gas analyses were obtained to determine ventilatory and lactate thresholds (VT, LT). Metabolic phases were defined in relation to VT1 and VT2 (aerobic: <VT1, aerobic-anaerobic transition: ≥VT1 and <VT2 and anaerobic: ≥VT2). We used linear mixed models to compare variables of PpIX-TSLT between metabolic phases and to analyze their associations with variables of gas exchange and capillary blood gas analyses.Results: MitoPO2 increased from the aerobic to the aerobic-anaerobic phase followed by a subsequent decline. A mitoPO2 peak, termed mitochondrial threshold (MT), was observed in most subjects close to LT2. MitoO2 increased during CPET, while no changes in mitoO2 were observed. MitoPO2 was negatively associated with partial pressure of end-tidal oxygen and capillary partial pressure of oxygen and positively associated with partial pressure of end-tidal carbon dioxide and capillary partial pressure of carbon dioxide. MitoO2 was associated with cardiovascular variables. We found no consistent association for mitoO2.Conclusion: Our results indicate an association between pulmonary respiration and cutaneous mitoPO2 during physical exercise. The observed mitochondrial threshold, coinciding with the metabolic transition from an aerobic to an anaerobic state, might be of importance in critical care as well as in sports medicine.

Monitoring mitochondrial PO2: the next step.

To fully exploit the concept of hemodynamic coherence in resuscitating critically ill one should preferably take into account information about the state of parenchymal cells. Monitoring of mitochondrial oxygen tension (mitoPO2) has emerged as a clinical means to assess information of oxygen delivery and oxygen utilization at the mitochondrial level. This review will outline the basics of the technique, summarize its development and describe the rationale of measuring oxygen at the mitochondrial level. Mitochondrial oxygen tension can be measured by means of the protoporphyrin IX-Triplet State Lifetime Technique (PpIX-TSLT). After validation and use in preclinical animal models, the technique has recently become commercially available in the form of a clinical measuring system. This system has now been used in a number of healthy volunteer studies and is currently being evaluated in studies in perioperative and intensive care patients in several European university hospitals. PpIX-TSLT is a noninvasive and well tolerated method to assess aspects of mitochondrial function at the bedside. It allows doctors to look beyond the macrocirculation and microcirculation and to take the oxygen balance at the cellular level into account in treatment strategies.

Non-invasive Assessment of Mitochondrial Oxygen Metabolism in the Critically Ill Patient Using the Protoporphyrin IX-Triplet State Lifetime Technique-A Feasibility Study.

The imbalance of oxygen delivery and oxygen consumption resulting in insufficient tissue oxygenation is pathognomonic for all forms of shock. Mitochondrial function plays an important role in the cellular oxygen metabolism and has been shown to impact a variety of diseases in the intensive care setting, specifically sepsis. Clinical assessment of tissue oxygenation and mitochondrial function remains elusive. The in vivo protoporphyrin IX-triplet state lifetime technique (PpIX-TSLT) allows the direct, non-invasive measurement of mitochondrial oxygen tension (mitoPO2) in the human skin. Our recently established measurement protocol for the Cellular Oxygen Metabolism (COMET) Monitor, a novel device employing the PpIX-TSLT, additionally allows the evaluation of oxygen consumption (mitoVO2) and delivery (mitoDO2). In the intensive care setting, these variables might provide new insight into mitochondrial oxygen metabolism and especially mitoDO2 might be a surrogate parameter of microcirculatory function. However, the feasibility of the PpIX-TSLT in critically ill patients has not been analyzed systematically. In this interim study analysis, we evaluated PpIX-TSLT measurements of 40 patients during the acute phase of sepsis. We assessed (a) potential adverse side effects of the method, (b) the rate of analyzable measurements, (c) the stability of mitoPO2, mitoVO2, and mitoDO2, and (d) potential covariates. Due to excessive edema in patients with sepsis, we specifically analyzed the association of patients' hydration status, assessed by bioimpedance analysis (BIA), with the aforementioned variables. We observed no side effects and acquired analyzable measurements sessions in 92.5% of patients (n = 37/40). Different measures of stability indicated moderate to good repeatability of the PpIX-TSLT variables within one session of multiple measurements. The determined limits of agreement and minimum detectable differences may be helpful in identifying outlier measurements. In conjunction with signal quality they mark a first step in developing a previously unavailable standardized measurement quality protocol. Notably, higher levels of hydration were associated with lower mitochondrial oxygen tension. We conclude that COMET measurements are viable in patients with sepsis. To validate the clinical and diagnostic relevance of the PpIX-TSLT using the COMET in the intensive care setting, future studies in critically ill patients and healthy controls are needed.

Non-invasive versus ex vivo measurement of mitochondrial function in an endotoxemia model in rat: Toward monitoring of mitochondrial therapy

Mitochondrial function has been predominantly measured ex vivo. Due to isolation and preservation procedures ex vivo measurements might misrepresent in vivo mitochondrial conditions. Direct measurement of in vivo mitochondrial oxygen tension (mitoPO2) and oxygen disappearance rate (ODR) with the protoporphyrin IX‐triplet state lifetime technique (PpIX-TSLT) might increase our understanding of mitochondrial dysfunction in the pathophysiology of acute disease.LPS administration decreased mitochondrial respiration (ODR) in vivo but did not alter mitochondrial function as assessed with ex vivo techniques (high resolution respirometry and specific complex determinations). PpIX-TSLT measures in vivo mitoPO2 and ODR and can be applied non-invasively at the skin.

Oxygen-dependent delayed fluorescence of protoporphyrin IX measured in the stomach and duodenum during upper gastrointestinal endoscopy.

Protoporphyrin IX‐triplet state lifetime technique (PpIX‐TSLT) is a method used to measure oxygen (PO2) in human cells. The aim of this study was to assess the technical feasibility and safety of measuring oxygen‐dependent delayed fluorescence of 5‐aminolevulinic acid (ALA)‐induced PpIX during upper gastrointestinal (GI) endoscopy. Endoscopic delayed fluorescence measurements were performed 4 hours after oral administration of ALA in healthy volunteers. The ALA dose administered was 0, 1, 5 or 20 mg/kg. Measurements were performed at three mucosal spots in the gastric antrum, duodenal bulb and descending duodenum with the catheter above the mucosa and while applying pressure to induce local ischemia and monitor mitochondrial respiration. During two endoscopies, measurements were performed both before and after intravenous administration of butylscopolamine. Delayed fluorescence measurements were successfully performed during all 10 upper GI endoscopies. ALA dose of 5 mg/kg showed adequate signal‐to‐noise ratio (SNR) values >20 without side effects. All pressure measurements showed significant prolongation of delayed fluorescence lifetime compared to measurements performed without pressure (P < .001). Measurements before and after administration of butylscopolamine did not differ significantly in the duodenal bulb and descending duodenum. Measurements of oxygen‐dependent delayed fluorescence of ALA‐induced PpIX in the GI tract during upper GI endoscopy are technically feasible and safe.

A pilot study of exercise-induced changes in mitochondrial oxygen metabolism measured by a cellular oxygen metabolism monitor (PICOMET)

Impaired tissue oxygenation is the key pathomechanism in the development of organ dysfunction in shock; mitochondrial impairment can aggravate the condition. However, measuring tissue oxygenation directly and non-invasively still poses a clinical challenge. A novel device (COMET) allows the assessment of mitochondrial oxygen metabolism using the Protoporphyrin IX Triplet State Lifetime Technique (PpIX-TSLT). Critically ill patients, especially in sepsis, often exhibit oedema which may interfere with the COMET measurement. Furthermore, patients' physical activity level differs significantly before and during hospitalisation. Thus, the aim of this study was to identify the effects of physical activity and body composition on mitochondrial oxygen tension (mitoPO2) and consumption (mitoVO2) in healthy controls (N = 40). Furthermore, the study tested the repeatability of the COMET variables and identified covariates. Multiple COMET measurements were performed before (T1, T2), during and after (T3, T4) ergometry. Body composition was assessed by bioimpedance analysis. Physiological variables (blood pressure, heart rate, oxygen saturation) were recorded. In the analytical sample (n = 26), physical activity significantly decreased mitoVO2; other COMET variables remained unchanged between T2 and T3. During ergometry, mitoPO2 increased significantly. The distribution of body water significantly influenced mitoVO2. In our setting, the method demonstrated moderate repeatability. Variables of fitness (heart rate recovery, phase angle and physical activity level), signal quality and duration of exposure to 5-aminolevulinic acid (obligatory for PpIX-TSLT) were identified as significant covariates of mitoVO2. Mitochondrial oxygen delivery (mitoDO2) was established as a new variable of COMET analysis. Results of this pilot study should be validated in future studies.

Subcellular Energetics and Metabolism: Potential Therapeutic Applications.

Part I of this review discussed the similarities between embryogenesis, mammalian adaptions to hypoxia (primarily driven by hypoxia-inducible factor-1 [HIF-1]), ischemia-reperfusion injury (and its relationship with reactive oxygen species), hibernation, diving animals, cancer, and sepsis, and it focused on the common characteristics that allow cells and organisms to survive in these states. Part II of this review describes techniques by which researchers gain insight into subcellular energetics and identify potential future tools for clinicians. In particular, P nuclear magnetic resonance to measure high-energy phosphates, serum lactate measurements, the use of near-infrared spectroscopy to measure the oxidation state of cytochrome aa3, and the ability of the protoporphyrin IX-triplet state lifetime technique to measure mitochondrial oxygen tension are discussed. In addition, this review discusses novel treatment strategies such as hyperbaric oxygen, preconditioning, exercise training, therapeutic gases, as well as inhibitors of HIF-1, HIF prolyl hydroxylase, and peroxisome proliferator-activated receptors.

A monitor for Cellular Oxygen METabolism (COMET): monitoring tissue oxygenation at the mitochondrial level

After introduction of the protoporphyrin IX-triplet state lifetime technique as a new method to measure mitochondrial oxygen tension in vivo, the development of a clinical monitor was started. This monitor is the “COMET”, an acronym for Cellular Oxygen METabolism. The COMET is a non-invasive electrically powered optical device that allows measurements on the skin. The COMET is easy to transport, due to its lightweight and compact size. After 5-aminolevulinic acid application on the human skin, a biocompatible sensor enables detection of PpIX in the mitochondria. PpIX acts as a mitochondrially located oxygen-sensitive dye. Three measurement types are available in the touchscreen-integrated user interface, ‘Single’, ‘Interval’ and ‘Dynamic measurement’. COMET is currently used in several clinical studies in our institution. In this first description of the COMET device we show an incidental finding during neurosurgery. To treat persisting intraoperative hypertension a patient was administered clonidine, but due to rapid administration an initial phase of peripheral vasoconstriction occurred. Microvascular flow and velocity parameters measured with laser-doppler (O2C, LEA Medizintechnik) decreased by 44 and 16% respectively, but not the venous-capillary oxygen saturation. However, mitochondrial oxygen tension in the skin detected by COMET decreased from a steady state of 48 to 16 mmHg along with the decrease in flow and velocity. We conclude that COMET is ready for clinical application and we see the future for this bedside monitor on the intensive care, operating theater, and testing of mitochondrial effect of pharmaceuticals.

Cutaneous Respirometry as Novel Technique to Monitor Mitochondrial Function: A Feasibility Study in Healthy Volunteers.

BackgroundThe 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.MethodsThe 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.ResultsOxygen-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.ConclusionThese 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.

Non-invasive monitoring of mitochondrial oxygenation and respiration in critical illness using a novel technique.

IntroductionAlthough 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.MethodsAnimals (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.ResultsAt 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.ConclusionThese 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.

In vivo assessment of mitochondrial oxygen consumption.

The protoporphyrin IX-triplet state lifetime technique (PpIX-TSLT) has been proposed by us as a potential clinical noninvasive tool for monitoring mitochondrial function. We have been working on the development of mitochondrial respirometry for monitoring mitochondrial oxygen tension (mitoPO2) and mitochondrial oxygen consumption (mitoVO2) in skin. In this work we describe the principles of the method in experimental animals.

Cutaneous mitochondrial respirometry: non-invasive monitoring of mitochondrial function.

The recently developed technique for measuring cutaneous mitochondrial oxygen tension (mitoPO2) by means of the Protoporphyrin IX-Triplet State Lifetime Technique (PpIX-TSLT) provides new opportunities for assessing mitochondrial function in vivo. The aims of this work were to study whether cutaneous mitochondrial measurements reflect mitochondrial status in other parts of the body and to demonstrate the feasibility of the technique for potential clinical use. The first part of this paper demonstrates a correlation between alterations in mitochondrial parameters in skin and other tissues during endotoxemia. Experiments were performed in rats in which mitochondrial dysfunction was induced by a lipopolysaccharide-induced sepsis (n = 5) and a time control group (n = 5). MitoPO2 and mitochondrial oxygen consumption (mitoVO2) were measured using PpIX-TSLT in skin, liver and buccal mucosa of the mouth. Both skin and buccal mucosa show a significant mitoPO2-independent decrease (P < 0.05) in mitoVO2 after LPS infusion (a decrease of 37 and 39% respectively). In liver both mitoPO2 and mitoVO2 decreased significantly (33 and 27% respectively). The second part of this paper describes the clinical concept of monitoring cutaneous mitochondrial respiration in man. A first prototype of a clinical PpIX-TSLT monitor is described and its usability is demonstrated on human skin. We expect that clinical implementation of this device will greatly contribute to our understanding of mitochondrial oxygenation and oxygen metabolism in perioperative medicine and in critical illness. Our ultimate goal is to develop a clinical monitor for mitochondrial function and the current results are an important step forward.

Measuring Mitochondrial Oxygen Tension

The protoporphyrin IX-triplet state lifetime technique (PpIX-TSLT) has been recently introduced as the first method to measure mitochondrial oxygen tension (mitoPO2) 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 mitoPO2 in humans. MitoPO2 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 PO2 by the Stern-Volmer equation. In vivo measurements of mitoPO2 in liver, heart, and skin of rats have revealed surprisingly high values of typically several tens of mm Hg. Clinical measurements of mitoPO2 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 mitoPO2 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.