The use of cardiopulmonary bypass (CPB) during cardiac surgery has a profound effect on the human body since circulation, respiration, homeostasis, and thermoregulation are all affected by the CPB. Maintaining an adequate tissue oxygen supply is essential to preserve cellular oxygenation and subsequently the preservation of organ function during CPB. The supply of oxygen is dependent on the red blood cell (RBC) count, as well as the macrovascular and microvascular perfusion. Inadequate macro-/micro-vascular perfusion and/or shortage of RBCs (anemia) can lead to an inadequate supply of oxygen, resulting in tissue hypoxia. It is widely understood that hypoxia is an independent risk factor for postoperative complications, such as acute kidney injury (AKI), postoperative cognitive dysfunction (POCD), and postoperative delirium (1, 2). Cardiac surgery-associated kidney injury (CSA-AKI) is unfortunately common as the kidneys are especially susceptible to periods of low mean arterial blood pressure and hypoxia due to their high metabolic activity (3). Currently, no individual estimate can determine the minimum mean arterial blood pressure or critical oxygen tension before tissue injury or ischemia occurs. It is known that there is an oxygen reserve that must be depleted before the onset of ischemia. Thus, only a longer period of low oxygen levels contributes to potential ischemia, resulting in acute kidney injury.

On the other hand, striving for a hyperoxic state during CPB has not proven clinically beneficial (4). This is due to the fact that hyperoxia can lead to the generation of oxygen radicals, which play a role in the development of ischemic reperfusion injury (4). Moreover, hyperoxia can lead to alveolar collapse, resulting in atelectasis, which is one of the main contributing factors to postoperative hypoxia (4). This highlights the fact that both hypoxia and hyperoxia can result in undesired outcomes, and thus, a narrow window for optimal oxygenation exists during CPB.

The current perioperative monitoring techniques seem to be sufficient for the maintenance of adequate tissue oxygenation in most patients. However, in high-risk patients, the optimal oxygenation window during CPB is narrow and not completely understood (5). This accentuates the need for novel monitoring techniques that can measure tissue oxygenation. The recently introduced COMET^® monitor (Photonics Healthcare, Utrecht) could be of significance for these patients.

The Cellular Oxygen METabolism (COMET^®) monitor enables non-invasive, in vivo measurements of cutaneous mitochondrial oxygenation (mitoPO₂). In order to measure cutaneous mitoPO₂, the COMET^® device utilizes the protoporphyrin IX–triplet state lifetime technique (PpIX-TSLT) (6–8). This technique relies on the accumulation of an oxygen-sensitive delayed fluorescent porphyrin, namely, protoporphyrin IX (PpIX), which is endogenously generated in the mitochondria through the administration of 5-aminolevulinic acid (6–8). Subsequent PpIX singlet excited-state intersystem crossing into triplet states results in long-lived triplet states that are quenched efficiently by molecular oxygen, which is the only known quencher in vivo (6–8). The pioneering work conducted by E.G. Mik validated that the delayed fluorescence quenching by molecular oxygen was able to consistently quantify mitoPO₂ in isolated cells, isolated organs, and in vivo (6). Moreover, the fact that mitochondria are the end-users of oxygen, mitoPO₂ reflects the local balance between oxygen supply and oxygen consumption within the cell (6, 7, 9). This technique has also been proven feasible in measuring the cutaneous mitoPO₂ in healthy volunteers (10–12).

Near-infrared spectroscopy (NIRS) is a technique utilized to monitor tissue oxygenation, especially for the prevention of cerebral hypoxia (13). However, Chan et al. conducted a systematic review of 11 randomly controlled trials and found that current reference values for NIRS are varied and that the information on any association between NIRS measurements and outcomes such as POCD is limited (14). In recent laboratory and clinical studies, cutaneous mitoPO₂ has shown to be more sensitive in measuring subtle changes in local oxygen supply and demand than the NIRS technique. This sensitivity was demonstrated for the first time in a porcine model. In this experiment, hemodilution in pigs resulted in an earlier decrease in the cutaneous mitoPO₂ compared to the NIRS tissue oxygenation saturation measurements (StO₂) (15). Second, a recent pilot study in chronic anemia patients (16) has shown variable individual responses to cutaneous mitoPO₂ measurements during red blood cell transfusion. This further supports the potential of cutaneous mitoPO₂ measurements to provide an insight into tissue oxygenation. Therefore, it is assumed that cutaneous mitoPO₂ measurements can be an earlier indicator for tissue hypoxia than StO₂.

This pilot study's primary aim was to examine the feasibility of this measurement technique during cardiothoracic surgical procedures requiring a CPB. The impact of hemodynamic changes during CPB and its run time on the cutaneous mitoPO₂ were also analyzed. Examining the impact of CPB run time on cutaneous mitoPO₂ is essential since it is widely understood that long CPB run times lead to worse clinical outcomes (17). This decrease in clinical outcome is primarily attributed to increased myocardial stunning and increased inflammatory mediators during long CPB run times. The resulting inflammatory reaction contributes to increased vascular permeability, leading to an efflux of intravascular fluid into the interstitial space, causing microvascular dysfunction and an increased distance from the blood circulation to the target cell mitochondria (18, 19). Therefore, the expectation was that long CPB run times are associated with reduced mitochondrial oxygenation, tissue hypoxia, and poor outcomes.

Moreover, the importance or superiority of pulsatile flow during CPB has remained controversial and unresolved (17). There are two major CPB methods, namely, continuous flow or pulsatile flow utilizing a rotary pulsatile flow pump. However, in this academic hospital, only a continuous flow pump is used. Therefore, in order to examine the effects of non-pulsatile flow on a mitochondrial level, cutaneous mitoPO₂ will be examined during the CPB aortic cross-clamp period (a period in which patients have no pulsatile flow) and will be contrasted to the CPB period in which the patients have their own cardiac output.

In conclusion, we provide an impression of the cutaneous mitoPO₂ and its behavior during CPB. Cutaneous mitoPO₂ was also compared to simultaneous StO₂ measurements. The correlation between perioperative cutaneous mitoPO₂ measurements and clinical outcomes in cardiac surgery has also been investigated.

COMET^® data were successfully collected from all 41 patients who underwent cardiothoracic surgery requiring CPB. In total, 0 patient was excluded or replaced, and this is illustrated by a flowchart in Figure 2. The INVOS^® device, which was used for StO₂ measurements, was not available for 10 patients; thus, they do not have any StO₂ data. This is due to the introduction of a new NIRS monitor which was not used for this study to ensure data compatibility. Table 1 describes the baseline patient characteristics.

This pilot study demonstrates that it is feasible to examine cutaneous mitoPO₂ in patients undergoing cardiothoracic surgery requiring a CPB. Cutaneous mitoPO₂ was successfully measured intraoperatively using the COMET^® device based on the PpIX-TLST to monitor oxygen delivery on a mitochondrial level (7). Moreover, it was also evident that it was logistically feasible to measure the cutaneous mitoPO₂ during cardiothoracic procedures as all 41 patients were measured successfully without any major incidents.

The COMET^® enables the non-invasive measurement of in vivo cutaneous mitoPO₂ values. In this study, the mean cutaneous mitoPO₂ level at the start of surgery was 62 ± 23 mmHg, which is in-line with the typically reported cutaneous mitoPO₂ levels under baseline circumstances of 40–70 mmHg (9, 23). This is substantially higher than the classically derived and theorized mitoPO₂ values, which are based on a gradual decline in PO₂ from the vascular, interstitial, and cytosolic compartments (24–26). Historically, most studies reported low compartmental/tissue PO₂ levels, with values ranging from 10 to 17 mmHg based on measurements using oxygen electrodes (27, 28). This has resulted in the classically derived and estimated mitoPO₂ values of several mmHg (24–26). However, after the introduction of less invasive measurement techniques, it was found that tissue oxygen levels were substantially higher than those measured by oxygen electrodes (29). This has been exemplified by Bodmer et al. through the use of phosphorescence-based microvascular PO₂ measurements of the liver in which they measured tissue oxygen values of 60 mmHg (30), as opposed to classical studies using oxygen electrode measurements in which the measured tissue oxygen was between 10 and 20 mmHg (31). These recent insights into microvascular PO₂ suggest that the classical estimations of mitoPO₂ are potentially underestimated and are likely to be in the order of several tens of mmHg (32–34). This is in line with the average cutaneous mitoPO₂ as measured by the COMET^®.

It is important to note that cutaneous mitoPO₂ as measured with the COMET^® monitor is an average mitochondrial oxygen tension in the measurement area under the COMET^® sensor and not the oxygen tension of a single mitochondrion. The oxygen tension in individual mitochondria is heterogenic because of the presence of an oxygen gradient necessary to drive the oxygen flux. The mitochondria in the measurement site have differing mitoPO₂ values influenced by the location of each mitochondrion and its oxygen consumption rate. Mitochondria in close proximity to large vessels can have a higher oxygen tension than capillaries, and mitochondria located further away from capillaries and large vessels have a lower mitoPO₂ (35). Our current understanding of mitoPO₂ advocates that it should be interpreted as an oxygen balance, depending on both oxygen supply and consumption (36).

The information emphasized in the previous two paragraphs is important for the interpretation and understanding of the cutaneous mitoPO₂. Until now, there is no clear threshold at which it is safe to say that the cutaneous mitoPO₂ as measured by the COMET^® is too low and at which point an intervention must be made to increase it. In general, our knowledge about in vivo mitochondrial oxygen tension and oxygen-dependent functional and metabolic adaptation is limited. In vitro studies have previously shown that tissue cells start adapting to available oxygen at higher mitochondrial oxygen levels than isolated mitochondria. Isolated mitochondria only experience oxygen-limited ATP production at very low mitochondrial oxygen levels (<2 mmHg). This is due to the high oxygen affinity of the mitochondrial respiratory chain (37). However, a study examining physiological hearts found that tissue cells started adapting to available oxygenation levels at values as high as 20–40 mmHg (38). This oxygen dependence is driven by oxygen-sensing mechanisms such as the hypoxia-inducible factor (38–40). Such mechanisms result in oxygen-limited ATP production at much higher oxygen levels than previously assumed. However, as mitoPO₂ is an averaged value of all the mitochondria measured using a sensor, a low mitoPO₂ value could indicate that some of these mitochondria are hypoxic and in a state of shock while others are not. In order to improve the understanding and interpretation of cutaneous mitoPO₂, multiple clinical trials are currently being conducted.

NIRS was included in this study as it is a clinically available means to non-invasively assess tissue saturation, a measure for tissue oxygenation but actually measured in the vasculature. Unlike NIRS, in which the tissue saturation values are not calibrated, the values of the cutaneous mitoPO₂ are calibrated (9). In contrast to the changes exhibited by the cutaneous mitoPO₂ during the CPB run times, StO₂ measured by NIRS did not show any significant changes. In our patient cohort, only during extreme intraoperative events such as deep hypothermia with circulatory arrest, StO₂ decreased. This apparent earlier or more sensitive response of cutaneous mitoPO₂ to microcirculatory alterations than NIRS has been previously demonstrated in a hemodilution study on pigs. During hemodilution, cutaneous mitoPO₂ dropped well before StO₂ did (15).

In this study, we found that cutaneous mitoPO₂ progressively decreases with the length of the CPB run time. This is further highlighted when comparing the baseline cutaneous mitoPO₂ at the start of the CPB and the cutaneous mitoPO₂ at the end of the CPB run, which was significantly reduced. However, after a CPB run time of 180 min, there were few patients left; this potentially resulted in a bias as the differences in cutaneous mitoPO₂ were especially pronounced after 180 min. As previously mentioned in the Introduction, long CPB run times are associated with poor clinical outcomes (17). This can be due to a myriad of factors, one of which is that patients with prolonged CPB run times are often in a worse preoperative clinical condition requiring more complex surgeries than those with shorter run times augmenting the effect of the CPB run times. However, it is widely understood that a CPB with extracorporeal circulation results in multiple physiological changes such as decreased blood pressure, hemodilution, hypothermia, hyperoxia, and cardiac arrest, and each of these negatively influences the microcirculation (41). Moreover, the heart–lung machine also influences the pharmacokinetics of drugs during CPB run times (42). The heart–lung machine itself negatively impacts the microcirculation as contact of the non-endothelialized synthetic surface with the patient's blood leads to an activation of the innate immune system; this immune reaction associated with CPB carries many similarities to the systemic immune response syndrome (19, 43, 44).

Currently, there is still no consensus on whether pulsatile or non-pulsatile flow is preferred during CPB mainly because no clinical difference in outcome can be found, and there is a lack of clinical monitoring techniques to monitor the differences. This study attempted to evaluate if mitoPO₂ could detect changes related to pulsatile flow. This was attempted by comparing two distinct surgery groups, namely, the CABG group and the valve surgery group. As previously discussed in the Methods section, these two groups have different states of flow after commencing CPB. Due to the differing aortic cross-clamping times, after cross-clamping, the continuous blood flow is a result of the CPB pump. Cutaneous mitoPO₂ decreased significantly when the aorta was cross-clamped and increased significantly upon release and re-establishment of partial cardiac output. Other studies demonstrated similar results to Koning et al., showing that the initiation of non-pulsatile flow reduced microcirculatory perfusion and oxygenation (45). Furthermore, when comparing on-pump vs. off-pump surgeries, studies have shown that off-pump CABG surgeries resulted in a less significant decrease in microcirculatory perfusion and oxygenation than on-pump CABG surgeries (41, 46). This can partially be explained by the fact that rhythmic flow variations allow for increased oxygen offloading in the microcirculation (41, 47). These aforementioned studies further reinforce the fact that non-pulsatile flow results in a poor microcirculatory environment, concurring with the cutaneous mitoPO₂ measurements found in this study.

The changes in cutaneous mitoPO₂ during aortic cross-clamping and before aortic cross-clamping were not only witnessed on an individual level, as previously mentioned, but also witnessed when comparing patients undergoing CABG and patients undergoing valve surgery. This is because these surgeries have different aortic cross-clamping times after commencing extracorporeal circulation and thus different moments in time when the aorta is cross-clamped. The cutaneous mitoPO₂ was significantly higher during the period in which cardiac output with extracorporeal circulation co-existed in the 15- to 35-min time frame after commencing CPB in the CABG cohort than during the immediate aortic cross-clamping and CPB flow during valve surgery. This further exemplifies that the patients' own cardiac output most likely improves the oxygen availability in the skin during cardiothoracic surgery requiring a CPB.

The development of AKI is a major postoperative complication after cardiothoracic surgery with an occurrence of approximately 18% (1). The causes of AKI are multifactorial in origin, with no clear single parameter or factor augmentation to prevent AKI. Hypotension and anemia, both of which result in decreased oxygen delivery capacity, will synergistically contribute to the development of AKI (48). However, the cutaneous mitoPO₂ measured the overall end result of oxygen delivery in which all the factors that potentially contribute to the development in AKI play an important role. Therefore, it is hypothesized that the cutaneous mitoPO₂ might just be sensitive enough to detect potential perfusion issues. This is mainly due to the fact that poor perfusion in the skin and gastrointestinal tract usually prelude poor perfusion in other organs (49). Cutaneous mitoPO₂ can be measured as the accumulated time below a certain low oxygen level threshold potentially, highlighting poor perfusion. In this study, a method has been developed to determine the amount of time in minutes below a set threshold of cutaneous mitoPO₂.

When comparing the duration of time that the cutaneous mitoPO₂ was below a certain threshold in the AKI and the non-AKI group, an association was found between the duration of time they were below certain thresholds. This is most evident when the cutaneous mitoPO₂ was below 20 mmHg where the AKI group spent more time below this threshold in absolute minutes and as a percentage of the operation's duration. However, there was a bias in these data as the more prolonged procedures also had generally lower cutaneous mitoPO₂ values, and the negative effects associated with prolonged CPB run times, such as an increased inflammatory response, also become more pronounced (17). It is also important to note that the eGFR in the AKI group was lower than that in the non-AKI group as a lower eGFR is a risk factor for the development of AKI. Nonetheless, as this study was not powered for an AKI and non-AKI comparison, it is not possible to draw any substantial conclusions, but this is of great interest in further research utilizing the COMET^®.

In conclusion, this pilot study was successful in demonstrating the feasibility of this measurement technique during cardiothoracic surgical procedures. Cutaneous mitoPO₂ seems more sensitive to hemodynamic alterations during these surgeries than StO₂. The COMET^® device could potentially become a vital supplementary monitoring technique during cardiothoracic surgeries; however, further specific research must be conducted to prove this.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

The studies involving human participants were reviewed and approved by Medisch Ethische Toetsings Commissie Erasmus MC. The patients/participants provided their written informed consent to participate in this study.

FH and MH conceived and planned the experiments. RU, FH, and ML carried out the experiments. CW, RU, FH, ML, and EM contributed to the interpretation of the results. FH, RU, and CW took the lead in writing the manuscript. All authors provided critical feedback and helped shape the research, analysis, and manuscript. All authors contributed to the article and approved the submitted version.

This study was funded by the Young Investigator Grant of the Dutch Society of Anesthesiology and the Department of Anesthesiology in the Erasmus Medical Center, Rotterdam, the Netherlands.

EM is listed as an inventor on patents related to mitochondrial oxygen measurements held by the Academic Medical Center Amsterdam and the Erasmus Medical Center Rotterdam, the Netherlands. He is the founder and shareholder of Photonics Healthcare, a company that holds exclusive licenses to these patents and that markets the COMET^® system. RU is a minority shareholder of Photonics Healthcare. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.