Mitochondrial oxygen monitoring with COMET: verification of calibration in man and comparison with vascular occlusion tests in healthy volunteers

Mitochondria are the primary consumers of oxygen and therefore an important location for oxygen availability and consumption measurement. A technique has been developed for mitochondrial oxygen tension (mitoPO2) measurement, incorporated in the COMET. In contrast to most textbooks, relatively high average mitoPO2 values have been reported. The first aim of this study was to verify the validity of the COMET calibration for mitoPO2 measurements in human skin. The second aim was to compare the dynamics of mitoPO2 to several other techniques assessing tissue oxygenation. Firstly, we performed a two-point calibration. Mitochondrial oxygen depletion was achieved with vascular occlusion. A high mitoPO2 was reached by local application of cyanide. MitoPO2 was compared to the arterial oxygen partial pressure (PaO2). Secondly, for deoxygenation kinetics we compared COMET variables with the LEA O2C, SenTec OxiVenT™ and Medtronic INVOS™ parameters during a vascular occlusion test. 20 healthy volunteers were recruited and resulted in 18 datasets (2 times 9 subjects). The lowest measured mitoPO2 value per subject had a median [IQR] of 3.0 [1.0–4.0] mmHg, n = 9. After cyanide application the mitoPO2 was 94.1 mmHg [87.2–110.9] and did not differ significantly (n = 9, p = 0.5) from the PaO2 of 101.0 [98.0–106.0] mmHg. In contrast to O2C, OxiVenT™ and INVOS parameters, mitoPO2 declined within seconds with pressure on the probe. The kinetics from this decline are used to mitochondrial oxygen consumption (mitoVO2). This study validates the calibration of the COMET device in humans. For mitoVO2 measurements not only blood flow cessation but application of local pressure is of great importance to clear the measurement site of oxygen-carrying erythrocytes.

Evaluation of Different Near-Infrared Spectroscopy Devices for Assessing Tissue Oxygenation with a Vascular Occlusion Test in Healthy Volunteers

Near-infrared spectroscopy devices can measure peripheral tissue oxygen saturation (StO₂). This study aims to compare StO₂ using INVOS® and different O3™ settings (O3^25:75 and O3^30:70). Twenty adults were recruited. INVOS® and O3™ probes were placed simultaneously on 1 side of forearm. After baseline measurement, the vascular occlusion test was initiated. The baseline value, rate of deoxygenation and reoxygenation, minimum and peak StO₂, and time from cuff release to peak value were measured. The parameters were compared using ANOVA and Kruskal-Wallis tests. Bonferroni’s correction and Mann-Whitney pairwise comparison were used for post hoc analysis. The agreement between StO₂ of devices was evaluated using Bland-Altman plots. INVOS® baseline value was higher (79.7 ± 6.4%) than that of O3^25:75 and O3^30:70 (62.4 ± 6.0% and 63.7 ± 5.5%, respectively, <i>p</i> &#x3c; 0.001). The deoxygenation rate was higher with INVOS® (10.6 ± 2.1%/min) than with O3^25:75 and O3^30:70 (8.4 ± 2.2%/min, <i>p</i> = 0.006 and 7.5 ± 2.1%/min, <i>p</i> &#x3c; 0.001). The minimum and peak StO₂ were higher with INVOS®. No significant difference in the reoxygenation rate was found between the devices and settings. The time to reach peak after cuff deflation was faster with INVOS® (both <i>p</i> &#x3c; 0.001). Other parameters were similar. There were no differences between the different O3™ settings. There were differences in StO₂ measurements between the devices, and these devices should not be interchanged. Differences were not observed between O3™ device settings.