Mark Wefers Bettink

Verification of calibration in man and comparison with vascular occlusion tests in healthy volunteers with COMET 4 77 This study also presents the comparison of different oxygen consumption measurements in the arm. The COMET was compared to O2C, INVOS and SenTec OxiVenT™ during an arterial occlusion test in nine subjects. During arterial occlusion all oxygenation parameters show a decline in a comparable rate. Also, the measured decline in NIRS saturation of 0.21%/sec (12.6%/min) in this study is comparable to previously found values of 10.8, 13.2, 22.8%/min during occlusion of an extremity [26]. However, during a dynamic measurement for measuring mitochondrial oxygen consumption (mitoVO 2 ), with pressure on the measurement probe, a faster decline is seen. This decline is seen in all but one subject in figure 4 A with an equilibrium at 15 mmHg. Since the curve of this individual shows similarities to the measurements without local pressure on the probe, we think that probably the effect of local pressure on the sensor was inadequate. We hypothesize that in this case an equilibrium emerges between the still saturated hemoglobin and mitochondrial respiration, similar to the situation in laboratory animals [27]. Also, when a mitoVO 2 procedure is done on the sternum we do not see such equilibrium at a high mitoPO 2 value. Therefore, we think that the pressure on the skin sensor did not adequately push away the erythrocytes in the measurement volume. MitoVO 2 measurements should preferably be done on skin above a bone structure to allow the buffer of erythrocytes to be pushed away. The arm is therefore not the preferred site because the skin is not located above a flat bone. This likely resulted in a relatively slow median mitoVO 2 of 2.1 mmHg/s in comparison to measurements done on the sternum with a median mitoVO 2 5.8 mmHg/s measured on healthy volunteers in our lab [10]. When the skin sensor is on top of a bone structure, with a little pressure the microcirculation is blocked and the erythrocytes are pushed out of the measurement volume. In this study the applied pressure was not measured but this could add to the standardization of a mitoVO 2 maneuver and improve the repeatability. However, arterial occlusion tests can only be done on an extremity and therefore the mitoVO 2 values are different from other healthy volunteer studies [6, 10]. Whilst the forearm is not a preferred measurement site a large difference in mitoVO 2 could be demonstrated if pressure is exerted onto the COMET Skin Sensor, 2.1 mmHg/s compared to 1.3 mmHg/s during arterial occlusion. The oxygen buffer available in microcirculation likely accounts for the difference of 0.8 mmHg/s. Both the mode of measurement (hemoglobin-based versus non-hemoglobin-based) and differences in tissue penetration depth per technique might account to the observed differences in oxygen disappearance rates. The COMET has a penetration depth of less than a mm, in contrast to infra-red optical techniques ( > 900nm wavelength) with