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Studies have suggested that tissue oxygenation (StO2) measured on the thenar muscle is not sensitive to track acute changes in hemodynamics due to reductions in central blood volume. We aimed to investigate the feasibility of StO2 measurements in the mouth as a quantitative indicator comparable with StO2 measurements obtained from the thenar eminence during changes in central blood volume (CBV).
We performed a head-up tilt (HUT) test in 10 healthy volunteers as an experimental model to reduce CBV. StO2 was continuously measured using two devices (InSpectra model 650): a multiple depth optical probe was placed over the thenar eminence (15 and 25 mm) and a 1-mm probe was placed in the mouth. Subjects were placed on an electrically driven tilt table with a footboard. After 5 minute baseline measurements in the supine position, the table was tilted up to 70° and returned to the supine position after 10 minutes. StO2 readings were analyzed at the lowest stroke volume value.
All subjects (mean age: 23 ± 6; six males) tolerated well the supine and head-up positions. Cardiac output significantly decreased in the HUT position; simultaneous decrease in StO2 was observed in the mouth, but not in the thenar. The general results of the HUT test are shown in Tables Tables11 and and22.
Near-infrared spectroscopy can be used in the mouth to track changes in central blood volume. Our results showed that buccal StO2 is an earlier indicator of acute hemodynamic responses to the HUT-induced central hypovolemia than thenar StO2.