Reproducibility of Near Infrared Spectroscopy (NIRS)‐Derived Peripheral Muscle Oxygenation Measurements at Rest and During Central Hypovolemia

Background Noninvasive muscle oxygen saturation (SmO 2 ) measurements from near‐infrared spectroscopy (NIRS) sensors have been demonstrated to track the severity of central hypovolemia. The reproducibility of these devices in detecting and tracking reductions in SmO 2 during central hypovolemia, however, has not been quantified. Methods 27 healthy human subjects (11 F, 16 M) were instrumented with a CareGuide 1100 muscle NIRS sensor (Reflectance Medical Inc.) on their right flexor carpi ulnaris muscle for assessment of SmO 2 , tissue pH, and the microcirculatory index (MCI, an estimate of peripheral resistance). Subjects were exposed to two trials (蠅 4 weeks intervening) of lower body negative pressure (LBNP) applied at a rate of 3 mmHg/min until presyncope or voluntary termination. SmO 2 , pH, and MCI were compared to stroke volume (SV) responses derived from a non‐invasive arterial pressure waveform. Results SV decreased by ~50% for both trials ( p =0.94), and time to LBNP termination was similar ( p =0.36). Both baseline and maximal SmO 2 , pH, and MCI were statistically indistinguishable between the two trials ( p 蠅0.17). Responses of SmO 2 , pH, and MCI were highly correlated between trials (r蠅0.91; p 蠄0.004; slopes蠅0.77), and each parameter tracked the reduction in SV (table). Conclusions NIRS‐derived measurements of SmO 2 , pH, and MCI were reproducible during central hypovolemia elicited by continuous application of LBNP. These findings support the use of SmO 2 to monitor blood loss. Funding Source: US Army MRMCTrial 1 Trial 2R 2P‐Value Slope R 2P‐Value SlopeSmO 2 vs. SV 0.94 <0.001 0.09 0.97 <0.001 0.12pH vs. SV 0.89 0.001 0.001 0.91 <0.001 0.001MCI vs. SV 0.82 <0.001 0.31 0.96 <0.001 0.30