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

Background: Noninvasive muscle oxygen saturation (SmO2) 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 SmO2 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 SmO2, tissue pH, and the microcirculatory index (MCI, an estimate of peripheral resistance). Subjects were exposed to two trials (≥ 4 weeks intervening) of lower body negative pressure (LBNP) applied at a rate of 3 mmHg/min until presyncope or voluntary termination. SmO2, 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 SmO2, pH, and MCI were statistically indistinguishable between the two trials (p≥0.17). Responses of SmO2, pH, and MCI were highly correlated between trials (r≥0.91; p≤0.004; slopes≥0.77), and each parameter tracked the reduction in SV (table). Conclusions: NIRS-derived measurements of SmO2, pH, and MCI were reproducible during central hypovolemia elicited by continuous application of LBNP. These findings support the use of SmO2 to monitor blood loss.