Interpretation of Near-Infrared Spectroscopy (NIRS) Signals in Skeletal Muscle.

Adeola A Sanni,Kevin K Mccully

doi:10.3390/jfmk4020028

Adeola A Sanni, Kevin K Mccully

Open Access

https://doi.org/10.3390/jfmk4020028

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Abstract

Near-infrared spectroscopy (NIRS) uses the relative absorption of light at 850 and 760 nm to determine skeletal muscle oxygen saturation. Previous studies have used the ratio of both signals to report muscle oxygen saturation. Purpose: The purpose of this pilot study is to assess the different approaches used to represent muscle oxygen saturation and to evaluate the pulsations of oxygenated hemoglobin/myoglobin (O2heme) and deoxygenated hemoglobin/myoglobin (Heme) signals. Method: Twelve participants, aged 20–29 years, were tested on the forearm flexor muscles using continuous-wave NIRS at rest. Measurements were taken during 2–3 min rest, physiological calibration (5 min ischemia), and reperfusion. Ten participants were included in the study analysis. Results: There was a significant difference in pulse size between O2heme and Heme signals at the three locations (p < 0.05). Resting oxygen saturation was 58.8% + 9.2%, 69.6% + 3.9%, and 89.2% + 6.9% when calibrated using O2heme, the tissue oxygenation/saturation index (TSI), and Heme, respectively. Conclusion: The difference in magnitude of O2heme and Heme pulses with each heartbeat might suggest different anatomical locations of these signals, for which calibrating with just one of the signals instead of the ratio of both is proposed. Calculations of physiological calibration must account for increased blood volume in the tissue because of the changes in blood volume, which appear to be primarily from the O2heme signal. Resting oxygen levels calibrated with Heme agree with theoretical oxygen saturation.

Highlights

Skeletal muscle contractions play an essential role in human locomotion by generating the forces required for movement [1]
This study found that physiological calibration includes a transient increase in blood volume, which appears to be entirely from the O2heme signal
Muscle oxygen levels provide an important window into the function of skeletal muscle

Summary

Introduction

Skeletal muscle contractions play an essential role in human locomotion by generating the forces required for movement [1]. A key to muscle contractions is the production of chemical energy (ATP) primarily from oxidative metabolism in muscle mitochondria [2,3]. Near-infrared spectroscopy (NIRS) has been increasingly used to study oxygen levels in skeletal muscle [4,5,6,7]. NIRS measurements of muscle oxygen level, blood flow, and metabolism are noninvasive and have shown good agreement with other measurement modalities [8]. Continuous-wavelength NIRS devices use physiological calibration (vascular occlusion) to 0%; this has been shown to allow accurate quantitative measurements in the skeletal muscle [4]

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