Analysis of NIRS-based muscle oxygenation parameters by inclusion of adipose tissue thickness.

The assessment of muscle oxygenation by non-invasive near-infrared spectroscopy generally assumes a homogeneous medium, and this is flawed for large adipose tissue layers underneath the skin. Here we summarize the influence of the adipose tissue thickness on the oxygenation data, show that the adipose layer can be measured by NIRS and indicate a possible correction algorithm. Spectroscopic evidence suggests the usefulness of this algorithm, however, not in all subjects.

Tissue oxygenation during exercise measured with NIRS: reproducibility and influence of wavelengths.

Near-infrared spectroscopy (NIRS) is widely used for the measurement of skeletal muscle oxygenation during exercise as it reflects muscle metabolism, and most studies report a large variability between subjects. Here we assess the data quality of tissue oxygen saturation (SO2) and oxygenated (oxyHb) and deoxygenated (deoxyHb) haemoglobin concentrations recorded during an incremental cycling protocol in nine healthy volunteers. The protocol was repeated three times on the same day and a fourth session on a different day to estimate the reproducibility of the method with a broadband, spatially resolved spectroscopy (SRS) system. We found that the inter-subject variation in SO2 (standard deviation ≈ 6 %) was considerably larger than the reproducibility (≈ 1.5 %) both for the same-day and different-day tests. The reproducibility of changes in SO2 was better than 1 %.

Optical measurement of adipose tissue thickness and comparison with ultrasound, magnetic resonance imging, and callipers.

Near-infrared spectroscopy is used to quantify the subcutaneous adipose tissue thickness (ATT) over five muscle groups (vastus medialis, vastus lateralis, gastrocnemius, ventral forearm and biceps brachii muscle) of healthy volunteers (n=20). The optical lipid signal (OLS) was obtained from the second derivative of broad band attenuation spectra and the lipid absorption peak (lambda=930 nm). Ultrasound and MR imaging as well as mechanical calliper readings were taken as reference methods. The data show that the OLS is a good predictor for ATT (<16 mm) with absolute and relative errors of <0.8 mm and <24%, respectively. The optical method compares favourably with calliper reading. The finding of a non-linear relationship of optical signal vs. ultrasound is explained by a theoretical two-layer model based on the diffusion approximation for the transport of photons. The crosstalk between the OLS and tissue hemoglobin concentration changes during an incremental cycling exercise was found to be small, indicating the robustness of OLS. Furthermore, the effect of ATT on spatially-resolved spectroscopy measurements is shown to decrease the calculated muscle hemoglobin concentration and to increase oxygen saturation.