Measuring oxygen saturation by physiological calibration

This simple approach can be applied to almost any mNIRS system. The principle is to expose the muscle to physiological conditions that produce a minimum and maximum oxygenation signal, and use these to define a 0–100% calibration range.

The minimum can be produced by exercise, arterial occlusion using a pressure cuff, or a combination of the two. Exercise increases muscle oxygen consumption, whilst arterial occlusion prevents oxygen delivery, allowing tissue oxygenation to fall towards a minimum.

The upper end of the calibration can also be defined in different ways. A commonly used approach is to release an arterial occlusion and use the peak oxygenation produced by the resulting reactive hyperemia as the 100% value. Earlier approaches have also used other physiological reference points.

The resulting 0–100% scale is therefore a physiological calibration rather than an absolute measurement of oxygen saturation. Exactly what the 0% and 100% points represent depends upon the calibration protocol used.

This was one of the first methods used to quantify muscle oxygenation.

Chance B, et al. “Recovery from exercise-induced desaturation in the quadriceps muscles of elite competitive rowers.” American Journal of Physiology 262 (1992): C766–C775.

Physiological calibration also has the advantage that it can partially correct for differences in the optical properties of individuals due to, for example, differences in adipose tissue thickness (ATT).

Ryan TE, et al. “Noninvasive evaluation of skeletal muscle mitochondrial capacity with near-infrared spectroscopy: correcting for blood volume changes.” Journal of Applied Physiology 113 (2012): 175–183.

It can also correct or minimise differences in StO₂ values between different mNIRS devices.

McManus CJ, et al. “Performance comparison of the MOXY and PortaMon near-infrared spectroscopy muscle oximeters at rest and during exercise.” Journal of Biomedical Optics 23 (2018): 015007.
https://doi.org/10.1117/1.JBO.23.1.015007