NIRS is a non-invasive method for the continuous monitoring of changes in the local muscle oxygenation that represents the dynamic balance between oxygen delivery and consumption. The original application was measuring in the brain, but over recent years using NIRS for monitoring muscle tissue oxygenation has gained popularity.
Measuring with NIRS
A probe is placed on the skin that enables monitoring of NIRS signals from small vessels, i.e. arterioles, capillaries and venules, deep within the muscle for a distinct volume of tissue as determined by the size of the probe. The probe includes two functional ends. At one end of the probe light is emitted at wavelengths of 680, 720, 760, and 800 nm into the muscle, then sensors at the other end measure the amount of that light absorbed by the different hemoglobin forms. These wavelengths cover the spectrum that includes those that are sensitive for oxy-hemoglobin (HbO2) and deoxy-hemoglobin (HHb).
Principles of NIRS
Fig 1. Near infrared light covering the wavelength spectrum from 680-800 nm traverses into the muscle and is reflected or absorbed. Oxyhemoglobin (HbO2) and deoxyhemoglobin (HHb) absorb light at different wavelengths. In this way the percent saturation (StO2%) is calculated as HbO2/(HbO2 + HHb) x 100. Total haemoglobin (HbT) is calculated as HbO2 + HbO. Here the 25-mm probe emits light that penetrates to a muscle tissue depth of approximately 12.5 mm (i.e. half of the distance between sending and receiving).
Attainable physiological parameters with NIRS
- Blood Flow – Slope increase in Hbt with proximal limb venous occlusion
- Oxygen consumption – Slope increase in HHb with proximal limb arterial occlusion
- Oxygen recovery – Slope increase in HbO2 following proximal limb arterial occlusion