Lactate levels show a consistent pattern when using a correlation heat map for the wavelength ratio between 1000-1100 nm and 640-680 nm

Lactate is produced during anaerobic energy production, and its measurement is crucial to assess oxygen delivery to tissue related to hypoxia, sepsis, and other conditions. Currently, the gold standard of lactate measurement is using blood sample. Hence, non-invasive measurements of lactate have attracted researchers around the globe. One promising modality comes from near-infrared spectroscopy approach that has been already studied to some extent. We used a spectral range of 600 nm to 1700 nm to study wavelength sensitive to lactate measured from finger. Ten young adults were recruited for a standard exercise test, where participants were cycling on a stationary bike while the resistance (load) was progressively increased until voluntary exhaustion occurred. During the test increased lactate levels were measured from blood samples taken from finger. Optical probe of the spectrometer was placed on the right forefinger using a ring holder. Inspired by the tissue oxygenation index estimation, we investigated lactate levels against the ratio from A to B, where A presents wavelengths in the range 900nm-1350nm, previously shown significance to lactate levels, whereas B presents wavelength range 600nm-780nm, dominantly absorbed by deoxyhaemoglobin. Since lactate levels have some correlation with oxyhaemoglobin, the range 810nm-900nm sensitive to this chromophore were excluded from analysis. The presented division is another way to normalise the signal because the entire spectrum would change in the same way due to different motion artefacts. A heat map was created for each subject to summarise the correlation between the calculated A/B ratios and lactate levels resulting in a consistent pattern, particularly in the ratio between wavelengths 100 nm-1100nm and 640nm-680nm.