Calibration and evaluation of a continuous wave multi-distance NIRS system in simulated desaturation investigations

The venous oxygen saturation reflects pathophysiological changes in oxygen delivery and consumption, where the saturation range beneath 50% may represent a clinical borderline to avoid irreversible damage. We present a model to simulate a desaturation investigation with 4 plateaus (PL₁ = 99 ± 0.3%; PL₂ = 75 ± 3%; PL₃ = 50 ± 3%, PL₄ = 25 ± 3%). A continuous wave multi-distance near-infrared spectroscopy sensor (wavelengths: 770,808 and 850 nm; 6 photodiodes (PD_1-6), linearly arranged, separated 6 mm each) was equipped to detect the saturation at these plateaus. The study was divided in 5 calibration experiments (1 day; 5 per day) and 25 experiments (5 days; 5 per day) with the calibrated sensor unit. The detected saturation was compared with the reference values from the CO-Oximetry unit (IL-682) while keeping the amount of total hemoglobin (tHb) and the other physiological parameters at constant levels (tHb = 12 ± 0.3 g dl^-1, blood temperature = 35.5 ± 0.6 °C, carbon dioxide partial pressure (pCO₂) = 39 ± 6 mmHg, glucose = 103 ± 9 mg dl^-1 and pH 7.4 ± 0.04). The oxygen saturation, in the range from 99% to 20%, could be detected reproducible with a mean absolute deviation of 3%.