The study aimed to examine the relationship between near-infrared spectroscopy (NIRS) signals and venous hemoglobin oxygen saturation (O2Hb%) and venous oxygen concentration (CvO2).Methods
Gastrocnemius muscles (GS) in six dogs were surgically isolated and pump perfused. NIRS signals were recorded, and venous blood samples were collected at constant flow rates (control flow, high flow, and low flow) at rest as well as during electrically stimulated tetanic muscle contractions at rates of one contraction per 2 s (1/2 s) and two contractions per 3 s (2/3 s). Similar data were also collected at three different inspired O2 percentages (12%, 21%, and 100%) with constant blood flow.Results
Complete data from five animals were analyzed; all data from one animal were deleted because of erratic oxy-NIRS signals. Venous O2Hb% ranged from 7.6% to 97.5% across the various experimental conditions. After the NIRS signals were normalized to the physiological range, a high linear correlation was seen between the deoxygenated heme signal (HHbMb%) and the venous O2Hb% (R2 = 0.92 ± 0.05), between the oxygenated heme signal (HbMbO2%) and the venous O2Hb% (R2 = 0.92 ± 0.03), between the HHbMb% and the CvO2 (R2 = 0.89 ± 0.06), and between the HbMbO2% and the CvO2 (R2 = 0.90 ± 0.05). The overall relationships between HHbMb%, HbMbO2%, and venous O2Hb% as well as between HHbMb%, HbMbO2%, and CvO2 were also linear and highly correlated with R2 values ranging from 0.81 to 0.90.Conclusion
In this controlled canine muscle model, NIRS signals are highly correlated with venous O2Hb% and CvO2 across a wide range of physiological conditions. The practical application of our results is that for an individual muscle or perhaps muscle group, normalized NIRS HHbMb and HbMbO2 signals accurately reflect the mean venous O2 saturation of the interrogated muscle tissue.