Relationship between muscle venous blood oxygenation and near-infrared spectroscopy: quantitative analysis of the Hb and Mb contributions

A linear relationship between skeletal muscle venous (C^T_ven) and oxygenated (DHbMbO_2,N) or deoxygenated (DHHbMb_N) near-infrared spectroscopy (NIRS) signals suggest a main hemoglobin (Hb) contribution to the NIRS signal. However, experimental, and computational evidence supports a significant contribution of myoglobin (Mb) to the NIRS. Venous and NIRS measurements from a canine model of muscle oxidative metabolism (Sun Y, Ferguson BS, Rogatzki MJ, McDonald JR, Gladden LB. Med Sci Sports Exerc 48(10):2013–2020, 2016) were integrated into a computational model of muscle O₂ transport and utilization to evaluate whether the relationship between venous and NIRS oxygenation can be affected by a significant Mb contribution to the NIRS signals. The mathematical model predicted well the measure of the changes of C^T_ven and NIRS signals for different O₂ delivery conditions (blood flow, arterial O₂ content) in muscle at rest (T1, T2) and during contraction (T3). Furthermore, computational analysis indicates that for adequate O₂ delivery, Mb contribution to NIRS signals was significant (20%–30%) even in the presence of a linear C^T_ven-NIRS relationship; for a reduced O₂ delivery the nonlinearity of the C^T_ven-NIRS relationship was related to the Mb contribution (50%). In this case (T3), the deviation from linearity is observed when O₂ delivery is reduced from 1.3 to 0.7 L kg^–1·min^–1 (C^T_ven < 10 mLO₂ 100 mL^–1) and Mb saturation decreased from 85% to 40% corresponding to an increase of the Mb contribution to DHHbMb_N from 15% to 50% and the contribution to DHbMbO_2,N from 0% to 30%. In contrast to a common assumption, our model indicates that both NIRS signals (DHHbMb_N and DHbMbO_2,N are significantly affected by Hb and Mb oxygenation changes.