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 ([Formula: see text]) and oxygenated (ΔHbMbO) or deoxygenated (ΔHHbMb) 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. 48(10):2013-2020, 2016) were integrated into a computational model of muscle O 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 [Formula: see text] and NIRS signals for different O delivery conditions (blood flow, arterial O content) in muscle at rest (T1, T2) and during contraction (T3). Furthermore, computational analysis indicates that for adequate O delivery, Mb contribution to NIRS signals was significant (20%-30%) even in the presence of a linear [Formula: see text]-NIRS relationship; for a reduced O delivery the nonlinearity of the [Formula: see text]-NIRS relationship was related to the Mb contribution (50%). In this case (T3), the deviation from linearity is observed when O delivery is reduced from 1.3 to 0.7 L kg·min ([Formula: see text] < 10 mLO 100 mL) and Mb saturation decreased from 85% to 40% corresponding to an increase of the Mb contribution to ΔHHbMb from 15% to 50% and the contribution to ΔHbMbO from 0% to 30%. In contrast to a common assumption, our model indicates that both NIRS signals (ΔHHbMb and ΔHbMbO are significantly affected by Hb and Mb oxygenation changes. Within the near-infrared spectroscopy (NIRS) signal, the contribution from hemoglobin is indistinguishable from that of myoglobin. A computation analysis indicates that a linear relationship between muscle venous oxygen content and NIRS signals does not necessarily indicate a negligible myoglobin contribution to the NIRS signal. A reduced oxygen delivery increases the myoglobin contribution to the NIRS signal. The integrative approach proposed is a powerful way to assist in interpreting the elements from which the NIRS signals are derived.