Effect of Blood Flow on Hemoglobin and Myoglobin Oxygenation in Contracting Muscle Using Near-Infrared Spectroscopy

Insufficient O₂ delivery to, and uptake by skeletal muscle can produce mobility limitations for patients with chronic diseases. Near-infrared spectroscopy (NIRS) can be used to noninvasively quantify the balance between skeletal muscle O₂ delivery and utilization during contraction. However, it is not clear how the oxygenated or deoxygenated NIRS signal should be used to assess muscle O₂ changes. This issue is related to the fact that the contributions of hemoglobin (Hb) and myoglobin (Mb) cannot be distinguished. This conundrum can be resolved by quantitative analysis of experimental data by computer simulations with a mechanistic, mathematical model. Model simulations distinguish dynamic responses of the oxygenated (HbO₂, MbO₂) and deoxygenated (HHb, HMb) contributions to the NIRS signal components (HbMbO₂, HHbMb). Simulations of muscle O₂ uptake and NIRS kinetics correspond closely to published experimental data (Hernández et al., J Appl Physiol 108: 1169-1176, 2010). Simulated muscle O₂ uptake and oxygenation kinetics with different blood flows indicate (1) faster O₂ delivery is responsible for slower muscle oxygenation kinetics; (2) Hb and Mb contributions to the HbMbO₂ are similar (40-60%); and (3) Hb and Mb contributions to the HHbMb are significantly different, 80% and 20%, respectively. The effect of slow blood flow kinetics on oxygenated Hb and Mb contributions is minimal. However, the effect on the imbalance between O₂ delivery and utilization rates causes significant overshoots and undershoots of deoxygenated Hb and Mb contributions. Model analysis in combination with NIRS measurements and information on hemodynamic and microvascular distribution can help to determine the use of NIRS signal in evaluating the factors limiting exercise tolerance in health and disease states.