Blood volume versus deoxygenated NIRS signal: Computational analysis of the effects muscle O₂ delivery and blood volume on the NIRS signals

Near-infrared spectroscopy (NIRS) signals quantify the oxygenated (DHbMbO₂) and deoxygenated (δHHbMb) heme group concentrations. δHHbMb has been preferred to DHbMbO₂in evaluating skeletal muscle oxygen extraction because it is assumed to be less sensitive to blood volume (BV) changes, but uncertainties exist on this assumption. To analyze this assumption, a computational model of oxygen transport and metabolism is used to quantify the effect of O₂delivery and BV changes on the NIRS signals from a canine model of muscle oxidative metabolism (Sun Y, Ferguson BS, Rogatzki MJ, McDonald JR, Gladden LB. Med Sci Sports Exerc 48: 2013-2020, 2016). The computational analysis accounts for microvascular (DHbO₂, DHHb) and extravascular (DMbO₂, DHMb) oxygenated and deoxygenated forms. Simulations predicted muscle oxygen uptake and NIRS signal changes well for blood flows ranging from resting to contracting muscle. Additional NIRS signal simulations were obtained in the absence or presence of BV changes corresponding to a heme groups concentration changes (DHbMb = 0-48 μM). Under normal delivery (Q = 1.0 Lkg_1min_1) in contracting muscle, capillary oxygen saturation (SO₂) was 62% with capillary DHbO₂and DHHb of ± 41 μM for DHbMb = 0. An increase of BV (DHbMb = 24 μM) caused a DHbO₂decrease (16μM) almost twice as much as the increase observed for DHHb (9 μM). When SO₂increased to more than 80%, only DHbO₂was significantly affected by BV changes. The analysis indicates that microvascular SO₂is a key factor in determining the sensitivity of DHbMbO₂and deoxygenated δHHbMb to BV changes. Contrary to a common assumption, the δHHbMb is affected by BV changes in normal contracting muscle and even more in the presence of impaired O₂delivery.