Blood volume vs. 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 (ΔHbMbO2) and deoxygenated (ΔHHbMb) heme group concentrations. ΔHHbMb has been preferred to ΔHbMbO2 in evaluating skeletal muscle oxygen extraction because it is assumed to be insensitive 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 O2 delivery and BV changes on the NIRS signals from a canine model of muscle oxidative metabolism (Med.Sci.SportsExerc.,48(10)2013-2020,2016). The computational analysis accounts for microvascular (ΔHbO2, ΔHHb) and extravascular (ΔHMb, ΔHMb) 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 (ΔHbMb=0-48μM). Under normal delivery (Q=1.0L kg-1min-1) of contracting muscle, capillary oxygen saturation (SO2) was 62% with capillary ΔHbO2 and ΔHHb of ±41μM for ΔHbMb=0. An increase of BV (ΔHbMb =24mM) caused a ΔHbO2 decrease (16mM) almost twice as much as the increase observed for ΔHHb (9μM). When SO2 increased to more than 80%, only ΔHbO2 was significantly affected by BV changes. The analysis indicates that microvascular SO2 is a key factor in determining the sensitivity of ΔHbMbO2 and 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 O2 delivery.