Introduction
Near-infrared spectroscopy (NIRS) has been used for measurement of cerebral hemoglobin (Hb) concentrations in neonates to study cerebral oxygenation and hemodynamics. In this study, measurements by portable three-wavelength NIR time-resolved spectroscopy (TRS) were performed in a piglet hypoxia model with various degrees of oxygenation to estimate the absorption coefficient (μa) and reduced scattering coefficient (μ's) of the head.
Methods
Measurements of absolute values of μa at three wavelengths enable estimation of Hb concentration and Hb oxygen saturation in the head (SO2). However, there is a problem of which background absorption should be used for estimation of Hb concentration in the head derived from μa at three wavelengths because it is different from a simple in vitro model. Therefore, we used two different background absorption values with the assumption that background absorption is due only to 85% (by volume) water or that background absorption is equal to absorption of the piglet head with blood exchange transfusion by fluorocarbon (FC), and we compared SO2 measured by TRS with arterial Hb oxygen saturation (SaO2) and sagittal sinus venous Hb oxygen saturation (SvO2) measured by a co-oximeter at several inspired fractional O2 (FIO2) concentrations.
Results and Discussions
It was found that SO2 values using the absorption of the piglet head with blood exchange transfusion by FC (abs of the head with BET by FC) were not significantly different from SO2 values using the water only background at FIO2 in the range of 15% to 100% but that the values using abs of the head with BET by FC were lower than the values using the water only background at FIO2 in the range of 12 to 4%. The SO2 values calculated from the water only background were higher than those of SaO2 at FIO2 in the range of 10% to 4%. However, SO2 values using the abs of the head with BET by FC were between those of SaO2 and SvO2 over the whole range of FIO2. Therefore, abs of the head with BET by FC is more useful for estimation of the absolute values of oxyHb and deoxyHb of the piglet head.