Non-invasive and quantitative near-infrared haemoglobin spectrometry in the piglet brain during hypoxic stress, using a frequency-domain multidistance instrument

The frequency-domain multiple-distance (FDMD) method is capable of measurin g the absolute absorption and reduced scattering coefficients of optically turbid media. Absolute measurement of absorption at two near-infrared (NIR) wavelengths makes possible the quantitation of tissue haemoglobin concentr ation and tissue haemoglobin oxygen-saturation (StO(2)). However, errors ar e introduced by the uncertainties of background absorption and the dissimil arities between real tissues and the simplified mathematical model on which these measurements are based. An FDMD-based tissue instrument has been use d for the monitoring of tissue haemoglobin concentration and oxygenation in the brain of newborn piglets during periods of hypoxia and hyperoxia. Thes e tissue haemoglobin saturation values were compared with arterial saturati on (SaO(2)) and venous saturation (SvO(2)) measured by blood gas analyses. A linear correlation was observed between StO(2) and the average of SaO(2) and SvO(2). However, StO(2) is not equal to any fixed weighted average of S aO(2) and SvO(2) unless we introduce an effective background tissue absorpt ion. The magnitude of the background absorption was about 0.08 cm(-1) at 75 8 nm and 0.06 cm(-1) at 830 nm, and it was nearly consistent between piglet s. The origin of this 'effective' background absorption may be real, an art efact caused by the application of a simplified model to a complex sample, or a combination of factors.