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Simultaneous Spectroscopic Determination of Forearm Muscle pH and Oxygen Saturation during Simulated Haemorrhage

Muscle oxygen saturation (SmO2) is a very early indicator of internal bleeding, but reaches a low and constant value when organ oxygen utilisation is at a maximum. With haemorrhage, muscle pH (pHm) continues to decline indicating the severity of injury. During resuscitation, it is important to continue treatment until acidosis is reversed, even after normal levels of tissue oxygen are restored. SmO2 and pHm can be determined continuously and non-invasively using near infrared (NIR) spectroscopy. Simultaneous determination of SmO2 and pHm from the same spectra are required for continuous monitoring of trauma patients. Acidosis enhances the release of oxygen from haemoglobin when there is insufficient oxygen available to the tissue (Bohr effect). An NIR spectroscopic-based patient monitor, however, must be capable of detecting independent variations in both pHm and SmO2. The results from an in vitro study of whole blood demonstrate spectral changes at the isosbestic point (800 nm) which were a strong function of pH, but independent of blood SO2 and scattering (cell size), suggesting that hydrogen ions directly affect the NIR spectrum in the region between 700 nm and 900 nm. Using a dual source, fibre optic-based system, NIR spectra (725–880 nm) were also collected from 40 human subjects undergoing lower body negative pressure as an experimental model of haemorrhage. SmO2 and pHm were calculated from the spectra collected from the forearm of each subject. Every muscle absorbance spectrum was fitted to a Taylor expansion attenuation model based upon Beer's law modified for a scattering component to calculate SmO2. The same spectra were corrected for subject variability using previously determined coefficients for principal component analysis loading correction. The corrected spectra were then used in a partial least-squares regression model previously developed from exercising subjects. pHm and SmO2 determined from the same spectra followed different time courses during simulated haemorrhage on human subjects. Taken together, results from these in vitro and in vivo studies show that the NIR spectroscopic-based monitor simultaneously and independently assesses SmO2 and pHm.