Simultaneous pO2and HbO2measurement to determine absolute HbO2concentration forin vivonear infrared spectroscopy

Purpose : Near infrared spectroscopy (NIRS) monitors absolute changes in concentration of chromophores from an unquantified baseline. A change in oxygenation is required to obtain information. Clinically, knowing initial chromophore concentration is desirable. We sought absolute initial concentrations by combining NIRS measurements with simultaneous Eppendorf histograph measurement (absolute values of tissue partial pressure of oxygen (pO 2 )). Methods : There were 22 trials on 9 occasions in 2 adult volunteers. Following local anaesthetic, the histograph probe was inserted into the forearm muscle and NIRS optodes applied. A sphygmomanometer cuff was inflated over the upper arm to produce ischaemia and hypoxia. Change in pO 2 was determined as a percentage of the initial pO 2 . Change in HbO 2 was measured, and the initial HbO 2 calculated by assuming the percentage change in HbO 2 equaled the percentage change in pO 2 . Results : Mean percentage change in pO 2 was 20.7 ± 9.6. Mean change in HbO 2 was 22.4 ± 3.4 µ mol/l(tissue). Calculated initial HbO 2 was 146.4 ± 108.7 µ mol/l(tissue), range 51 to 508 µ mol/l(tissue). Correlation ( r 2 ) between pO 2 and HbO 2 was 0.97 ± 0.03. Conclusion : Although changes in pO 2 and HbO 2 were highly correlated, calculation of initial absolute concentration of HbO 2 was not achieved by this method. A reliable method of obtaining absolute concentrations of HbO 2 in vivo remains to be found.