Simultaneous pO₂ and HbO₂ measurement to determine absolute HbO₂ concentration for in vivo near infrared spectroscopy

Abstract

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 (pO₂)). 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 pO₂ was determined as a percentage of the initial pO₂. Change in HbO₂ was measured, and the initial HbO₂ calculated by assuming the percentage change in HbO₂ equaled the percentage change in pO₂. Results: Mean percentage change in pO₂ was 20.7 ± 9.6. Mean change in HbO₂ was 22.4 ± 3.4 µmol/l(tissue). Calculated initial HbO₂ was 146.4 ± 108.7 µmol/l(tissue), range 51 to 508 µmol/l(tissue). Correlation (r²) between pO₂ and HbO₂ was 0.97 ± 0.03. Conclusion: Although changes in pO₂ and HbO₂ were highly correlated, calculation of initial absolute concentration of HbO₂ was not achieved by this method. A reliable method of obtaining absolute concentrations of HbO₂in vivo remains to be found.