Liquid Blood Phantoms to Validate NIRS Oximeters: Yeast Versus Nitrogen for Deoxygenation

Liquid blood phantoms are a tool to calibrate, test and compare near-infrared spectroscopy (NIRS) oximeters. They comprise a mixture of saline, blood and Intralipid, which is subsequently oxygenated and deoxygenated to assess the entire range of tissue oxygen saturation (StO₂) from 0% to 100%. The aim was to investigate two different deoxygenation methods: yeast versus nitrogen (N₂) bubbling. The phantom was oxygenated with pure O₂ in both experiments, but deoxygenated by bubbling N₂ in the first and by addition of yeast and glucose in the second experiment. A frequency domain NIRS instrument (OxiplexTS) was used as reference and to monitor changes in the reduced scattering coefficient (μ_s') of the phantom. Both deoxygenation methods yielded comparable StO₂ values. The deoxygenation was slower by a factor 2.8 and μ_s' decreased faster when bubbling N₂. The constant bubbling of N₂ mechanically stresses the Intralipid emulsion and causes a decrease in μ_s', probably due to aggregation of lipid droplets. Deoxygenation by N₂ requires a more complex, air tight phantom. The gas flow cools the liquid and temperature needs to be monitored more closely. Consequently, we recommend using yeast for phantom deoxygenation.