Electro-optical plethysmography for non-invasive estimation of hemoglobin concentration

Novel sensors and instrumentation are currently being investigated with the intended application of determining the concentration of hemoglobin and other optically absorbing compounds in blood using non-invasive methods. In order to measure concentration, the mass or amount of a compound must be known in addition to the volume of liquid. In principle, it may be possible to estimate hemoglobin concentration from a change in optical absorbance occurring over the cardiac cycle divided by a corresponding change in measured blood volume during the cycle measured from peripheral tissue, e.g. the finger or ear. Electrical and optical sensors were evaluated in vitro using a tissue phantom and an absorbing liquid medium. The effect of changes in optical absorption and pulse pressure on the capacitance and optical absorbance were studied.