Standard pulse oximetry only measures the functional derivatives oxyhemoglobin (O2Hb) and deoxyhemoglobin (HHb) to calculate the arterial oxygenation. However, the two dysfunctional hemoglobin derivatives carboxyhemoglobin (COHb) and methemoglobin (MetHb) are of much interest. The gold standard detecting abnormal concentration of COHb or MetHb is the blood gas analysis (BGA). In this paper a non-invasive method for measuring these derivatives and a setup for validation is presented.
A standard pulse oximeter utilizes a sensor with two LEDs at the wavelengths 660 nm and 905 nm. For achieving information about the other hemoglobin derivatives, the new sensor is equipped with additional wavelengths between 500 nm and 1400 nm. An artificial blood-flow-model is used to simulate the blood flow through tissue. In this model a variation of the concentrations O2Hb, MetHb and COHb is feasible. The reflection of emitted light is measured for wavelengths between 500 nm and 600 nm, whereas transmission is measured for emitted light above 600 nm. An algorithm was designed to calculate the different hemoglobin derivatives independently. Therefore, the perfusion for each wavelength is computed and combined in a linear regression model.
An in-vivo animal study with 18 pigs confirmed previous simulations. The pigs were ventilated with a respiratory minute volume of 4 l. To induce hypoxia (minimal SaO2 70 %), the inhaled air was mixed with nitrogen. Ventilating with 100 % oxygen reoxygenated the pigs. An increase of MetHb up to 35 % was induced by sodium nitrite. Afterwards methylene blue decreased the concentration. Carbon monoxide was mixed with the inhaled air to raise the COHb concentration up to 40 %. Different combinations of concentrations were performed. The references were achieved via BGA.
It is shown that this setup is capable of measuring COHb, MetHb and O2Hb non-invasively. The setup is accurate even when manipulating hemoglobin derivatives in parallel.