For in-vivo measurement and to analyze small blood gases a small compatible sensor is required. Sensor should have ability to be inserted in the blood flow of the artery. They are passed through arterial cannula and are placed in the same place for several days. Therefore, blood gas analyzers based on fiber optics are developed along with latest oxygen sensitive dyes. A fiber optic based blood gas analyzer is shown below. Electro-optic monitor is interfaced with sensor. A monochromatic source such as diode laser is used as excitation light. A broadband source such as xenon lamp also acts as a source. Monitor provides the source of excitation. Two wavelengths of light are used. One wavelength remains sensitive to any changes in the species that is measured. Another wavelength is not affected due to analyte concentration change.
Hence, this wavelength acts as reference and used to compensate for output fluctuations and efficiency. The light output coming from the monitor is connected to the fiber optic cable via lens and connectors.
Inside the sensor assembly, there are three optical fibers. Each fiber is used for measuring blood O2, CO2 and pH. The length of the fiber is around 10 cm. A thermocouple and thermistor are present along the side of the fiber to measure temperature near sensor tip. In the sensing material, solubility of gases is function of temperature.
The optical parameters also changes along with temperature. The fibers and temperature sensors are enclosed in a tubing to prevent breakage.
Each fiber is almost as thin as human hair and the tip is coated with a chemical dye. When the light falls on the dye, dye fluoresces and produces light at a different wavelength. This fluorescent emission changes the intensity and concentration of analyte solution in blood. The emitted output light travels through fiber and reaches the monitor. At the monitor, the light is converted into electrical signal using solid-state detector or a photomultiplier. At the next stage, the signal is amplified and given to the digitizer. A microprocessor is used to process the light intensity in relation to the analyte concentration and digitally displays it.
After the development of fiber-optic blood gas sensors, continuous self-monitoring devices were used more. One such development is mounting sensor on a single cassette. A syringe adapter allows automatic aspiration of the sample from the syringe. When adapter is removed from the capillary allows direct sample aspiration. Calibration of sensor is verified after insertion of cassette. After entire sample analysis cassette is removed.