Design and development of pulse oximeter

A pulse oximeter is a medical electronic instrument that measures the oxygen saturation (SaO₂) of arterial blood and pulse rate by non-invasive techniques. It enables prompt recognition of hypoxemia. In the transmittance type pulse oximeter that CSIO has developed, the absorption of light by oxygenated and reduced hemoglobin is measured at two wavelengths 660 nm (red) and 940 nm (infrared). At each wavelength, the light detected by the photodiode consists of a cardiac synchronous AC signal arising from arterial blood volume pulsations, superimposed on a DC level. The DC level depends on LED intensity, tissue absorption, path length, and detector sensitivity. The tissue absorption and path length may vary widely between individuals and probe site. The ratio of red and infrared signals after normalisation is calculated and is related to arterial oxygen saturation. The SaO₂ is finally calculated using the well known Mendelson and Kent equation which is derived based on Beer Lambert law. In CSlO´s pulse oximeter, a Nellcor finger clip probe is employed. The system operates at 400 Hz which is locked with power line frequency. Constant current feedback circuits are employed for driving the LEDs in a particular sequence. The detector output is given to sample and hold circuits for demultiplexing the three signals namely red, ambient and infrared. Our oximeter has provision for high and low alarm settings of SaO₂ from 50% to 100% and pulse rate setting from 30 BPM to 250 BPM. It also gives alarm conditions if the probe is accidentally disconnected from the finger. The SaO₂ is correctly measured in the range of 65 to 100% within ±2% accuracy and pulse rate within ±3 bpm