A pulse oximeter is a non-invasive optoelectronic medical device that monitors a patient's arterial blood oxygen saturation (SpO2) and pulse rate in real time. Standard clinical practice historically required invasive arterial blood gas (ABG) sampling via painful needle punctures to evaluate blood oxygen levels. Invented in 1974 by Japanese bioengineer Takuo Aoyagi at Nihon Kohden, the modern pulse oximeter transformed intensive care, anesthesiology, emergency medicine, and outpatient home healthcare by providing immediate, continuous, and painless diagnostic assessments of cardiopulmonary function using a simple clip placed over a fingertip, toe, or earlobe.
The physical operation of a pulse oximeter combines two complementary scientific principles: Spectrophotometry (measuring the relative light absorption characteristics of chemical compounds at specific wavelengths) and Photoplethysmography (measuring volume fluctuations in blood vessels during the cardiac cycle). Hemoglobin, the iron-containing metalloprotein in red blood cells that transports oxygen from the lungs to peripheral tissues, exists in two primary optical forms: oxygenated hemoglobin (HbO2) and deoxygenated hemoglobin (Hb). The device houses two distinct light-emitting diodes (LEDs): one emitting red light at a wavelength of approximately 660 nanometers, and another emitting near-infrared light at approximately 940 nanometers.
These two forms of hemoglobin exhibit starkly contrasting absorption spectra. Deoxygenated hemoglobin absorbs significantly more red light at 660 nanometers than near-infrared light, whereas oxygenated hemoglobin absorbs substantially more infrared light at 940 nanometers than red light. As light passes through the pulsating capillary bed of the finger, an opposing photodetector records transmitted light intensity. The device's internal microprocessor isolates the variable, pulsatile arterial blood component (AC signal) from static background tissue, bone, and venous blood (DC signal). By calculating the ratio of red to infrared light absorbance, the device uses a calibrated algorithmic formula to calculate SpO2, where normal healthy values range between 95 and 100 percent. Readings dropping below 90 percent alert medical personnel to hypoxemia, enabling swift therapeutic interventions before tissue hypoxia causes cellular damage.