Oximetry

Oximetry is the measurement of oxygenation in blood and body tissues. The most widely used form is pulse oximetry, a noninvasive method that estimates arterial oxygen saturation (SpO2) and heart rate by analyzing light transmitted through a fingertip, earlobe, or toe.

Pulse oximetry relies on absorbance of two wavelengths of light, typically red and near-infrared. Oxygenated and deoxygenated hemoglobin absorb these wavelengths differently. By measuring the pulsatile arterial signal, a photodetector computes the ratio of absorptions and converts it to an SpO2 value using calibration curves.

In addition to pulse oximetry, other oximetry modalities include transcutaneous oxygen tension (TcPO2), which assesses tissue oxygenation through heated skin sensors, and near-infrared spectroscopy (NIRS), which provides regional tissue oxygenation estimates. Invasive alternatives include co-oximetry from arterial blood gas analysis, which can measure SaO2 and dyshemoglobins such as carboxyhemoglobin and methemoglobin.

Limitations and sources of error include motion, poor perfusion, ambient light, skin pigmentation, and nail polish. Abnormal dyshemoglobins or carbon monoxide exposure can bias SpO2 readings. In neonates and patients with circulatory compromise, readings may be unreliable.

Clinical use centers on continuous monitoring during anesthesia, critical care, and recovery, as well as home monitoring for respiratory diseases, sleep apnea evaluation, and high-altitude acclimatization.

The development of pulse oximetry is credited to Takuo Aoyagi in the 1970s, with commercial devices appearing in the 1980s. Modern pulse oximeters are compact, battery-powered, and incorporate algorithms to reject artifacts and improve accuracy.