pulse oximetry

Hold a torch against your fingertip and the flesh glows red — light passes through you. Pulse oximetry turns that everyday fact into a measurement: a small clip shines light through a fingertip (or earlobe) and reads how much comes out the other side, working out the oxygen saturation of the blood without any needle. It is one of the most useful, painless tools in all of medicine.

The trick relies on two ideas. First, oxygen-rich and oxygen-poor hemoglobin absorb red and infrared light differently — bright red, fully loaded blood lets more red light through — so the ratio of absorbed colours reveals saturation. Second, the device watches only the pulsing component of the signal, the part that throbs with each heartbeat, which comes from arterial blood; this lets it ignore skin, bone and steady venous blood and report the arterial saturation, shown as SpO2.

Its convenience makes it ubiquitous, but its limits deserve respect. It can mislead when the pulse signal is weak (cold or poorly perfused fingers), when nail polish or motion interferes, and notably with carbon monoxide poisoning, where the oximeter reads falsely high because it cannot tell carbon-monoxide-bound hemoglobin from oxygen-bound. It also reports only saturation, never carbon dioxide — so a normal reading does not rule out dangerous under-breathing.