Anaes · Measurement & monitoring physics

Oxygen measurement

Also known as Pulse oximetry · Beer-Lambert law · Oxyhaemoglobin dissociation curve · Clark electrode · Paramagnetic oxygen analyser · Galvanic fuel cell

Oxygen measurement is the single most monitored quantity in anaesthesia — the pulse oximeter on the finger, the oxygen analyser in the breathing circuit, the arterial blood gas in the machine, and the tissue-oxygenation probes on the forehead all answer the same question from different angles: is enough oxygen reaching the patient's cells? The framework rests on six exam-critical ideas. First, the PULSE OXIMETER exploits the BEER-LAMBERT LAW — that the absorbance of light by a solution is proportional to the concentration of the absorbing species — at two wavelengths, red 660 nanometres and infrared 940 nanometres; oxyhaemoglobin absorbs more infrared and deoxyhaemoglobin absorbs more red, and the RATIO of the two absorbances yields the saturation. Because only the pulsatile component of the absorbance is used, the device isolates arterial blood from venous blood and tissue. Second, the SIGNAL is processed into an AC (pulsatile) component riding on a DC (baseline) component, and the calibration curve that converts the absorbance ratio into a saturation is empirical, built from studies in healthy volunteers. Third, the device has well-defined LIMITATIONS: carboxyhaemoglobin is read as oxyhaemoglobin so the reading is falsely high; methaemoglobin absorbs almost equally at both wavelengths and pulls the reading toward 85 per cent; dyes (methylene blue, indocyanine green) and nail polish lower the reading; poor perfusion, motion artefact, dark skin pigmentation and high ambient light all degrade accuracy. Fourth, the OXYHAEMOGLOBIN DISSOCIATION CURVE relates the arterial partial pressure of oxygen (PaO2) to the saturation (SaO2) in a sigmoid that is flat at the top (so saturation is a poor index of PaO2 in the normal range) and steep at the bottom; the P50 — the PaO2 at which haemoglobin is half saturated — is about 3.5 kPa (26.8 mmHg) at 37 degrees and pH 7.40, and shifts LEFT (increased affinity) with alkalosis, hypothermia, low 2,3-DPG, fetal haemoglobin and carbon monoxide, and RIGHT (reduced affinity, better unloading) with acidosis, hyperthermia and high 2,3-DPG. Fifth, the PARTIAL PRESSURE of oxygen is measured directly by electrochemical cells: the CLARK (polarographic) electrode, in which oxygen diffuses through a membrane and is reduced at a platinum cathode held at a polarising voltage, the resulting current being proportional to PO2, used in the blood-gas analyser and consuming oxygen; and the GALVANIC (fuel cell) sensor, self-generating with a gold cathode and lead anode, used in portable analysers and also consuming oxygen. Sixth, the CONCENTRATION of oxygen in a gas mixture is measured by the PARAMAGNETIC analyser — oxygen is one of the few paramagnetic gases (attracted into a magnetic field) while most other gases are diamagnetic, and the dumb-bell or dual-chamber sensor on the anaesthetic machine exploits this for the inspired-oxygen monitor with its low-concentration alarm. Beyond these, tissue oxygenation is assessed by near-infrared spectroscopy (NIRS) for the regional saturation, by the mixed venous oxygen saturation (SvO2) from the pulmonary artery catheter and the central venous saturation (ScvO2), and by lactate as a global surrogate. Built on the pulse-oximetry perioperative review (Moon 2026), the Critical Care pulse-oximetry review (Jubran 2015), the oxygenation-monitoring history (Severinghaus 2011), the Clark-electrode and oxygen-measurement histories (Severinghaus 1986, two papers), the melanin-corrected tissue-oxygen-saturation work (Kubo 2026), the pulse-oximetry hypoxaemia-overestimation study (Atuar 2026), the SvO2-transfusion cardiac-ICU study (Okamoto 2026), the P50 carbon-dioxide-and-temperature study (Woyke 2021), the near-infrared cytochrome-c-oxidase monitoring work (Ward 2026), and the tissue-oxygenation monitoring during hyperbaric-oxygen study (Kmiec 2026).