Measurement & monitoring physics

Also known as Measurement physics · Transducer · Pulse oximetry physics · Capnography physics · Beer-Lambert law · Paramagnetic oxygen analyser · Thermistor

The measurement physics underlies every number on the anaesthetic monitor. The framework rests on five exam-critical ideas: the principle of transduction (the conversion of the physiological variable into an electrical signal, via the strain gauge and the Wheatstone bridge for the pressure); the flow measurement (the rotameter and the pneumotachograph); the gas analysis (the paramagnetic oxygen analyser and the infrared carbon dioxide analyser); the pulse oximetry (the Beer-Lambert law, the two wavelengths, and the pulsatile addition); and the temperature (the thermistor and the thermocouple). The clinical application is the standards for the basic monitoring and the awareness of every monitor's limitations.

medium7 referencesUpdated 26 July 202612 min readVerification in progress

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8 MCQs

The one-line answer

Every number on the anaesthetic monitor is a transduced signal — pressure, flow, gas concentration, light or temperature converted to an electrical value. The exam-critical physics (taught device physics — [1] is a pulse-oximetry review and owns only the oximetry lines below): pressure by the strain gauge and the Wheatstone bridge (the arterial line), flow by the rotameter and the pneumotachograph, oxygen by its unique paramagnetism, carbon dioxide by infrared absorption (the Beer-Lambert law), saturation by two-wavelength pulse oximetry, and temperature by the thermistor. Know where each fails — the pulse oximeter over-reads in carbon-monoxide poisoning, lags the true oxygenation by the circulation time, and is biased by dark skin pigmentation (review-exact failure modes [1][2][3]).^[1]^[2]^[3]

Meet the patient

A patient pulled from a house fire arrives confused, with a pulse-oximeter reading of 97 per cent that looks reassuring. The arterial blood gas tells a different story — a high carboxyhaemoglobin. The pulse oximeter lied, because with only two wavelengths it assumes haemoglobin is oxy- or deoxy- and cannot tell oxyhaemoglobin from carboxyhaemoglobin (taught vignette — the mechanism is review-exact [1][2]).^[1]^[2]

The two questions that govern every monitor are: how is the number derived? (the physics of the transduction) and where does it fail? (the dyshemoglobins, the lag, the damping, the aliasing). Hold those two and the monitors become tools you trust with your eyes open (taught monitor discipline). ^[1]

References7Show

[1]Moon K, et al. Pulse Oximetry-A Perioperative Perspective Diagnostics (Basel), 2026.PMID 42351472
[2]León-Valladares D, Barrio-Mateu LA, Cortés-Carmona N, et al. Determining factors of pulse oximetry accuracy: a literature review Rev Clin Esp (Barc), 2024.PMID 38599519
[3]Cotton SA, et al. Do Differences in Skin Pigmentation Affect Detection of Hypoxemia by Pulse Oximetry: A Systematic Review of the Literature Clin Nurs Res, 2025.PMID 41045137
[4]Ishihara T, et al. Time-Based Capnography to Diagnose Airway Obstruction During Lung Lobectomy in a Dog Animals (Basel), 2025.PMID 41375427
[5]Jogie J, et al. Perioperative Temperature Monitoring in Anesthesia: A Review of Current Evidence and Clinical Practice Cureus, 2026.PMID 42147589
[6]Yu H, et al. Depth of anesthesia monitoring: Current evidence, clinical impact, and future directions J Int Med Res, 2026.PMID 42333677
[7]Huang X, et al. Effects of EEG-based monitoring of depth of anesthesia on postoperative delirium, cognitive dysfunction, and long-term neurocognitive outcomes: a meta-analysis Front Med (Lausanne), 2025.PMID 41907691