Anaes · Measurement & monitoring physics
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.
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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 carboxyhaemoglobin of 25 per cent. The pulse oximeter lied, because it cannot tell oxyhaemoglobin from carboxyhaemoglobin.[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.[1]
You have read the opening of this topic. The complete unit — every section and its primary-source references — is part of the Anaesthesia fellowship atlas.
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- [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