Measurement & monitoring physics

Overview & definition

The measurement physics is the science of converting the physiological variables (the pressure, the flow, the gas concentration, the temperature, the electrical activity) into the numbers and the waveforms on the anaesthetic monitor. Every monitor is a transducer — a device that converts one form of energy (the mechanical, the chemical, the thermal, the optical) into another (usually the electrical) that can be processed, displayed, and recorded. The understanding of the measurement physics is an exam-critical topic in the Primary physics and the foundation of the safe monitoring: knowing how each number is derived, and where it can fail, is what prevents the dangerous over-reliance on a single monitor. [1]

The principle of transduction

The transducer converts the physiological variable into the electrical signal. The conversion typically uses a physical effect: the strain (the resistance change of a stretched wire — the strain gauge), the piezoelectric effect (the charge generated by a compressed crystal), the photoelectric effect (the light absorbed by a tissue — the pulse oximeter), the thermoelectric effect (the voltage generated by a temperature difference — the thermocouple), and the chemical effect (the reaction at an electrode — the blood gas). [1]

The transducer's signal is small and must be amplified, filtered, converted from the analog to the digital, and displayed. The quality of the measurement depends on the static characteristics (the accuracy, the precision, the linearity, the range, the sensitivity, the drift) and the dynamic characteristics (the frequency response, the damping, the time delay).[1]

Pressure measurement — the Bourdon gauge and the strain gauge

The pressure is measured by two principal devices:[1]

• The Bourdon gauge — a mechanical device for the high pressures. A curved, hollow metal tube (the Bourdon tube) is connected to the gas; the pressure straightens the tube, and the movement is transmitted through a linkage to a needle on a dial. The Bourdon gauge measures the gauge pressure (relative to the atmosphere) and is used for the cylinder pressures and the pipeline pressures (the oxygen cylinder up to 200 bar).
• The strain gauge and the Wheatstone bridge — the electronic device for the lower and the dynamic pressures. A diaphragm is exposed to the pressure; the pressure deforms the diaphragm, and the strain gauges bonded to the diaphragm change their resistance. The four strain gauges form a Wheatstone bridge (a circuit of four resistors); the small resistance change unbalances the bridge and produces a voltage proportional to the pressure. This is the basis of the intra-arterial blood pressure transducer and the airway pressure transducer. [1]

The intra-arterial pressure transducer

The intra-arterial cannula connects via a fluid-filled column (the rigid cannula, the stiff tubing, and the saline flush) to the pressure transducer (a strain-gauge diaphragm). The arterial pressure is transmitted through the fluid column to the diaphragm, which converts it to the electrical signal displayed as the arterial waveform. The system must be zeroed to the atmosphere (the transducer opened to air and set to zero at the level of the patient's heart — the phlebostatic axis) and levelled (the transducer at the right atrium for the absolute pressure reading). The dynamic accuracy depends on the natural frequency and the damping of the system: an under-damped system overshoots (the spurious high systolic, the resonance), and an over-damped system under-reads (the low systolic, the slurred upstroke). The optimal damping and the short, stiff tubing minimise the distortion.[1]

Flow measurement — the rotameter and the pneumotachograph

The flow is measured by two principal devices:[1]

• The rotameter (the variable-orifice flowmeter) — the device on the anaesthetic machine for the fresh gas flow. A gas flows upward through a vertical tapered tube, lifting a bobbin (the float). The bobbin floats at the height where the upward force of the gas (the flow) balances the downward weight of the bobbin. The tube is tapered (wider at the top), so the annular area around the bobbin increases with the height; the flow is read at the top of the bobbin. The rotameter is calibrated for a specific gas (the density and the viscosity differ) and is read at the level of the eye.
• The pneumotachograph — the electronic device for the respiratory flow. The gas flows through a fine mesh (a resistance); the pressure drop across the mesh is proportional to the flow (the laminar flow). A differential pressure transducer measures the drop, and the flow is integrated to give the volume. The pneumotachograph measures the inspiratory and the expiratory flows and is the basis of the spirometry. [1]

Gas analysis — the paramagnetic oxygen analyser

The oxygen is uniquely paramagnetic (it is attracted to a magnetic field, unlike most gases which are diamagnetic). The paramagnetic oxygen analyser exploits this: a pair of nitrogen-filled glass spheres (dumb-bells) is suspended in a magnetic field; the oxygen drawn into the field exerts a force on the spheres, rotating them against a restoring torque. The rotation is measured optically (a mirror and a light beam) and is proportional to the oxygen concentration. The paramagnetic analyser is accurate and fast and is used for the inspired and the expired oxygen measurement in the modern anaesthetic machine.[1]

Gas analysis — the infrared carbon dioxide analyser and the capnography

The carbon dioxide (and the nitrous oxide and the volatile agents) absorbs the infrared light at specific wavelengths (the CO2 absorbs at about 4.3 micrometres). The infrared analyser passes the infrared light through the gas sample to a detector; the absorption (the Beer-Lambert law — the absorbance is proportional to the concentration) gives the CO2 concentration. The analyser is configured as the mainstream (the sensor on the airway, fast, no sample drawn) or the sidestream (the sample drawn through a tube to the analyser, slower, used in the routine theatre). The capnography displays the CO2 against time, and the waveform (the baseline, the upstroke, the alveolar plateau, the end-tidal value, and the inspiratory downstroke) diagnoses the airway obstruction, the circuit leak, the malignant hyperthermia, and the circuit disconnect (the flat trace).[4]

The pulse oximetry — the Beer-Lambert law and the wavelengths

The pulse oximeter measures the arterial oxygen saturation non-invasively. It uses two light-emitting diodes — one red (about 660 nanometres) and one infrared (about 940 nanometres) — shone through a pulsatile tissue bed (the finger, the ear). The oxyhaemoglobin and the deoxyhaemoglobin absorb the two wavelengths differently (the oxyhaemoglobin absorbs more infrared, the deoxyhaemoglobin absorbs more red). The Beer-Lambert law (the absorbance is proportional to the concentration) relates the absorption to the saturation. The pulse oximeter isolates the pulsatile component (the arterial inflow) from the constant component (the venous, the tissue, the skin), so it measures the arterial saturation. The two wavelengths allow the solution for the two unknowns (the oxy- and the deoxyhaemoglobin).[1]

The pulse oximetry — the limitations

The pulse oximeter has important limitations that must be known:[1][2]

• The dyshemoglobins. The pulse oximeter uses only two wavelengths, so it assumes the haemoglobin is either oxy- or deoxyhaemoglobin. The carboxyhaemoglobin (the carbon monoxide poisoning) absorbs at the red wavelength like the oxyhaemoglobin, so the pulse oximeter over-reads (it reads near normal despite the severe hypoxaemia). The methaemoglobin absorbs both wavelengths, driving the reading toward about 85 per cent. The pulse oximeter cannot detect these, and the arterial blood gas (the co-oximetry) is required.
• The time lag. The pulse oximeter lags the actual oxygenation by the circulation time from the lungs to the finger — about one to two minutes in the apnoeic or the poorly perfused patient. A normal reading does not guarantee the current oxygenation.
• The accuracy and the skin pigmentation. The pulse oximeter is calibrated between 70 and 100 per cent and is less accurate below 80 per cent. The motion, the poor perfusion (the vasoconstriction, the hypotension), the nail polish, and the ambient light interfere. The dark skin pigmentation biases the reading, overestimating the saturation and missing the hypoxaemia, especially in the 85 to 90 per cent range — a clinically important equity issue.[2][3]

Temperature measurement — the thermistor and the thermocouple

The temperature is measured by two principal devices:[5]

• The thermistor — a semiconductor whose resistance falls markedly and predictably with the temperature (a large, negative temperature coefficient). The thermistor is small, fast, and accurate over the clinical range, and is the common temperature probe in the theatre (the nasopharyngeal, the oesophageal, the rectal).
• The thermocouple — two dissimilar metals joined at a junction; the temperature difference between the measuring junction and the reference junction generates a small voltage (the Seebeck effect). The thermocouple is robust and wide-range but less sensitive than the thermistor. [1]

The site of the measurement matters: the nasopharyngeal and the oesophageal (the lower third) estimate the core temperature; the rectal, the bladder, and the skin estimate less well. The core temperature monitoring is a standard of the basic monitoring because the perioperative hypothermia is common and harmful (the wound infection, the coagulopathy, the shivering, the prolonged recovery).[5]

The depth of anaesthesia — the processed EEG and the BIS

The depth of the anaesthesia monitoring uses the processed electroencephalogram (the EEG). The raw EEG is complex; the processed monitors reduce it to a dimensionless index (the Bispectral Index, the BIS, from 0 to 100; the entropy). The index is derived from the EEG features (the frequency, the synchronisation, the burst suppression) and correlates with the level of the hypnosis. The BIS of 40 to 60 is the recommended range for the general anaesthesia.[6]

The clinical value is debated: the processed EEG reduces the explicit recall but does not eliminate it, and it guides the titration to avoid the over- and the under-dosing. The recent systematic reviews suggest that the EEG-based depth monitoring may reduce the postoperative delirium in the high-risk elderly patients, though the evidence is mixed. The monitors are confounded by the neuromuscular blockers (the artefactual suppression), the ketamine and the nitrous oxide (the paradoxical rise), and the electrical interference.[6][7]

The display and the sampling — the aliasing and the time delay

The digital display introduces two artefacts:[1]

• The aliasing — if the signal is sampled at a rate less than twice its highest frequency (the Nyquist criterion), the high-frequency components appear as spurious low-frequency signals (the aliasing). The arterial waveform, for example, must be sampled at a high rate to avoid the distortion.
• The time delay and the averaging — the monitor averages the signal over a window and updates the display at intervals, so the displayed value lags the actual value. The pulse oximeter and the capnograph have the characteristic update delays. The alarm thresholds must account for these delays. [1]

The standards for the basic monitoring

The professional bodies (the ASA, the ANZCA) define the standards for the basic anaesthetic monitoring that must be present for every anaesthetic: the continuous presence of the anaesthetist; the oxygen analyser in the breathing system (with a low-oxygen alarm); the pulse oximetry; the capnography (the end-tidal CO2); the blood pressure and the heart rate; and the temperature monitoring when the clinically significant changes are intended or anticipated. The capnography is now required for all the anaesthetics, including the moderate and the deep sedation, because it detects the airway and the ventilation problems early. The adherence to these standards reduces the morbidity and the mortality.[1][4]

The calibration and the safety

The monitors require the periodic calibration (the comparison to a known standard) to maintain the accuracy. The gas analysers are calibrated with the known gas mixtures; the pressure transducers are zeroed to the atmosphere; the pulse oximeters have no user calibration (they are factory-calibrated, which is a limitation in the unusual haemoglobin states). The safety also depends on the alarm settings (the appropriate thresholds, the audible alarms, the alarm fatigue avoidance) and the recognition that every monitor fails — the clinical observation of the patient is the final safeguard.[1][2]

[1] [1]

Red flags

[1] [1] [1] [1] [1]

References

1. [1]Moon K, et al. Pulse Oximetry-A Perioperative Perspective Diagnostics (Basel), 2026.PMID 42351472
2. [2]Leon-Valladares D, et al. Determining factors of pulse oximetry accuracy: a literature review Rev Clin Esp (Barc), 2024.PMID 38599519
3. [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. [4]Ishihara T, et al. Time-Based Capnography to Diagnose Airway Obstruction During Lung Lobectomy in a Dog Animals (Basel), 2025.PMID 41375427
5. [5]Jogie J, et al. Perioperative Temperature Monitoring in Anesthesia: A Review of Current Evidence and Clinical Practice Cureus, 2026.PMID 42147589
6. [6]Yu H, et al. Depth of anesthesia monitoring: Current evidence, clinical impact, and future directions J Int Med Res, 2026.PMID 42333677
7. [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