Anaes · Measurement & monitoring physics

Pressure and temperature measurement

Also known as Pressure measurement · Pressure transducer · Arterial line · Damping · Fast-flush test · CVP waveform · Pulmonary artery catheter · Temperature measurement · Thermistor · Thermocouple · RTD

Blood pressure and body temperature are the two most frequently measured physiological variables in anaesthesia, and the physics of how they are measured determines whether the numbers on the screen are trustworthy. The framework rests on six exam-critical ideas. First, PRESSURE is force per unit area, with a defined set of clinical units and conversions; it is measured by mechanical devices (the Bourdon gauge for high-pressure cylinders, the aneroid gauge and the liquid manometer for anaesthetic-machine and venous pressures) and by electronic transducers for invasive blood pressure. Second, invasive blood pressure is measured by a PRESSURE TRANSDUCER — a device that converts the mechanical pressure of a fluid column into an electrical signal, typically a piezoresistive strain gauge bonded to a diaphragm whose resistance changes with deformation, detected by a Wheatstone bridge. Third, the transducer must be ZEROED to atmospheric pressure and LEVELLED to the mid-axillary line at the right atrium, because each centimetre of height difference introduces a hydrostatic pressure error of about 0.75 to 1 mmHg. Fourth, the dynamic response of the catheter-tubing-transducer system depends on its NATURAL FREQUENCY and DAMPING, assessed by the FAST-FLUSH test: under-damping overshoots (spurious high systolic), over-damping lags (under-read systolic, over-read diastolic), and mean pressure is preserved in both. Fifth, temperature is measured by several physical principles — a THERMISTOR (resistance falls as temperature rises), a THERMOCOUPLE (Seebeck voltage from two dissimilar metals), a platinum RTD (resistance rises with temperature, the laboratory reference), and an INFRARED detector — and the choice of monitoring SITE matters because core temperature is best estimated from oesophageal, nasopharyngeal, pulmonary-artery, tympanic or bladder sites while skin lags and under-reads. Sixth, general anaesthesia abolishes thermoregulation and produces the three phases of perioperative heat loss — redistribution (the steep first-hour fall), linear decline, and plateau — and prevention of inadvertent hypothermia by prewarming and forced-air warming is a quality indicator. Built on the dynamic-response requirements paper (Gardner 1981), the fast-flush test validation (Kleinman 1992), the arterial catheter complications review (Scheer 2002), the AHA blood-pressure-measurement statement (Pickering 2005), the transradial-access ultrasound guidance (Hamilton 2024), the PAC-Man trial (Harvey 2005), the perioperative thermoregulation reviews (Sessler 2008, 2016), the peripheral-thermometer accuracy meta-analysis (Niven 2015), the continuous-temperature-methods comparison (Ehlers 2025), and the four landmark hypothermia-outcomes trials (Frank 1997, Kurz 1996, Schmied 1996, Heier 1991).