Pressure and temperature measurement

Why this matters to the anaesthetist

Blood pressure and body temperature are the two constants of anaesthetic monitoring, and the physics of how they are measured determines whether the numbers on the screen are trustworthy. An under-damped arterial line over-reads the systolic by 20 mmHg and may provoke unnecessary antihypertensives; an un-levelled transducer introduces a systematic error of nearly a millimetre of mercury for every centimetre of height; a tympanic thermometer under-reads in shock when skin perfusion fails. Understanding the transducer, the damping, the zeroing and levelling, the central venous waveform, and the temperature-sensing principles is the foundation of reliable monitoring — and a favourite of the physics viva [1][9].

The topic divides cleanly into two halves. The first is the measurement of pressure: the definition and units, the mechanical gauges, the electronic pressure transducer and its dynamic response, and the waveforms of the arterial, central venous and pulmonary artery catheters. The second is the measurement of temperature: the temperature scales, the four sensing principles, the monitoring sites and their accuracy, and the three phases of perioperative heat loss and its prevention. Each half carries a classic examination trap. [1]

Pressure: definition and units

Pressure is defined as force acting perpendicularly per unit area, P = F/A. The SI unit is the pascal (Pa), one newton per square metre, but the pascal is inconveniently small for clinical use, so physiological and anaesthetic pressures are quoted in kilopascals (kPa), millimetres of mercury (mmHg), centimetres of water (cmH2O), bar or atmosphere. Knowing the conversions is a guaranteed viva question. [1]

A useful clinical anchor: a normal mean arterial pressure of about 90 mmHg is about 12 kPa; a central venous pressure of 8 mmHg is about 11 cmH2O; a cylinder of oxygen at 137 bar (gauge) holds, with the gas laws, the equivalent of many hundred litres. [1]

There are three ways of expressing any pressure, and the distinction matters for cylinders and airway pressures. Absolute pressure is referenced to a perfect vacuum (so atmospheric pressure at sea level is about 101 kPa absolute). Gauge pressure is referenced to atmospheric pressure, so a gauge reading of zero means the pressure equals atmospheric — most physiological pressures, cylinder contents and ventilator airway pressures are gauge pressures. Differential pressure is simply the difference between two pressures. The full oxygen cylinder marked 137 bar is a gauge pressure; its absolute pressure is about 138 bar [4].

Mechanical pressure-measuring devices

Before the electronic transducer, pressure was measured mechanically, and three mechanical devices still appear in every operating theatre and on the physics viva [4].

The Bourdon gauge measures high pressures — the contents gauge on an oxygen or nitrous-oxide cylinder. It is a curved, flattened metal tube (the Bourdon tube) sealed at one end and open at the other to the gas being measured. As pressure rises inside the tube it tends to straighten, and the sealed free end moves; a linkage and gear convert that motion into the rotation of a needle across a dial. It is robust, simple, needs no power, and reads gauge pressure. Its limitation is that it measures only relatively high pressures and is not linear enough for the physiological range. [1]

The aneroid gauge measures moderate pressures — the pressure gauge on the anaesthetic machine (showing pipeline and cylinder supply pressures) and the dial of the aneroid sphygmomanometer. It uses a sealed, partially evacuated metal capsule (aneroid = without fluid) that expands and contracts as the external pressure changes; a lever and gear system amplifies the capsule's deformation into needle movement. It is more sensitive than the Bourdon gauge and still needs no power, but it must be periodically recalibrated against a mercury column because the capsule drifts. [1]

The manometer is the simplest and most direct — a liquid column in a U-tube or vertical tube whose height is proportional to the pressure. A mercury manometer (density 13.6 g/mL) measures blood pressure in mmHg and is the calibration reference standard for all other BP devices; a water manometer measures venous and airway pressures in cmH2O (CVP manometers, the older "bubble" CVP sets). The height of the column IS the measurement, read directly off a scale. Its limitations are that it is bulky, slow, spills a toxic liquid (mercury), and cannot follow a rapidly changing pressure [4].

The electronic pressure transducer

The invasive blood-pressure transducer is the workhorse of modern monitoring, and its principle is the most examined single device in anaesthesia physics. It converts the hydraulic pressure of a fluid column into an electrical signal proportional to the pressure [1].

The commonest design is the piezoresistive strain-gauge transducer. The fluid presses on a thin diaphragm, which deforms by a few micrometres. Bonded to the diaphragm are strain gauges — fine wires or, in modern devices, diffused silicon elements whose electrical resistance changes when they are stretched or compressed (the piezoresistive effect). To maximise sensitivity and cancel temperature effects, four strain gauges are arranged in a Wheatstone bridge: two are placed where the diaphragm stretches and two where it compresses, so the resistance changes are equal and opposite and the bridge output is doubled. A small excitation voltage drives the bridge; the out-of-balance voltage across the bridge is proportional to the diaphragm's deformation and hence to the pressure. This voltage is amplified, filtered and displayed as a continuous waveform and numerical readout. The system is accurate, fast and linear across the physiological range [1][2].

The alternative modern design is the capacitive transducer, in which the diaphragm forms one plate of a capacitor and moves relative to a fixed plate as pressure changes; the changing capacitance is the measure of pressure. Capacitive sensors are less temperature-sensitive and are used in some disposable transducers, but the piezoresistive strain-gauge design remains dominant. [1]

The arterial line: components and setup

The arterial line is a continuous fluid-filled path from the artery to the monitor, and the physics of every component in that path determines whether the displayed waveform is faithful [1][3].

The flush deserves a moment. The flush bag is pressurised to about 300 mmHg above the patient's systolic pressure (in practice, a 300 mmHg cuff on a 500 mL bag of saline, or a pre-pressurised disposable set). At that high driving pressure the flow through the fine restrictor is constant at about 3 mL/hr regardless of the patient's arterial pressure, because the patient's pressure (up to about 200 mmHg) is a negligible fraction of the 300 mmHg head. The slow continuous flush keeps the column of saline moving and (when heparinised) prevents clotting at the cannula tip. A spring-loaded valve in the flush device allows a fast flush (the square-wave test, see below) and to clear blood from the line [3].

Two setup steps are mandatory for an accurate invasive pressure. Zeroing opens the transducer to atmosphere (turning the three-way tap so the transducer sees only room air) and tells the monitor 'this is zero', removing the offset of atmospheric pressure so that all subsequent readings are relative to atmosphere. Zeroing need only be done once per session (at setup, and after any electrical discontinuity) because atmospheric pressure is constant during a case. Levelling aligns the transducer's zero-pressure reference point (the air-fluid interface, marked on the transducer) with the physiological reference point — the level of the right atrium, taken as the mid-axillary line in the fourth intercostal space. The transducer must be re-levelled whenever the operating table or the patient is moved [4].

The mechanical reason is the hydrostatic pressure of a column of fluid: a column of height h and density rho exerts a pressure rho·g·h. A 10 cm column of blood (density about 1060 kg/m3) exerts about 7.8 mmHg. This is also why a transducer below the heart over-reads (the column adds to the measured pressure) and above the heart under-reads. [1]

The dynamic response: natural frequency and damping

A perfectly static pressure would be reproduced exactly by the transducer, but the arterial pulse is dynamic — it contains frequencies from about 1 to 5 Hz with harmonics up to about 10 to 20 Hz, and the measuring system must be able to follow them. Two properties of the catheter-tubing-transducer system govern its dynamic response, and Gardner's classic paper set out the requirements [1][2].

The natural frequency is the frequency at which the system oscillates freely when disturbed. For faithful reproduction the natural frequency must be well above the highest frequency present in the pulse — if the two are close, the system resonates and the waveform is distorted. The natural frequency is raised by short, stiff, wide tubing, a small fluid volume in the system, and a stiff diaphragm; it is lowered (made worse) by long tubing, compliant (stretchy) tubing, large fluid volumes, and air bubbles. Modern disposable systems achieve a natural frequency of about 20 to 40 Hz, comfortably above the pulse's harmonic content [1].

The damping is the dissipation of the oscillatory energy — how quickly a disturbed system returns to rest. It is quantified by the damping coefficient (zeta). An under-damped system (zeta less than about 0.2) oscillates freely many times before settling; the waveform overshoots and rings, so the systolic is spuriously high and the diastolic spuriously low. An over-damped system (zeta approaching 1) returns slowly without oscillating; the waveform is slurred, the upstroke slowed, the systolic under-read and the diastolic over-read. An optimally damped system (zeta about 0.6 to 0.7, the practical optimum) settles after one or two small oscillations and reproduces the true waveform with the best fidelity [1][2].

The crucial clinical point is what damping does to each displayed value: [1]

The fast-flush (square-wave) test

The dynamic response is assessed at the bedside by the fast-flush test, validated by Kleinman as measuring the response of the entire system (cannula, tubing, transducer and monitor together) [2]. The operator briefly opens and then closes the valve to the high-pressure flush bag, sending a near-square-wave pressure perturbation through the system. The system's response to that square wave reveals the damping:

• Optimal damping — one or two small oscillations after the square wave, then a clean return to the arterial trace. Trust the systolic and diastolic.
• Under-damping — many oscillations of large amplitude (ringing) above and below the baseline before settling. The systolic reads falsely high; do not act on it.
• Over-damping — a slurred deflection with no oscillation and a slow crawl back up to the trace. The systolic reads falsely low. [1]

The causes of over-damping at the bedside are the ones to act on: an air bubble in the tubing or transducer dome (air is compressible and absorbs the pulse), a kink in the line or cannula, a blood clot at the tip, long or compliant tubing, and a narrow cannula. The fixes are to flush out or aspirate the air bubble, straighten kinks, aspirate and reflush clot, shorten and stiffen the tubing, and (if all else fails) re-site to a larger cannula. Under-damping, when it occurs, is corrected by adding a damping resistor or by accepting the mean value [1][2].

The arterial waveform and what it tells you

Beyond the numbers, the shape of the arterial waveform carries useful physiological information, and the viva examiner will expect a candidate to read it. A normal radial-arterial waveform has a rapid upstroke (the contraction phase, an index of contractility), a peak systolic, a dicrotic notch (closure of the aortic valve), and a smooth diastolic runoff down to the diastolic pressure. The area under the curve is proportional to stroke volume, and beat-to-beat variation of that area with respiration is the basis of pulse-contour cardiac-output analysis and of pulse-pressure variation as a dynamic test of fluid responsiveness. [1]

The site of measurement matters: a femoral waveform is sharper and gives a higher systolic than a radial waveform because the pressure pulse is amplified and distorted as it travels peripherally (the pulse-pressure amplification), so central and peripheral arterial lines are not interchangeable in the systolic value — but the mean is preserved. After cardiopulmonary bypass or in vasopressor use the radial may under-read for a time (radial-artery compression or vasoconstriction), and a femoral line is more reliable in shock. [1]

Arterial line complications

The arterial line is invasive and carries a defined set of complications, reviewed by Scheer and colleagues [3]. The common ones are temporary occlusion, haematoma and bleeding; the rarer but serious ones are thrombosis, distal ischaemia, infection and pseudoaneurysm. The risk factors are prolonged cannulation, a larger cannula, peripheral vascular disease, diabetes, hypertension, Raynaud phenomenon, female sex and shock. Allen's test is traditionally performed before radial cannulation to confirm ulnar collateral flow, though its predictive value is debated; distal perfusion and capillary refill must be checked regularly. The routine use of ultrasound guidance improves first-attempt success and reduces attempts and complications, particularly in patients with difficult anatomy [5].

The central venous pressure waveform

The central venous pressure (CVP) is measured by a catheter in the right atrium or superior vena cava, connected to the same kind of transducer. Its waveform is far richer than the arterial trace, because the atrial mechanical events are written directly onto it, and reading it is a classic viva task. [1]

The CVP trace has three positive waves and two descents, each corresponding to an event in the cardiac cycle and timed to the ECG [4]:

The pathological patterns are the high-yield exam material: [1]

The CVP itself (the mean value) is an index of right atrial pressure and, in the absence of tricuspid disease, of right ventricular preload. It is influenced by intrathoracic pressure (it rises with positive-pressure ventilation and tension pneumothorax), by venous return, and by right ventricular compliance. It is a poor predictor of fluid responsiveness in isolation — a single CVP reading tells you little; the change in CVP with a fluid challenge tells you more. This is the reason the CVP has fallen out of favour as a sole guide to resuscitation. [1]

The pulmonary artery (Swan-Ganz) catheter

The pulmonary artery catheter (PAC) extends central venous monitoring into the right heart and pulmonary circulation, giving the right-sided pressures and, by thermodilution, the cardiac output. It is floated from a central vein (internal jugular or subclavian) through the right atrium, right ventricle and pulmonary artery, and (with the balloon inflated) into a wedge position [6][7].

As it is advanced, the pressure trace changes characteristically with the chamber or vessel, and the approximate insertion depths (from a right internal jugular approach in an adult) are a common viva question: [1]

The pulmonary artery occlusion (wedge) pressure — obtained by inflating the balloon to occlude a branch of the PA, so the column of blood becomes static from the catheter tip through to the left atrium — approximates the left atrial pressure and so reflects left ventricular preload. It is the catheter's main diagnostic value, because in critical illness the right-sided filling pressures (CVP) can diverge markedly from the left-sided (wedge) pressures. The PAC also measures cardiac output by thermodilution (a bolus of cold saline injected in the right atrium; the temperature change measured by a thermistor at the PA tip is integrated by the Stewart-Hamilton equation), mixed venous oxygen saturation (SvO2, sampled from the PA), and from these the systemic and pulmonary vascular resistance can be derived. [1]

The PAC's place in practice has narrowed because its use has not shown a mortality benefit in general ICU patients and it carries real risks — arrhythmia, infection, pulmonary artery rupture, knotting and thrombosis. The PAC-Man trial (Harvey, Lancet 2005) found no overall survival benefit from PAC use in general ICU patients, and the post-hoc analysis reinforced that the catheter should be reserved for specific indications (cardiac surgery, shock of mixed or uncertain aetiology, pulmonary hypertension, and the management of complex fluid balance) and used by trained operators [6][7]. Less-invasive alternatives — echocardiography, arterial pulse-contour analysis (PiCCO, LiDCO), and oesophageal Doppler — have displaced it for routine perioperative cardiac-output monitoring.

Non-invasive blood pressure

When an arterial line is not warranted, blood pressure is measured non-invasively by oscillometry (the standard automated theatre NIBP) or by auscultation of Korotkoff sounds [4].

The oscillometric method inflates the cuff above systolic and then slowly deflates. As the cuff pressure passes through the arterial range, the arterial wall under the cuff is free to oscillate, and those oscillations are transmitted to the cuff and detected by a pressure sensor. The point of maximum oscillation amplitude is the mean arterial pressure — this is the directly measured value. The systolic and diastolic are algorithm-derived from the envelope of oscillation amplitudes (the systolic is taken where the oscillations first appear as the cuff deflates, the diastolic where they disappear). This is why an oscillometric device reports a mean that is more reliable than its systolic or diastolic, and why it can struggle with arrhythmia and very high or very low pressures. [1]

The auscultatory method uses a cuff and a stethoscope over the brachial artery. As the cuff deflates, the turbulent flow first produces the Korotkoff sounds: phase I (first clear tapping) = systolic; phases II-IV progressive muffling; phase V (disappearance) = diastolic. In some guidelines (and in children and high-output states) phase IV (muffling) rather than phase V (disappearance) is taken as the diastolic. Mercury sphygmomanometry is the reference standard against which all other devices are calibrated [4].

The sources of NIBP error are all viva material. A cuff too small (the bladder should cover 80 per cent of the arm circumference) over-reads; a cuff too large under-reads slightly. Arrhythmia (especially atrial fibrillation) degrades oscillometry; movement and shivering cause artefact; very high or very low pressures and a poor-quality pulse reduce accuracy. The cuff should be at heart level for the same hydrostatic reason as the transducer. NICE and the AHA recommendations codify these [4].

Temperature: scales and definition

Temperature is the second of the two constants of anaesthetic monitoring. Two scales are used clinically. The Celsius scale sets water's freezing point at 0 and boiling point at 100 (at 1 atm); the Kelvin scale is the SI unit, with its zero at absolute zero, so 0 K = -273.15 degrees C, and the size of one kelvin equals one Celsius degree. The conversion is K = C + 273.15; a normal body temperature of 37 degrees C is 310.15 K. Normal core temperature is about 37 degrees C (oral, range 36.5 to 37.5) with a diurnal variation of about 0.5 to 1 degree C and a thermoneutral zone of about 28 degrees C for a naked adult. [1]

Temperature measurement: the four sensing principles

Temperature is measured by four distinct physical principles, and the distinction between them is a guaranteed examination question [9][11].

The thermistor is a metal-oxide semiconductor whose electrical resistance falls as temperature rises — a negative temperature coefficient (NTC). It is small, fast, accurate (to about 0.1 to 0.2 degrees) and cheap, and it is the basis of most clinical temperature probes: the oesophageal, nasopharyngeal, rectal and bladder probes, and the skin probes. Its non-linear resistance-temperature curve is linearised by the monitor's software. The thermistor is the workhorse of perioperative temperature monitoring. [1]

The thermocouple exploits the Seebeck effect: when two wires of dissimilar metals (typically copper-constantan or type T, suitable for human use) are joined at a junction, a small voltage is generated that is proportional to the temperature difference between the measuring junction (in the patient) and a reference junction (the cold junction). Thermocouples are extremely small (a welded bead at the tip), cheap, robust and fast, and they cover a very wide range; they are used in some disposable skin and rectal probes and in industrial measurement. They are less accurate than thermistors in the physiological range (about 0.5 degrees) and require a stable reference junction. [1]

The resistance thermometer or RTD (resistance temperature detector) uses a fine coil of pure platinum wire (a Pt100, 100 ohms at 0 degrees C) whose resistance rises linearly with temperature — a positive temperature coefficient. The RTD is the most accurate and stable of the four (to about 0.01 degrees), it is the laboratory reference standard, and its output is almost perfectly linear. Its disadvantages are that it is larger, slower and more expensive than a thermistor, so it is used where accuracy matters more than speed — in calibration and laboratory work rather than at the bedside. [1]

The infrared detector measures the blackbody radiation emitted by a surface (the Stefan-Boltzmann relation: the radiated power is proportional to the fourth power of the absolute temperature). It does not touch the patient, so it is non-contact and instant. It is the principle of the tympanic membrane thermometer (the eardrum approximates core temperature and shares its blood supply from the carotid artery) and the temporal artery scanner. Its limitation is that it reads a surface, so it is confounded by earwax, a poor seal, a dirty lens, and — most importantly — by poor skin perfusion in shock, when the surface no longer reflects the core. [1]

Temperature monitoring sites and accuracy

The choice of monitoring site determines how closely the measurement reflects the core temperature, and this is a frequent source of clinical error. Core temperature is the temperature of the vital organs — the brain, heart and abdominal viscera — and it is best measured at a site with a high blood flow and good thermal insulation from the environment [9][11][12].

The core sites are the gold standard. The oesophagus (lower third) and the nasopharynx are the practical intraoperative core sites: they lie close to the great vessels and the blood in the heart, they track core temperature rapidly, and they are accurate provided the probe is correctly placed (the oesophageal probe behind the heart, the nasopharyngeal probe near the soft palate; the nasopharyngeal can be falsely low if it is cooled by fresh gas flow). Pulmonary artery blood is the truest core temperature (it is the mixed central blood), but it requires a PAC and is reserved for cardiac surgery and critical care. The tympanic membrane measured with a contacting probe is accurate and tracks core well, sharing the carotid blood supply with the hypothalamus; the bladder follows core with a lag, the lag lengthening at low urine flow. The Ehlers comparison of continuous ICU methods confirms these are the reliable sites [12].

The intermediate and peripheral sites are less reliable. The rectum lags behind core (the rectal wall is insulated by faeces and cooled by the lower-body blood returning from the periphery) and reads falsely high if there is faecal insulation or bacterially active stool; it should not be relied on during rapid temperature change. Skin sites (forehead, axilla, toe) are peripheral, not core: they lag markedly, are cooled by the environment, and under-read core by 1 to 2 degrees, more in vasoconstriction. The temporal artery scanner and the tympanic infrared device are convenient but are less accurate than a true core probe. The systematic review by Niven and colleagues quantified this: peripheral thermometers (tympanic, temporal, axillary, oral) are less accurate than core, with a pooled mean difference of around 0.5 degrees but wide limits of agreement, so they are acceptable for screening but not when accuracy matters — as in malignant hyperthermia, targeted temperature management, or the monitoring of perioperative hypothermia [11].

Perioperative heat loss: the three phases

General and regional anaesthesia abolish the body's thermoregulatory defences and the patient becomes poikilothermic, the core temperature drifting toward ambient. The intraoperative fall follows a characteristic three-phase pattern described by Sessler, and understanding it is the key to prevention [8][9].

Phase 1, redistribution, dominates the first hour and is the steepest fall, about 1 to 1.5 degrees C. It is NOT true heat loss — anaesthesia abolishes the vasoconstrictor tone that normally keeps the warm core blood centralised, and the core heat redistributes to the cool periphery. The body's total heat content is unchanged; only its distribution has altered, with the core falling and the periphery rising. Because it is redistribution and not loss, phase 1 cannot be treated by active warming once it has occurred — it can only be prevented. [1]

Phase 2, the linear decline, follows over the next 2 to 3 hours at about 0.5 to 1 degree C per hour. This IS true heat loss: heat is lost to the environment (by radiation, convection, evaporation from open cavities, and conduction) faster than the anaesthetised patient can generate it. Phase 2 is reduced (though not abolished) by active warming. [1]

Phase 3, the plateau, begins after about 3 to 4 hours. The core temperature stabilises when vasoconstriction returns (as the core falls enough to trigger the lowered vasoconstriction threshold under anaesthesia) and heat loss again equals heat production. The patient reaches a new, lower steady state. [1]

The prevention of phase 1 is the single most effective intervention, and it is achieved by prewarming — warming the periphery for 10 to 30 minutes before induction, so the core-to-peripheral gradient is abolished at the moment anaesthesia removes the vasoconstrictor tone. With no gradient, there is nothing to redistribute, and the steep first-hour fall is prevented. Phase 2 is addressed by active warming throughout the case [8][10].

Consequences of inadvertent perioperative hypothermia

Mild perioperative hypothermia — a core below 36 degrees C — is not benign. It is defined as mild (35.0 to 35.9 degrees C), moderate (34.0 to 34.9) and severe (below 34), and even mild hypothermia carries measurable harm, established in four landmark trials [8][9].

The mechanisms are worth knowing. The cardiac harm is driven by a cold-induced sympathetic surge (noradrenaline rises, causing vasoconstriction and hypertension) together with shivering, which raises oxygen consumption by 200 to 500 per cent at the moment cardiac reserve is most limited — precipitating demand ischaemia in patients with fixed coronary stenoses, and below about 34 degrees predisposing to arrhythmia. The coagulopathy is from impaired platelet function and slowed coagulation-enzyme kinetics. The infection risk is from cold-induced vasoconstriction lowering subcutaneous oxygen tension, impairing neutrophil oxidative killing and collagen deposition. The prolonged drug action is from slowed hepatic metabolism and renal clearance, and a left-shift of the oxyhaemoglobin curve impairing tissue oxygen unloading. Shivering itself is unpleasant, consumes oxygen and produces carbon dioxide, and degrades monitoring [13][14][15][16].

Prevention of perioperative hypothermia: warming strategies

Because hypothermia is harmful and predictable, its prevention is now a quality indicator in anaesthesia, codified in the NICE guideline CG65 and the Society for Ambulatory Anesthesia consensus. The aim is to keep the core at or above 36.0 degrees C [8][10].

The reverse scenario — a patient who fails to cool and instead overheats — raises the differential of malignant hyperthermia, covered in detail in the thermoregulation topic. The recognition is an unexplained, rapidly rising end-tidal carbon dioxide despite adequate ventilation (the earliest and most reliable sign), with tachycardia, rigidity, hyperkalaemia, acidosis and a late rising temperature; the management is to stop all triggers, switch to a non-triggering technique, give intravenous dantrolene 2.5 mg/kg, and actively cool. Temperature is a late sign in MH — never wait for it. [1]

Special populations

In neonates and infants, the large surface-area-to-mass ratio, limited substrate stores, and reliance on brown-fat non-shivering thermogenesis make hypothermia rapid and severe; the ambient temperature must be raised (24 to 26 degrees), the head (a large fraction of the surface area) insulated, and a forced-air warmer and warmed fluids used from induction. In the elderly, impaired vasoconstriction and shivering, reduced muscle mass, and blunted thermal perception increase the risk and the consequences (especially the cardiac harm), so active warming and continuous monitoring are even more important. In pregnancy, the altered thermoregulation and the risk to the fetus make warming routine for caesarean section under neuraxial anaesthesia, where hypothermia is common and can cause fetal distress. In cardiac and major-vascular surgery, where bypass and large open cavities maximise heat loss, a pulmonary artery temperature catheter and aggressive warming are standard [9][12].

Pitfalls and common errors

The examination-relevant pitfalls of this topic are concentrated and worth listing explicitly. Trusting an under-damped systolic and giving an antihypertensive to a spurious 240 mmHg is the classic iatrogenic error; the defence is the fast-flush test and trusting the mean. Forgetting to re-level after moving the table or the patient leaves a systematic error. Using a tympanic infrared probe in shock under-reads because the tympanic surface is underperfused. Reading a peripheral skin temperature as core under-reads by 1 to 2 degrees. Forgetting that the oscillometric NIBP measures mean directly but derives systolic and diastolic, so it struggles in arrhythmia. Confusing thermistor with thermocouple — the thermistor's resistance falls, the thermocouple produces a voltage. And treating phase 1 redistribution with intraoperative warming alone, when only prewarming can prevent it [1][8][11].

Exam pearls

A candidate who carries these one-liners will answer most of the pressure-and-temperature viva: pressure is force per unit area; the conversions are 1 kPa = 7.5 mmHg = 10.2 cmH2O and 1 atm = 101.325 kPa = 760 mmHg; the invasive transducer is a diaphragm + strain gauges + Wheatstone bridge; zero to atmosphere, level to the mid-axillary line, and each cm of error is about 0.75 to 1 mmHg; the fast-flush test shows under-damped rings, over-damped slurs, optimal settles in 1 to 2, and mean survives all three; the CVP waves are a (P wave), c (QRS), v (T wave) with cannon a = heart block, large v = TR, no a = AF; the PA catheter gives RA, RV, PA, wedge at about 30, 40, 50, 60 cm, and wedge approximates left atrial pressure; oscillometry measures mean directly, systolic and diastolic are derived; thermistor resistance falls, thermocouple voltage, RTD resistance rises; core sites are oesophageal, nasopharyngeal, pulmonary artery, bladder, tympanic contact; the three phases are redistribution (1-1.5 degrees, first hour, prevented by prewarming), linear, plateau; and the four hypothermia trials are Frank (cardiac), Kurz (infection), Schmied (bleeding), Heier (vecuronium) [1][8][9][13].