Temperature measurement

Regarding temperature measurement, the following statements are true EXCEPT

which one?

a) Rectal temperature tends to be higher than oesophageal temperature

b) Oesophageal probes most commonly incorporate a thermistor to transduce

temperature to electrical changes

c) A thermopile is a collection of thermocouples connected in parallel

d) Tympanic membrane thermometers often employ the Seebeck effect

e) Miniaturised temperature measurement probes typically have response times of

around one second

Answer: e

Explanation

Given the importance of intraoperative temperature management and temperature

measurement incorporated into other devices, familiarity with thermometers will not

be limited to the Primary exam. Local bacterial fermentation in the rectum renders its

temperature slightly higher than core temperature. Oesophageal temperature may also

be falsely lowered if the probe (which does indeed incorporate a thermistor) is too

superficially inserted, as the proximal oesophagus is subject to the cooling effect of

respiratory gas exchanges in the trachea. The lower third (sometimes quoted as 35 to

45 cm insertion depth) is in closer proximity to the great vessels and thus the core

temperature of interest. Nasopharyngeal temperature measurement is accurate if an

airway management device has eliminated cooling gas flow in the nasopharynx that

might otherwise lower measured temperature. Tympanic membrane thermometers

transduce infrared radiation (produced by most objects at around body temperature) to

measurable electrical change either via a thermopile or a pyroelectric sensor. A thermocouple

employs the Seebeck effect (the induction of a potential difference at the junction of

two dissimilar metals that varies with temperature). When connected in parallel the

sensitivity and response time are improved and this is called a thermopile. A pyroelectric

sensor involves a crystal of variable measurable polarisation where the degree of polarisation

varies with the level of infrared radiation to which it is exposed. Temperature

probes miniaturised for use in, for example, intravascular catheters must have a much

faster response time (e.g. 100ms) in order to be useful for thermodilution techniques.