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Temperature

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Temperature measurement

Non-electrical methods

Mercury thermometer

- commonest method in clinical use
- depends on expansion of mercury with increase in temperature
- 2 main clinical disadvantages:

  • takes 2-3 mins for complete thermal equilibrium between mercury and its surroundings
  • risk of breaking

- alcohol can be used instead of mercury but relationship between volume and temperature less linear. Values of temperature that can be measured are lower

Dial thermometers

- either use bimetallic strip or Bourdon gauge
- Bourdon gauge actually measures pressure in a sensing chamber containing a small volume of mercury or a volatile liquid. Gauge is calibrated in units of temperature

Electrical techniques

- response time depends mainly on size with smaller probes having shorter response times due to lower heat capacities
- response times: 0.1-15 sec

Resistance wire thermometer

- relies on a linear increase in resistance of metal wire (usually platinum) with increase in temperature
- usually incorporated into Wheatstone bridge circuit to increase sensitivity

Thermistor

- thermistors are semi-conductor temperature sensors that have advantage that their change in resistance is greater with temperature than resistance wire thermometer. However change is not entirely linear and calibration may drift with ageing. May be extremely small; small enough to be attached to tympanic membrane
- usual type is a small bead of metal oxide resistance of which falls exponentially as temperature rises
- special thermistors are available in which resistance rises with temperature
- often used in Wheatstone bridge circuit
- cheaper to manufacture than platinum wire resistor
- calibration liable to change if thermistor is subjected to severe changes of temperature (eg in heat sterilization)

Thermocouple thermometers

- rely on Seebeck effect
- potential difference occurs at a junction between 2 dissimilar metals when they complete an electrical circuit
- magnitude of potential difference depends on temperature of metals
- voltage is very small and needs either amplification or a very sensitive galvanometer.
- second junction needed to form complete electrical circuit. Temperature dependent potential difference will also develop at this junction. For thermocouple to be used as a thermometer one of the junctions must be kept at a constant temperature while the other junction acts as a measuring probe. Alternatively equipment may provide electrical compensation for for changes in reference junction temperature
- usually made of copper and constantan (alloy of copper and nickel)
- advantages:

  • all couples made of the same material will behave identically and therefore calibration will not change should couple be replaced
  • measuring junction can be manufactured and used in the form of a needle

Other methods

Infrared ear thermometers

  • detect radiant energy from tympanic membrane and external auditory canal through an otoscopic probe
  • inaccurate if either structure inflammed or if there is obstruction of the external canal

Measuring sites

Rectum

- risk of perforation of or trauma to anus or rectum. Particular problem in small babies, neutropaenic or coagulopathic patients or patients who have recently undergone rectal surgery
- rectal temperature slow to equilibrate with core temperature in adults
- rectal temperature measurements have been implicated in the spread of enteric organisms eg Clostridium difficile, vancomycin resistant Enterococcus

Oesophagus

- important to use lower oesophagus as upper oesphagus may be cooled by cold inspiratory air in adjacent trachea
- in cardiothoracic surgery oesophageal temperature may not reflect core temperature due to cooling as a result of open chest and ice/cardioplegia

Nose

- only useful in intubated patients when nasal passages are not cooled by inspired air

Oral

  • Only suitable for alert and cooperative patients and therefore not usually suitable for ICU patients
  • Inaccuracies may result from mouth breathing, heated gases, hot/cold fluids
  • Can damage oral mucosa

Axilla

  • Correlates poorly with core temperature
  • Poor reproducibility

Pulmonary artery

  • Considered by many to be standard against which other measuring sites should be compared

Further reading

Parbrook GD, Davis PD and Parbrook EO, Basic physics and measurement in anaesthesia, 3rd ed. Butterworth-Heinemann, Oxford, 1990

O'Grady, N.P., Barie, P.S., Bartlett, J.G., Bleck, T., Garvey, G., Jacobi, J., Linden, P., Maki, D.G., Nam, M., Pasculle, W., Pasquale, M.D., Tribett, D.L., and Masur, H. Practice guidelines for evaluating new fever in critically ill adult patients. Clinical Infectious Diseases 26:1042-1059, 1998.


© Charles Gomersall December 1999

 

©Charles Gomersall, April, 2014 unless otherwise stated. The author, editor and The Chinese University of Hong Kong take no responsibility for any adverse event resulting from the use of this webpage.
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