Temperature Monitors (Thermometer, Thermistor, Thermocouple,
Thermopile, Liquid Crystal)
Three general techniques can be used for measuring temperature:
those based on expansion of a material as the temperature of the material increases,
those based on changes in electrical properties with temperature, and those based
on optical properties of a material. As heat is added to most substances (gases,
liquids, or solids), motion of the molecules increases, and the volume of the material
expands at constant pressure. Depending on the material, this expansion can be calibrated
linearly to changes in temperature. Liquids are most commonly used, specifically,
mercury, because its effective range extends from its freezing point of -39°C
to approximately 250°C.[27]
Mercury thermometers
have two general disadvantages. They require 2 to 3 minutes for complete thermal
equilibration (mercury has a high specific heat). In addition, they are enclosed
in a glass tube, which may break and injure the patient.[28]
Thermometers based on the expansion of gas (Bourdon tube) or metal (bimetallic strip)
are frequently used as thermostats because they respond slowly to transient changes
in temperature.
Electrical techniques for measuring temperature can be subdivided
into three categories: resistance thermometers, thermistors, and thermocouples.
Resistance thermometers operate on the principle that the electrical resistance
of metals increases with temperature. These devices most frequently use a platinum
wire as the temperature-sensitive resistor, a battery, and a galvanometer to measure
current, which can be calibrated to temperature. The platinum wire is incorporated
into a Wheatstone bridge circuit, which accurately measures very small changes in
resistance ( Fig. 30-36
).
When compared with a platinum thermometer, a thermistor is a semiconductor
that displays the opposite behavior with regard to electrical resistance. Specifically,
as the thermistor is heated, its resistance decreases. Thermistors, being solid-state
devices, can be manufactured as extremely small devices and therefore have a fast
response to change in tempeature (i.e., little heat is needed to increase their temperature).
Most of the temperature probes used in anesthesia, from the ones at the end of Swan-Ganz
catheters to esophageal probes, are thermistors.
Physical problems with thermistors are few: broken wires lead
to high resistance and improper interpretation of temperature. More common are poor
probe placement and misinterpretation of the value, such as placing the esophageal
probe in the oral pharynx and measuring airway temperature, not core temperature.
The optical properties of materials can be used to measure temperature in two ways:
(1) the infrared emission of a body can be measured by using a device known as a
thermopile and the emission then converted to temperature, and (2) a liquid crystal
"matrix" can be placed in direct contact with the desired zone and an optical change
can be observed (a change in color). The most commonly encountered example of infrared
temperature measurement involves the tympanic membrane temperature monitor used in
pediatrics and on hospital wards.[29]
The infrared
detector produces an electrical signal that is proportional to the fourth power of
the difference in absolute temperatures of the objects. The following formula describes
the specifics: Q1,2
= K (T1
4
−
T2
4
), where Q1,2
is net heat transfer
(W/cm2
),
Figure 30-36
Wheatstone bridge. A Wheatstone bridge is an electronic
circuit designed so that an unknown resistance can be calculated by knowing two sets
of variables: (1) the voltage drop across the bridge and (2) the other resistances
in the circuit (see Appendix 3
).
K is the Stefan-Boltzmann constant, and T1
and T2
are the absolute
temperatures of the two objects (°K). In theory, this method of measuring the
core temperature should be accurate. However, in practice, improper placement and
lack of calibration, but not cerumen, contribute to real-world errors.[30]
[31]
[32]
Additionally,
the probe is rather large for intraoperative use.
Liquid crystal measures of temperature (commonly found in "mood
rings") are often used for skin temperature monitoring. Just as the molecules in
a liquid crystal watch display change their optical properties with a small electric
current (becoming polarized and dark), the molecules in a liquid crystal temperature
device change their optical properties with temperature (resulting in a rainbow of
colors). The crystal matrix is sensitive to pressure as well as temperature. (By
touching a broken LCD display panel, one can note the visual rainbow of changing
colors.) The reported clinical accuracy of liquid crystal devices varies.[33]
[34]
In conclusion, new monitors are being developed
almost continuously, but new physical principles are revealed only rarely. Physics
as a science is trying to simplify and render the universe in absolute terms. The
terms and principles outlined here will be referred to multiple times in subsequent
chapters.
