Methods of Temperature Measurement
The Mercury-glass thermometer is the standard to which other methods of thermometry are compared. Today in most clinical circumstances electronic thermometers with digital displays and disposable sleeves are used. A beep or the cessation of blinking indicates the reading. The device does not measure tissue temperature, as does the glass thermometer, but rather the temperature of the probe after contact with target tissue. It does not wait for
an equilibrium between the probe and target tissue; rather, it predicts the equilibrium temperature with algorithmic calculations. Although subject to error, it has proved reliable.
The probe should be under the tongue touching the sublingual gland with the mouth tightly closed for at least 4 minutes with the glass thermometer and an appropriate amount of time for the electronic probe. Factors affecting an oral temperature include rapid breathing, the ingestion of hot or cold beverages, and incorrect placement of the thermometer tip.
The axillary temperature requires a closed axilla for 10 minutes before placement of the thermometer or electronic probe, and four minutes is required after placement. Because of the potential for error this is usually impractical. It is used in the trauma patient when other sites are unavailable, and occasionally in pediatrics.
The rectal temperature is often avoided because of patient discomfort and the extra effort required. Minimal problems occur if the probe is well lubricated. The rectal temperature is least influenced by error. The patient is positioned on the left side with the right thigh and knee flexed. Small children and infants are positioned on their backs and the thighs and knees are flexed while they are held. Children can sometimes be held on their hands and knees. The probe or thermometer is inserted 1-1/2 inches for adults, 1 inch for children, 1/ 2 inch for infants, and the thermometer is read in 3 minutes.
Contraindications are hemorrhoids and rectal disease. Rectal temperatures may be hazardous in newborns because of the possibility of perforation.
The vaginal temperature, similar to the rectal range, is used when other sites are unavailable, as in trauma. It is contraindicated in the last trimester of pregnancy and in uterine and vaginal disease.
The tympanic thermometer assesses infrared emissions from the tympanic cavity. A heat-sensitive probe, covered by a disposable plastic speculum or a plastic-wrap type of material, is inserted into the auditory canal, similar to how one uses an otoscope. The scan button is pressed, the signal is digitized through a computerized processor, and in 2 seconds the temperature is displayed on a small liquid-crystal screen. The probe must make a tight seal in the auditory canal or the reading will be in error. It is useful in the pediatric population, and in situations such as a wired jaw, the multiple trauma patient, the obtunded elderly and the comatose patient. The only contraindication is extensive bilateral auricular malformation/disease.
Other areas are used from time to time, such as forehead temperatures in
children. Plastic strips had been used, but were considered unreliable. Recently
a forehead temporal artery device was evaluated but was also considered