A thermometer is a device used to measure temperature.
A thermometer is used in health care to measure and monitor body temperature. In an office, hospital or other health care facility, it allows a caregiver to record a baseline temperature when a patient is admitted. Repeated measurements of temperature are useful to detect deviations from normal levels. Repeated measurements are also useful in monitoring the effectiveness of current medications or other treatments.
The patient's temperature is recorded to check for pyrexia or monitor the degree of hypothermia present in the body.
All health care professionals use thermometers. All health care facilities have thermometers. Most homes also have thermometers.
A thermometer can use any of several methods to register temperature. These include mercury; liquid-in-glass; electronic with digital display; infrared or tympanic; and disposable dot matrix. A thermometer can be used in a clinical or emergency setting or at home. Thermometers can record body temperatures in the mouth (oral), armpit (axillary), eardrum (tympanic membrane), or anus (rectal).
A mercury thermometer consists of a narrow glass stem approximately 5 in (12.7 cm) in length with markings along one or both sides indicating the temperature scale in degrees Fahrenheit, Centigrade or both. Liquid mercury is held in a reservoir bulb at one end and rises through a capillary tube when the glass chamber is placed in contact with the body. Mercury thermometers are not used in modern clinical settings.
Electronic thermometers can record a wide range of temperatures between 94°F and 105°F, (35°C and 42°C) and can record oral, axillary, or rectal temperatures. They have temperature sensors inside round-tipped probes that can be covered with disposable guards to prevent the spread of infection. The sensor is connected to a container housing the central processing unit. The information gathered by the sensor is then shown on a display screen. Some electronic models have such other features as memory recall of the last recording or a large display screen for easy reading. To use an electronic thermometer, the caregiver places the probe under the patient's arm or tongue, or in the patient's rectum. The probe is left in place for a period of time that depends on the model used. The device will beep when the peak temperature is reached. The time required to obtain a reading varies from 3–30 seconds.
A tympanic thermometer has a round-tipped probe containing a sensor that can be covered with a disposable guard to protect against the spread of ear infections. It is placed in the ear canal for 1 sec while an infrared sensor records the body heat radiated by the eardrum. The reading then appears on the unit's screen.
Digital and tympanic thermometers should be used in accordance with the manufacturer's guidelines.
Disposable thermometers are plastic strips with dots on the surface that have been impregnated with temperature-sensitive chemicals. The strips are sticky on one side to adhere to the skin under the armpit and prevent slippage. The dots change color at different temperatures as the chemicals in them respond to body heat. The temperature is readable after two to three minutes, depending on the instrument's guidelines. These products vary in length of use; they may be disposable, reusable, or used continuously for up to 48 hours. Disposable thermometers are useful for children, as they can record temperatures while children are asleep.
Caregivers should be given training appropriate for the type of device used in their specific clinical setting.
The patient should sit or lie in a comfortable position to ensure that temperature readings are taken in similar locations each time and to minimize the effects of stress or excitement on the reading.
The manufacturer's guidelines should be followed when taking a patient's temperature with a digital, tympanic, or disposable thermometer. Dot-matrix thermometers are placed next to the skin and usually held in place by an adhesive strip. With the tympanic thermometer, caregivers should ensure that the probe is properly inserted into the ear to allow an optimal reading. The reading will be less accurate if the sensor cannot accurately touch the tympanic membrane or if the ear canal is clogged by wax or debris.
A mercury thermometer can be used to monitor a temperature in three body locations:
Before recording a temperature using a mercury thermometer, the caregiver shakes the mercury down by holding the thermometer firmly at the clear end and flicking it quickly a few times with a downward wrist motion toward the silver end. The mercury should be shaken down below 96°F (35.5°C) before the patient's temperature is taken.
In axillary placement, the silver tip of the thermometer is placed under the patient's right armpit, with the patient's arm pressing the instrument against the chest. The thermometer should stay in place for six to seven minutes. The caregiver can record the patient's other vital signs during this waiting period. After the waiting period has elapsed, the caregiver removes the thermometer and holds it at eye level to read it. The mercury will have risen to a level indicating the patient's temperature.
The procedure for taking a patient's temperature by mouth with a mercury thermometer is similar to the axillary method except that the silver tip of the thermometer is placed beneath the tongue for four to five minutes before being read. In both cases, the thermometer is wiped clean and stored in an appropriate container to prevent breakage.
To record the patient's rectal temperature with a mercury thermometer, a rectal thermometer is shaken down as described earlier. A small amount of water-based lubricant is placed on the colored tip of the thermometer to make it easier to insert. Infants must be positioned lying on their stomachs and held securely by the caregiver. The tip of the thermometer is inserted into the rectum no more than 0.5 in (1.3 cm) and held there for two to three minutes. The thermometer is removed, read as before, and cleansed with an antibacterial wipe. It is then stored in an appropriate container to prevent breakage. This precaution is important as mercury is poisonous when swallowed.
Liquid-in-glass thermometers contain alternatives to mercury (such as colored alcohol), but are used and stored in the same manner as mercury thermometers.
Many digital and infrared thermometers are self-calibrating and need relatively little care. To ensure accuracy, mercury thermometers should be shaken down prior to every use and left in place for at least three minutes. They require careful storage to prevent breakage and thorough cleaning after each use to prevent cross-infection.
As of early 2003, there is a nationwide initiative to ban the sale of thermometers and blood pressure monitors containing mercury. Health activists are concerned about mercury from broken or unwanted instruments contaminating the environment. A mercury thermometer contains 0.7g (0.025 oz) of mercury; 1 g of the substance is enough to contaminate a 20-acre lake. Several states have banned the use of products containing mercury. Most retail stores have stopped selling mercury thermometers. In October 1999, the Environmental Protection Agency (EPA) advised using alternative products to avoid the need for increased regulations in years to come and to protect human health and wildlife by reducing unnecessary exposure to mercury. According to a 2001 study by the Mayo Clinic, mercury-free devices can monitor information without compromising accuracy.
A thermometer should be cleaned, disinfected and placed in an appropriate container for storage.
Breakage of a glass thermometer creates a risk of cuts from broken glass and possible mercury poisoning. Improper operation of a tympanic thermometer can cause injury to the middle ear. As digital devices have replaced glass thermometers, however, the number of injuries has declined.
An additional risk is that old or broken thermometers may give inaccurate results.
A normal body temperature is defined as approximately 98.6°F (37°C). Body temperature is not constant throughout a 24-hour period. Some variation (0.3°F) is normal. Individuals also vary in their basal temperatures (0.3°F). A fever is defined as a temperature of 101°F or higher in an infant younger than three months or above 102°F for older children and adults. Hypothermia is recognized as a temperature below 96°F (35.5°C).
Injuries caused by properly inserted and normally functioning thermometers are extremely rare.
There are no convenient alternatives to using a thermometer to measure body temperature.
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L. Fleming Fallon, Jr., MD, DrPH
Most health professionals are trained in the proper operation of thermometers used in clinical settings. Most families have and use thermometers in the home.