Which is the best site for a nurse to measure body temperature in an unconscious patient?

Interpretation

Normal values for body temperature are affected by the following variables: (1) site and methods used for measurement; (2) perfusion; (3) environmental exposure; (4) pregnancy, (5) activity level; and (6) time of day. Clinicians must interpret body temperature with knowledge of the range of normal values at the intended site of measurement. Although core body temperature remains nearly constant (37.0°C ± 0.6°C or 98.6°F ± 0.18°F), surface temperature rises and falls with changes in ambient temperature, exercise, and time of day. The definition of fever varies by the site of measurement and is defined by a temperature greater than 2 standard deviations above the mean. Fever has been defined as an oral temperature of 37.8°C or higher (100.0°F),246 a rectal temperature of 38.0°C or higher (100.4°F),254 or an IR ear temperature of 37.6°C or higher (99.6°F).246 Based on measurement of temperatures in normal, healthy infants, it is recommended that fever be defined as a rectal temperature of 38°C or higher in infants younger than 30 days, 38.1°C or higher in infants 30 to 60 days (1 to 2 months), and 38.2°C or higher in infants 60 to 90 days old (2 to 3 months).255 Hypothermia has been defined as a core body temperature lower than 35°C (<95°F), and hyperthermia has been defined as a core body temperature higher than 41°C (>105.8°F) with accompanying symptoms and signs.256 A useful nomogram and formulas for conversion of centigrade to Fahrenheit are provided inFig. 1.6.

Temperature probes that depend on transfer of heat energy from local tissues to the probe require a period of equilibration and reliable tissue contact at the intended body site. Acceptable equilibration times for mercury-in-glass thermometers in oral, rectal, and axillary sites are 7, 3, and 10 minutes, respectively. Used in a predictive mode, electronic digital thermometers generally require 30 seconds for oral or rectal temperature equilibration. The predictive mode uses temperature changes versus time to predict an equilibration temperature.

Normal ranges and suggested febrile thresholds for common body sites and methods should be considered in the interpretation of temperature values (Table 1.5). Interpretation of temperature measurements during clinical assessment must consider the use of antipyretics, level of activity, pregnancy, environmental exposure, and patient age. Body temperature is increased during sustained exercise, pregnancy, and the luteal phase of the menstrual cycle. Temperature also increases in the late afternoon because of diurnal variation. Body temperature is generally reduced with advanced age, and age may have an impact on the magnitude of fever. Axillary temperatures have a low sensitivity but a high specificity for fever. Axillary temperatures should not be used to screen for fever.

Additional Example 1: CI on Precision of Thermometer

We are investigating the reliability of a certain brand of tympanic thermometer (temperature measured by a sensor inserted into the patient’s ear)110. The standard deviation will give us an indication of its precision. We want a 95% confidence interval on this precision, that is, a range outside of which the standard deviation would lie no more than 5 times in 100 random readings. Sixteen readings (°F) were taken on a healthy patient at intervals of 1 minute. Data were 95.8, 97.4, 99.3, 97.1, …, yielding s=1.23. s2=1.232=1.51. We use Eq. (7.12). The chi-square value from Table III for 97.5% area except right is χ2R=27.49 for 15 df, and from Table IV for area except left is χ2L=6.26. We find

P[s2×df/χ2R<σ2<s2×df/χ2 L]=P[1.51×15/27.49<σ2<1.51×15/6.26 ]=P[0.8239<σ2<3.6182]=0.95.

Taking square roots within the bracket, we obtain

P[0.9077<σ<1.9022]=0.95.

We are 95% sure that the standard deviation of this thermometer will lie within the range 0.9–1.9°F.

Infrared tympanic thermometer

Temporal Artery Thermometer

A noninvasive scanning thermometer was developed by Exergen Corporation as an alternative to the tympanic thermometer for measuring core body temperature, essentially eliminating the discomfort caused by a mouth, ear, or rectal thermometer (Figure 10.25). The measurement is based on scanning the area above the temporal artery using an IR detector similar to the sensor used in the tympanic thermometer. The superficial temporal artery extends directly from the external carotid artery and travels in front of the ear. Anatomically, it is lying approximately 1 mm below the skin, readily accessible, and maintains good blood perfusion. Assuming zero thermal loss to the environment, the skin surface over the temporal artery area would be at the same temperature as the arterial blood in the aorta, which is essentially equal to core body temperature. Since ambient temperature is normally lower than core body temperature, there is a cooling effect at the skin surface due to the radiative heat loss to the surrounding air. To account for errors due to the natural heat loss, the hand-held scanning thermometer measures ambient temperature at the same time it measures the absolute temperature of the skin surface over the temporal artery and computes arterial temperature using a heat balance equation.

Types of Thermometers

For many years, glass thermometers containing mercury were the most common type of thermometer used to measure body temperature. This type of thermometer is reasonably accurate for most clinical purposes. Although they are still available, use of mercury-containing thermometers has diminished greatly because of environmental concerns about mercury exposure from broken or discarded thermometers. These thermometers have been replaced largely by digital thermometers or glass thermometers containing liquids other than mercury.

Electronic thermometers (often referred to as digital thermometers) previously were used primarily in the hospital and office setting. As their cost has decreased, they are used more frequently in the home as well. Electronic thermometers have the advantage over mercury thermometers of requiring a significantly shorter dwell time—that is, the time they must remain in situ to obtain an accurate reading. Hospital-grade electronic thermometers typically have two modes: monitor and predictive. In the monitor mode, these thermometers function similarly to mercury thermometers in that they must remain in place until equilibration occurs, a process that may require several minutes. In the predictive mode, a complex algorithm is used to estimate the final temperature based on measurements made during the first few seconds. Because the predictive mode produces a temperature reading within seconds, it is the mode used most often in clinical settings. Determinations of temperatures using these two modes have been found to correlate well.25,60

Infrared thermometers are a more recent addition to the clinician's armamentarium. Devices that determine the temperature by detecting infrared radiation emitted from the eardrum are used most frequently. Tympanic temperature should provide an accurate estimation of the core temperature because its blood supply is derived from the carotid artery. Additional advantages of this type of thermometer are its speed, acceptance by patients, and decreased risk of cross-contamination compared with oral or rectal thermometers. Studies of the accuracy of tympanic thermometers have yielded mixed results, with numerous studies finding tympanic thermometers inaccurate compared with mercury in glass or electronic thermometers.14,56 A recent meta-analysis showed pooled sensitivity of 70% and specificity of 86% as compared to rectal thermometry in diagnosing pediatric fever.101 Discrepancies seem to be particularly common in infants who are in the first few months of life.83 Tympanic thermometers should not be used in young infants because of the importance of fever in making management decisions in these patients.17

Even more recently, the temporal artery thermometer has been introduced. These thermometers use an infrared sensor to determine skin temperature as the device is passed across the forehead and temporal area. The site of highest measured temperature is assumed to represent that of the temporal artery. An algorithm is applied to the measured temperature to estimate the core temperature. Studies to date suggest that temporal artery thermometer temperatures correlate significantly better with rectal and core temperatures than with temperatures determined by tympanic thermometers. The data also suggest that these thermometers are more sensitive at detecting fever in children than in adults. Temporal artery thermometers do not seem to correlate well enough with rectal or core temperature measurements, however, to replace rectal thermometry in clinical situations in which accurate measurement of fever is crucial for making decisions about management. Correlation with rectal thermometry may be particularly limited in children younger than 36 months of age, with a reported sensitivity as low as 27% to 53%.32 The accuracy of temporal artery thermometer readings is adversely affected by sweating and may be affected by vascular constriction or dilation. In addition, data comparing temporal artery and axillary thermometry are lacking.29,89

Multiple Sclerosis.

In patients with MS, Uthoff's phenomenon, an adverse reaction to external heat or increased body temperature, can occur with exercise, resulting in extreme fatigue and worsening of symptoms. An ear thermometer can therefore be used to monitor temperature before, during, and after exercise in patients with MS. Precooling with cold water immersion,287 cooling garments,288 ice packs, or by the patient consuming iced drinks or having the patient exercise in an air-conditioned room can prevent excessive gains in core temperature with physical work and may allow heat-sensitive individuals with MS to exercise without adverse effects. A randomized controlled study of 84 patients with MS found that wearing a liquid cooling garment (Fig. 17-12) for 1 hour a day for a month resulted in significant improvements in functional abilities as measured by the MS Functional Composite scale.289 Patients also reported less fatigue during the month of daily cooling. Because core body temperature is lowest in the morning, patients with MS may tolerate exercise better in the morning.

A few studies have examined the effects of aerobic exercise in patients with MS.139,258,290-292 Petajan and colleagues evaluated the impact of 15 weeks of aerobic training (40 minutes of exercise 3 times per week) on fitness and quality of life in a group of individuals with MS with an EDSS score of less than 6.0.292 This training program resulted in a 22% increase in VO2 max, a 48% increase in maximal power output, and significant improvements in body composition and serum triglyceride levels for the exercise group. Depression and anger scores as measured by the Profile of Mood States (POMS) were reduced at weeks 5 and 10. Furthermore, quality of life as measured by Sickness Impact Profile (SIP) scores improved, with the exercise group having significantly improved physical dimension scores and significant improvements in social interaction, emotional behavior, home management, and recreation and pastime scores.

Additional Example: Is a Treatment for Dyspepsia in the ED Too Variable?

In the additional example of Section 12.2, an emergency medicine physician tested the effectiveness of a “GI cocktail” (antacid plus viscous lidocaine) to treat emergency dyspeptic symptoms as measured on a 1–10 pain scale90. He concluded that the treatment was effective on average, but was it more variable? This question implies a one-tailed test. He will sample 15 patients, so df = 14. Table 14.2 provides a portion of Table III. From Table 14.2 or Table III with 14 df, the critical value of χ2 for α = 0.05 is 23.69. The population standard deviation for the scoring difference upon being seen minus that after a specified number of minutes for a large number of patients without treatment was σ = 1.73. He treated n = 15 patients, measuring the difference in pain before minus after treatment. Data were 6, 7, 2, 5, 3, 0, 3, 4, 5, 6, 1, 1, 1, 8, 6. He calculated m = 3.87 and s = 2.50. He tested the variance of his sample (s2 = 6.27) against the variance of untreated patients (σ2 = 2.99) at the α = 0.05 level of significance. By substituting in Eq. (14.1), he found

χ2=df×s2σ2=14×4.97292.99=29.36.

The calculated χ2 is greater than the critical value of 23.69. Indeed, it is slightly greater than the 29.14 value associated with α = 0.01. He has evidence that his sample is more variable than is the population. From a statistical software package, p = 0.009.

Exercise 14.1. Is the variability of readings from a tympanic thermometer too large? We are investigating the reliability of a certain brand of tympanic thermometer (temperature measured by a sensor inserted into the patient’s ear). Sixteen readings (°F) were taken on a healthy patient at intervals of 1 minute110, left and right ears pooled. (Data are given in Exercise 11.3.) In our clinical judgment, a reliable thermometer will be no more than 1°F off 95% of the time. This implies that 1°F is about 2σ, so that σ = 0.5°. From the data, s = 1.230. At the α = 0.05 level of significance, is the variability of the tympanic thermometer reading on a patient unacceptably large?

Tympanic Membrane Thermometers.

Tympanic membrane thermometers use an otoscope-like probe that is inserted into the external auditory canal to detect and measure thermal infrared energy emitted from the tympanic membrane (Fig. 22-2). A scan button is pressed to start the measurement, and an audible signal indicates that the temperature is ready to be recorded from the digital display.

Tympanic thermometers are minimally invasive, record temperatures in approximately 3 seconds, register temperatures in the range of 25°-43° C, have no direct contact with mucous membranes, and work only if the disposable probe cover is in place.4 The probe lens, however, can be easily damaged if not handled carefully. It is important to check the lens before each use and to replace its protective cover when not in use.4 Operator handedness, patient position, and ear (right or left) have been shown not to produce clinically significant variability,5 although mean tympanic measurements from a single ear were found to agree less than the mean of both ears when compared to temperature measurement by pulmonary artery catheter.6 Obstruction of the tympanic membrane by cerumen may lower tympanic measurements.7

Tympanic thermometers have been found to be accurate, easily usable clinically,8 and satisfactory for routine intermittent temperature measurement.9 They are as accurate as indwelling rectal probes and are suitable for estimating core temperature when a pulmonary artery catheter is not in place or is contraindicated.10 Tympanic membrane thermometers are the most sensitive noninvasive devices for measuring body temperature greater than 37.5° C and are better for detecting temperature shifts after acetaminophen than single-use or mercury-in glass-thermometers.11 Some authors find tympanic and oral electronic thermometer measurements equally acceptable if pulmonary artery catheter and rectal temperatures are not available or contraindicated.12

Significant variations in temperature measurement have been found among different types and makes of tympanic thermometers,10,13,14 which may be attributed at least in part to different people using the devices.5 Even though the tympanic thermometer produces more variable results, the mean readings are not significantly different from those taken with a mercury-in-glass thermometer.15 Inaccuracies are more likely in children if the thermometer is calibrated for an adult, the incorrect size probe is used, or if the child is less than 1 year old where even the smallest tip available is likely to fit poorly.14,16 Additionally, inaccuracies may occur with incorrect positioning,10 leading some researchers to recommend that an electronic oral measurement be taken before a tympanic measurement to first check if they correlate.17

Body Temperature

Body temperature is regulated by the hypothalamus to maintain the core temperature of approximately 37°C (98.6°F).31,32 Body temperature can be affected by diurnal fluctuations throughout the day (e.g., lowest in the morning, highest in the late afternoon to early evening), hormones, exercise, smoking, consumption of hot or cold beverages, and age.32 Measurement of body temperature can be obtained by an oral, rectal, axillary, or tympanic thermometer.

An oral thermometer is ideal for alert patients. The body temperature can be obtained by placing the oral thermometer under the tongue in either of the sublingual pockets with the lips closed. Hot or cold liquids and smoking can alter the temperature reading. As such, it is advised to delay the temperature measurement by 10–15 min after consuming beverages or smoking. The rectal thermometer may be considered in comatose patients or patients who are unable to close their mouth (e.g., intubation, wired mandible, facial surgery). The temperature reading through the rectal route may be 0.5°C (0.9°F) higher than the oral reading. The axillary route may be ideal for infants and small children. This reading may be 0.5°C (0.9°F) lower than the oral route, and it is obtained by placing the thermometer under the arm into the center of the axilla with the patient's arm folded over the chest to keep it in place for 5–10 min. A tympanic thermometer can be considered for unconscious patients, emergency departments, and labor and delivery units. These readings may be 0.8°C (1.4°F) higher than the oral route. Temperature readings are obtained by gently pulling the ear up and back to straighten the ear canal (if under 3-years old, the ear is pulled downward and back) and placing the probe into the ear canal. Most digital tympanic thermometers can provide a reading in 2–3 s. Additionally, noncontact forehead thermometers are also available and may be more advantageous for reading temperatures in children.33,34

A normal body temperature reading is 35.8–37.3°C (96.4°F to 99.14°F).31,32 A higher than normal temperature reading can be a result of pyrexia (i.e., fever) caused by infection, tissue breakdown (e.g., MI), or neurological (e.g., brain tumor). A high-temperature reading can also be a result of hyperthermia, in which heat production or external heat exposure exceeds heat loss, and the body is unable to thermoregulate. This can occur due to heat stroke, drugs (e.g., serotonin syndrome), and hyperthyroidism. A lower than normal temperature reading can be a result of hypothermia in which heat loss exceeds heat production. This can occur due to prolonged exposure to cold or intentional induction (e.g., cardiovascular surgery).