Temperature Regulation and the Pathogenesis of Fever

in the rectal temperature after a reduction in the colonic bacterial content. [18] There is also concern
that stool in the rectum acts as a heat sink to delay or mitigate changes in the rectal temperature,
particularly so if the thermometer is inserted directly into stool. [19] During shock, perfusion of the
rectum may be markedly impaired, causing the rectal temperature to lag significantly behind a
rapidly rising or falling core temperature. [20] For this reason, Houdas and Ring have concluded that
the rectal temperature provides a reliable approximation of the core temperature only if the patient
is in thermal balance. [21] In neonates, even in the absence of shock, the rectal temperature
(measured by standard technique) has been reported to correlate poorly with the core temperature
(as measured by a deep rectal probe). [22] Although generally safe, such measurements are
associated with a small risk for rectal perforation—especially in neonates and very young
infants [23][24] —and if proper infection control measures are not followed, may be a source of
nosocomial infection. [25]
Of the three sites most commonly used for clinical thermometric measurement (rectum, mouth, and
tympanic membrane), the mouth is usually preferred, because it is accessible, responds promptly
to changes in the core temperature, and has a long tradition of use in monitoring body temperature
in clinical practice. The temperature of the sublingual pocket may be especially relevant clinically,
because its main artery is a branch of the external carotid artery and, like its parent artery,
responds quickly to changes in the core temperature. [18] However, because oral temperature
measurements require the cooperation of the subject being examined, not all patients (e.g., young
children, uncooperative adults, and intubated individuals) are amenable to such measurements.
It has long been suspected that the ingestion of hot or cold food or beverages and smoking
influence oral temperature readings. In a study of 22 healthy young adults, Rabinowitz and
associates showed that mastication and smoking cause both significant and persistent increases in
the oral temperature, whereas drinking ice water causes a significant but much more transient
decrease in the oral temperature. [26]
There is controversy regarding the effect of tachypnea on the accuracy of oral thermometric
readings. In studies employing electronic thermometers, Tandberg and Sklar obtained average
rectal temperature readings that were 0.96° F (0.6° C) higher than simultaneous oral temperatures
in patients with respiratory rates of 20 per minute or less, compared with 1.67° F (1.0° C) higher in
patients with respiratory rates of greater than 20 per minute. [27] A more recent study of 78 subjects
by Neff and co-workers that controlled for open- and closed-mouth breathing and used tympanic
rather than rectal temperature as a reference concluded that sublingual temperature changes do
not correlate with the respiratory rate or depth but do depend on whether the mouth is open or
closed. [28] As noted earlier, the probe location and equilibration time are two additional variables
that can alter the results of oral temperature measurements.
The right atrium is the ideal site for measuring core temperature, because it is the nexus at which
venous blood from all anatomic regions joins. However, because it is relatively inaccessible, the
temperatures of other sites are more often used as approximations of core temperature. The
tympanic membrane (TM) temperature is felt by some to be particularly useful in this regard,
because the TM is perfused by a tributary of the artery that supplies the body’s thermoregulatory
center. [29] This fact, and the ease with which TM measurements can be obtained using modern