Page 542 - Encyclopedia of Nursing Research

THERMAL BALANCE n 509

Lenz and colleagues’ theory of unpleasant vasoconstriction, shivering, and increased
symptoms. metabolic activity. Each physiological
response augments or inhibits the transfer T
Shirley M. Moore of heat by affecting the thermodynamics
of conduction, convection, radiation, and
evaporation. Vasodilation warms the skin
where heat is more easily lost to air, contact
Thermal Balance surfaces, or liquids. Vasoconstriction creates
a poorly perfused insulative layer of tissue
that conserves heat. In infants, cold expo-
Thermal balance is defined as a thermal sure causes metabolic breakdown of brown
“steady state” in which the loss of body heat fat to generate heat. In older children and
is equal to the heat gain. In health, this bal- adults, the primary means of heat genera-
ance produces a thermoneutral state, opti- tion is shivering.
mal for cellular function. In humans, this Nurses have recognized the impor-
state averages about 37°C ± .05 for inter- tance of assessing thermal balance as a
nal temperatures and 33.5°C ± .05 for skin. vital health indicator for as long as the
Variations in body temperature respond to profession has existed. Body temperature
both homeostatic and circadian influences provides an important vital sign of meta-
(Holtzclaw, 2001). Circadian rhythm of bolic, neurological, and infectious activity.
core temperature is regulated by a remark- Circadian rhythms, monthly cycles, and
ably stable endogenous “clock,” which has daily body temperature ranges are assur-
helped to make it the most widely used ances of healthy variations. The pregnant
circadian indicator. There is evidence that mother provides heat exchange both for
circadian rhythms begin in fetal develop- herself and the fetus; therefore, high mater-
ment but there is also research support for nal body temperatures, from fever, hyper-
maternally derived prenatal and postnatal thermia, or prolonged “hot tub” use, put the
influences on rhythm (Weinert, 2005). The unborn infant at risk for neurological dam-
tendency of older people to go to bed and age. Temperature elevations in the acutely
wake up earlier than younger people has ill and injured may indicate either fever or
generated studies with a lack of consensus hyperthermia. Each has its own dynamics
about whether a phase advance (shifted ear- and treatment. Fevers are manifestation of
lier) of circadian rhythms occurs in later life the host response to pyrogens and are usu-
(Yoon et al., 2003), Hypothalamic thermo- ally self-limiting. By contrast, thermoregu-
regulatory controls keep internal tempera- latory control is lost during hyperthermia
tures fairly stable, despite environmental and requires aggressive cooling treatment.
changes and the propensity of heat to escape Temperatures above 42°C can cause irre-
to cooler regions. Metabolic and physical versible neural cellular damage. Conductive
activity continually generates heat, even cooling blankets, ice packs, and cooling
as it is constantly lost to the cooler envi- fans are used to lower core temperatures.
ronment. Current theory is that elaborate In immunosuppression associated with
thermoregulatory control systems maintain cancer treatment, fevers may indicate ful-
temperatures within the optimal set point minating systemic infection. However, the
range. Compensatory cooling or warming immunosuppressed HIV-infected patient
mechanisms respond to deviations above or may become febrile from high cytokine lev-
below this range. Temperatures rising above els, without obvious secondary infection. In
this range evoke vasodilation and sweat- both groups, constant assessment of other
ing, whereas falling temperatures cause indicators is necessary to rule out infection.