ZOLL Thermogard XP IVTM Physician Manual - Manual (Page 8)

ZOLL IVTM™ System Physicians' Manual
600248-001 Rev 3 7
Thermoregulation
Human beings are mammals: as such their physiology operates to set and maintain
body temperature within a narrow band about a set-point, nominally 37
o
± 1
o
C.
Normal Control of Body Temperature
1
The body temperature is a reflection of the equilibrium state between the body and its
environment. Within an environmental range of approximately 13
o
C to 54
o
C, a normal
unclothed human can maintain a core body temperature somewhere between 36
o
C
and 37.9 o C [1].
Heat is generated within the body via chemical and physical processes of the body.
The physical processes include both bodily activity and cellular respiration. Heat is a
byproduct of cellular respiration–most of this heat is generated in skeletal muscle
and, to a lesser extent, in brown fat and in the liver. Seventy five percent or more of
total energy input is released back to the environment directly as heat (depending
upon the level of physical activity). Shivering is a specific example of muscular
activity to produce heat.
Heat loss is via conduction to materials in direct contact with the body, via convection
to the air, and via infrared emissions. We use clothing to help minimize this heat loss.
Respiration and sweating are specific evaporative/ convective mechanisms (heat is
conducted to the surface layer of water where it then drives a phase change–the
movement of unsaturated air accelerates the process); the latter being specifically
variable in response to body temperature. Typical sources of human heat loss in a
room at normal temperatures are shown in the table below [1].
Table 1. Human Heat Loss by Source.
Source Percent
Radiation 60%
Evaporation 22%
Conduction to objects 3%
Convection/conduction to air 15%
In general, humans have a central control mechanism that seeks to maintain body
temperature in reference to a set-point. This set-point can be varied by both internal
and external mechanisms. For a given set-point, the body will act to maintain a
temperature (see following). For example, with a fever, attempts to withdraw heat will
be resisted until the set-point for that febrile body temperature is reset.
Central Set-Point
Temperature regulation is centered in the hypothalamus. The preoptic area of the
hypothalamus seems to serve as the thermostatic center for the body.
1 Unless otherwise stated, the general references used in this chapter are
Guyton and Hall, 2001 [1] and Schonbaum and Lomax, 1991 [2].