PERIOPERATIVE THERMAL MANIPULATIONS
Intraoperative thermoregulatory vasoconstriction, once triggered,
is remarkably effective in preventing further
core hypothermia.[38]
[46]
Nonetheless, most patients are poikilothermic during surgery because they do not
become sufficiently hypothermic to trigger thermoregulatory responses.[21]
[22]
[24]
[25]
Therefore, intraoperative hypothermia can be minimized by any technique that limits
cutaneous heat loss to the environment as a result of cold operating rooms, evaporation
from surgical incisions, and conductive cooling produced by the administration of
cold intravenous fluids.
Mean body temperature will decrease when heat loss to the environment
exceeds metabolic heat production. Heat production during anesthesia is approximately
0.8 kcal/kg/hr. Because the specific heat of the human body is about 0.83 kcal/kg,
[151]
body temperature decreases approximately 1°C/hr
when heat lost to the environment exceeds metabolic production by a factor of 2.
Normally, about 90% of metabolic heat is lost through the skin surface. During
anesthesia, additional heat is lost directly from surgical incisions and by the administration
of cold intravenous fluids.
Effects of Vasomotor Tone on Heat Transfer
Thermoregulatory vasodilation causes the initial core-to-peripheral
redistribution of body heat[57]
; similarly, re-emergence
of vasoconstriction in patients becoming sufficiently hypothermic produces a core
temperature plateau.[46]
It is thus evident that
vasomotor tone alters intercompartmental heat transfer. In addition to thermoregulatory
arteriovenous shunt status, arteriolar tone is directly modulated by anesthetics
per se.[54]
Both factors potentially influence
the speed with which peripherally applied heat reaches the core thermal compartment.
Thermoregulatory vasoconstriction slightly impairs induction of
therapeutic hypothermia during neurosurgery.[152]
However, arteriovenous shunt tone has little effect on intraoperative cooling[153]
or heating.[154]
Intraoperative vasoconstriction
thus only slightly impedes peripheral-to-core transfer of cutaneous heating and cooling.
Little clinical effect presumably results because intraoperative thermoregulatory
vasoconstriction is opposed by direct anesthetic-induced peripheral vasodilation.
During postanesthetic recovery, however, the situation differs
markedly. Here, anesthetic-induced peripheral dilation[54]
[155]
dissipates, with thermoregulatory vasoconstriction
left unopposed. As might be expected, this vasoconstriction then becomes an important
factor and significantly impairs transfer of peripherally applied heat to the core
thermal compartment. Patients with a residual spinal anesthetic block thus warm
considerably faster than those recovering from general anesthesia alone ( Fig.
40-17
).[156]
Heat balance studies indicate
that core warming is slowed because vasoconstriction constrains up to 30 kcal in
peripheral tissues.[157]
Because postoperative thermoregulatory vasoconstriction decreases
peripheral-to-core transfer of heat, applied warming is most effective during surgery
when patients are vasodilated. From a practical point of view, this means that it
is easier to maintain intraoperative normothermia (when most patients are vasodilated)
than to rewarm them postoperatively (when virtually all hypothermic patients
Figure 40-17
Intraoperative and postoperative core temperatures in
patients assigned to general anesthesia (n = 20)
and spinal anesthesia (n = 20). All patients were
warmed with forced air during the postoperative period. Core temperature did not
differ significantly during surgery, but it increased significantly faster postoperatively
in patients given spinal anesthesia (1.2 ± 0.1°C/hr
versus 0.7 ± 0.2°C/hr, means ± SD). (Redrawn
from Szmuk P, Ezri T, Sessler DI, et al: Spinal anesthesia only minimally increases
the efficacy of postoperative forced-air rewarming. Anesthesiology 87:1050–1054,
1997.)
are vasoconstricted). In addition to being more effective, intraoperative warming
is more appropriate than postoperative treatment of hypothermia because it prevents
the complications resulting from hypothermia.[106]
[117]
[122]
Patients
unavoidably becoming hypothermic during surgery should nonetheless be actively heated
postoperatively to increase thermal comfort, decrease shivering, and hasten rewarming.
