THERMOREGULATION DURING GENERAL ANESTHESIA
Behavioral regulation is not relevant during general anesthesia
because patients are unconscious and frequently paralyzed. All general anesthetics
tested thus far markedly impair normal autonomic thermoregulatory control. Anesthetic-induced
impairment has a specific form: warm-response thresholds are elevated slightly whereas
cold-response thresholds are markedly reduced. Consequently, the interthreshold
range is increased from its normal
Figure 40-3
Major autonomic thermoregulatory response thresholds
in volunteers given desflurane, alfentanil, dexmedetomidine, or propofol. All the
anesthetics slightly increase the sweating threshold (triggering core temperature)
while markedly and synchronously decreasing the vasoconstriction and shivering thresholds.
Standard deviation bars smaller than the data markers have been deleted. (Data
from references [21]
[22]
[23]
[24]
)
values near 0.2°C to approximately 2°C to 4°C.[21]
[22]
[23]
[24]
[25]
The gain and maximum intensity of some responses
remain normal,[8]
whereas others are reduced by
general anesthesia.[26]
[27]
Response Thresholds
Propofol,[21]
alfentanil,[22]
and dexmedetomidine[23]
all produce a slight linear
increase in the sweating threshold combined with a marked linear decrease in the
vasoconstriction and shivering thresholds. Isoflurane[25]
and desflurane[24]
also slightly increase the sweating
threshold; however, they decrease the cold-response thresholds nonlinearly. Consequently,
the volatile anesthetics inhibit vasoconstriction and shivering less than propofol
does at low concentrations, but more than propofol does at typical anesthetic doses.
In all cases (except during meperidine administration[28]
),
vasoconstriction and shivering decrease synchronously, thereby maintaining their
normal approximately 1°C difference.
The dose-dependent response thresholds for four anesthetic drugs
are shown in Figure 40-3
.
The combination
of increased sweating thresholds and reduced vasoconstriction thresholds increases
the interthreshold range about 20-fold, from its normal value near 0.2°C to around
2°C to 4°C. Temperatures within this range do not
trigger thermoregulatory defenses; by definition, patients are thus poikilothermic
within this temperature range.
Halothane,[29]
enflurane,[30]
and the combination of nitrous oxide and fentanyl[31]
decrease the vasoconstriction threshold 2°C to 4°C from its normal value
near approximately 37°C. However, the effect of these drugs on sweating or shivering
remains unknown. Clonidine synchronously decreases cold-response thresholds[32]
while slightly increasing the sweating threshold.[33]
Nitrous oxide decreases the vasoconstriction[34]
and shivering[35]
thresholds less than equipotent
concentrations of volatile anesthetics do. The only sedative or anesthetic drug
tested that minimally influences thermoregulatory control is midazolam.[36]
Painful stimulation slightly increases vasoconstriction thresholds[30]
;
consequently, thresholds will be somewhat lower when surgical pain is prevented by
simultaneous local or regional anesthesia.
