Shivering

Shivering-like tremor in volunteers given neuraxial anesthesia is always preceded by core hypothermia and vasoconstriction (above the level of the block).^[61] Furthermore, electromyographic analysis indicates that

Figure 40-16 Overall, heat balance was only slightly negative (loss exceeding production) before induction of anesthesia and subsequently changed little. To separate the contributions of decreased overall heat balance and internal redistribution of body heat to the decrease in core temperature, the change in overall heat balance was divided by body weight and the specific heat of humans. The resulting change in mean body temperature ("mean body") was subtracted from the change in core temperature ("core"), thus leaving the core hypothermia specifically resulting from redistribution ("redistribution"). After 1 hour of anesthesia, core temperature had decreased 0.8°C ± 0.3°C, with redistribution contributing 89% to the decrease. During the subsequent 2 hours of anesthesia, core temperature decreased an additional 0.4°C ± 0.3°C, with redistribution contributing 62%. Redistribution thus contributed 80% to the entire 1.2°C ± 0.3°C decrease in core temperature during the 3 hours of anesthesia. The increase in the "redistribution" curve before induction of anesthesia indicates that thermoregulatory vasoconstriction was constraining metabolic heat to the core thermal compartment. Such constraint is, of course, the only way in which core temperature could increase while body heat content decreased. Induction of epidural anesthesia is identified as elapsed time zero. Results are presented as means ± SD. (Redrawn with modification from Matsukawa T, Sessler DI, Christensen R, et al: Heat flow and distribution during epidural anesthesia. Anesthesiology 83:961–967, 1995.)

It remains probable, though, that tremor during regional anesthesia in pregnant women has a different etiology. Similarly, the shivering-like tremor so often observed during labor without neuraxial anesthesia has yet to be adequately characterized. In both cases, there appears to be a significant incidence of shivering-like tremor in normothermic and vasodilated patients. This observation suggests that some tremor is nonthermoregulatory because shivering should always be preceded by hypothermia and arteriovenous shunt vasoconstriction.

Spinal thermal receptors have been detected in every mammal and bird tested. Experimental stimulation of

1581

The risk of shivering during neuraxial anesthesia is markedly diminished by maintaining strict normothermia.^[61] However, there is a distinct incidence of low-intensity, shivering-like tremor that occurs in normothermic patients and is not thermoregulatory.^[85] The cause of this muscular activity remains unknown, but it is associated with pain and may thus result from activation of the sympathetic nervous system.

Shivering during neuraxial anesthesia can sometimes be treated by warming sentient skin. Such warming increases cutaneous thermal input to the central regulatory system, thus increasing the degree of core hypothermia tolerated. ^[86] Because the entire skin surface contributes 20% to thermoregulatory control^[5] and the lower part of the body contributes about 10%,^[68] sentient skin warming is likely to compensate for only small reductions in core temperature. The same drugs that are effective for postanesthetic tremor are also useful for shivering during regional anesthesia; these drugs include meperidine (25 mg intravenously [IV] or epidurally),^[87] clonidine (75 µg IV),^[88] ketanserin (10 mg IV),^[88] and magnesium sulfate (30 mg/kg IV).^[89]