Heat Balance

Hypothermia is common during regional anesthesia and may be nearly as severe as during general anesthesia.^{[74] [75] [76]} Core temperature typically decreases 0.5°C to 1.0°C shortly after induction of anesthesia. However, the vasodilation induced by regional anesthesia only slightly increases cutaneous heat loss. Furthermore, metabolic heat production remains constant or increases because of shivering thermogenesis. This rapid decrease in core temperature, similar to that noted after induction of general anesthesia, also results from an internal core-to-peripheral redistribution of body heat ( Fig. 40-16 ). ^[77] As during general anesthesia, redistribution hypothermia during regional anesthesia can be minimized by cutaneous warming before induction.^[78]

Subsequent hypothermia results simply from heat loss exceeding metabolic heat production. Unlike patients given general anesthesia, however, core temperature does not necessarily plateau after several hours of surgery. Not only is the vasoconstriction threshold centrally impaired by regional anesthesia,^{[62] [63]} but more importantly, vasoconstriction in the legs is also directly prevented by nerve block.^{[79] [80]} Because the legs constitute the bulk of the thermal compartment, an effective plateau cannot develop without vasoconstriction in the legs and the resulting decrease in cutaneous heat loss and constraint of metabolic heat to the core.

The importance of intraoperative leg vasoconstriction is illustrated during combined regional/general anesthesia. Consistent with the impairment in thermoregulatory responses with regional anesthesia alone,^{[62] [63]} vasoconstriction during combined regional/general anesthesia is triggered at a core temperature approximately 1°C less than during general anesthesia alone. Furthermore, once triggered, vasoconstriction produces a core temperature plateau during general anesthesia alone but not during combined regional/general anesthesia. The result is that core temperature during combined regional/general anesthesia continues to decrease throughout surgery.^[81] Consequently, core temperature monitoring and thermal management are particularly important in patients given simultaneous regional and general anesthesia.