Thermal balance and tremor patterns during epidural anesthesia

Five healthy, nonpregnant volunteers were studied before and after induction of lumbar epidural anesthesia to determine the cause of central hypothermia during epidural anesthesia. Cutaneous heat loss was measured from 10 area-weighted sites using thermal flux transducers. Oxygen consumption was measured and converted to heat production in watts (W). After a 2-h control period at approximately 20 degrees C, epidural anesthesia was induced by injection of 30-50 ml 3% chloroprocaine. Additional boluses were given to extend the sensory blockade to at least the T5 dermatome. Tremor during epidural anesthesia was compared with normal shivering induced by rapid central venous infusion of approximately 4 l iced saline in six unanesthetized volunteers. Average skin temperature and cutaneous heat loss decreased during the control period, while tympanic membrane temperature remained stable. During the 1st h of epidural blockade, tympanic membrane temperature decreased 1.1 +/- 0.3 degrees C, and average skin temperature increased 0.9 +/- 0.5 degrees C. Cutaneous heat loss increased 16 +/- 6% (15 +/- 5 W), but metabolic heat production increased even more (and was associated with a shivering-like tremor). Tremor during epidural anesthesia and shivering induced by iced saline infusion had similar synchronous waxing-and-waning patterns. No abnormal EMG patterns were detected during epidural anesthesia. We conclude that central hypothermia during the 1st h of epidural anesthesia does not result from heat loss to the environment in excess of metabolic heat production, but results primarily from redistribution of body heat from central to peripheral tissues. Analysis of the tremor patterns suggests that oscillations recorded during epidural anesthesia in nonpregnant individuals is normal thermoregulatory shivering. Shivering occurred sooner and was more intense during iced saline infusion than during epidural anesthesia, despite comparable central hypothermia. The low intensity of shivering during epidural anesthesia, and in some individuals the delay in onset, may result from blockade of afferent cutaneous cold signals.