SHIVERING DURING EPIDURAL-ANESTHESIA

The authors tested the hypotheses that during anesthesia: 1) shivering-like tremor is primarily normal thermoregulatory shivering; 2) hypothermia does not produce a subjective sensation of cold; and 3) injectate temperature does not influence tremor intensity. An epidural catheter was inserted into ten healthy, nonpregnant volunteers randomly assigned to skin-surface warming below the T10 dermatome (warmed group) or no extra warming (unwarmed group). Each volunteer was given two 30-ml epidural injections of 1% lidocaine (16.0 ± 4.7°C and 40.6 ± 0.7°C at the catheter tip), in random order separated by at least 3 h. Skin-temperature gradients (forearm-fingertip) and tympanic membrane and average skin temperatures were recorded; significant vasoconstriction was prospectively defined as a gradient ≥ 4°C. Integrated electromyographic (EMG) intensity was recorded from four upper-body muscles. Overall thermal comfort was evaluated using a visual analog scale. Tympanic membrane temperatures decreased significantly in the unwarmed group (n = 6). Tremor occurred following ten of 12 injections in unwarmed volunteers, but only following one of eight injections in the warmed group. Integrated EMG intensity did not differ significantly following epidural injection of warm and cold lidocaine: tremor started when tympanic membrane temperature decreased about 0.5°C and continued until central temperature returned to within 0.5°C of control. Tremor always was preceded by hypothermia and vasoconstriction in the arms. Thermal comfort increased in both groups after epidural injections, with maximal comfort occurring at the lowest tympanic temperatures. These data suggest that: 1) tremor during epidural anesthesia is primarily normal thermoregulatory shivering; 2) epidural injectate temperature does not influence tremor intensity; and 3) central hypothermia does not necessarily produce a subjective sensation of cold.