Warming 0.5% bupivacaine to 37 degrees C increases duration of spinal anesthesia

The effect of warming glucose-free 0.5% bupivacaine to 37 degrees C before injection for spinal anesthesia was studied in 20 patients having total knee replacement. Twenty additional patients having spinal anesthesia for the same procedure were given glucose-free 0.5% bupivacaine at room temperature (approximately 20 degrees C). Onset, maximum cephalad spread, quality of sensory anesthesia, and duration and degree of motor blockade were the same in both groups. However, the duration of sensory anesthesia was significantly prolonged in patients who received bupivacaine at 37 degrees C. The mechanism by which warming bupivacaine prolongs the duration of sensory spinal anesthesia is uncertain. However, a decrease in the dissociation constant (pKa) of bupivacaine owing to increasing the temperature to 37 degrees C may account for some of this effect.