Effect of warmed ropivacaine solution on onset and duration of axillary block

BACKGROUND Bicarbonate, as an adjunct increasing the non-ionized form of local anesthetics, can reduce latency and prolong duration of regional nerve block. Warming of local anesthetics decreases pKa and also increases the non-ionized form of local anesthetics. We warmed ropivacaine to body temperature (37℃) and evaluated the sensory block onset time, motor block onset time and analgesic duration of axillary block. METHODS Patients were consecutively allocated to two groups of 22 patients each. Ropivacaine 150 mg (30 ml) at 20℃ (room temperature) and 150 mg (30 ml) at 37℃ (body temperature) was injected in group 1 and group 2, respectively. Sensory block and motor block was assessed every 5 minutes, for 30 minutes after injection. The duration of analgesia was recorded after operation. RESULTS In group 2, the onset times of both sensory and motor block of the radial, ulnar, median and musculocutaneous nerves were significantly reduced, compared to group 1. Also, the number of blocked nerves was increased in group 2, within 30 minutes after injection. Analgesia lasted for 2 hours longer in group 2, compared to group 1, but the difference was not statistically significant (P > 0.05). CONCLUSIONS Warming of ropivacaine to 37℃ can reduce the onset time of both sensory and motor block, during axillary block.