Influence of Injection Speed on the Subarachnoid Distribution of Isobaric Bupivacaine 0.5

The purpose of this study was to compare the anesthetic characteristics after two radically different speeds of intrathecal injection of isobaric 0.5% bupivacaine during continuous spinal anesthesia. Forty consenting patients, undergoing hip surgery using continuous spinal anesthesia, were allocated randomly to two groups of 20 each according to the rate of injection of 2 mL (10 mg) of isobaric 0.5% bupivacaineFI (fast injection = 2 mL during 2 to 3 s or ∼0.75 mL/s) or SI (slow injection = 1 mL/min). No difference was observed between the two groups in terms of sensory and motor block or hemodynamic changes. However, the onset time to maximal sensory level was significantly shorter in the SI group (16 ± 9 min vs 24 ± 6 min; P < 0.05). Those patients requiring reinjection entered the second part of the study (n = 23; 15 in the FI group and 8 in the SI group). Each of these remaining patients was used as his or her own proper control. Those in the FI group received a slow reinjection and those in the SI group received a fast reinjection which consisted in all cases of 10 mg (2 mL) of isobaric 0.5% bupivacaine. When looking at the anesthetic characteristics after reinjection, maximal sensory levels, as well as onset times, were very similar in both groups. At all times, the maximal sensory level obtained after reinjection was two dermatomes higher than after the initial injection. Duration of sensory block, which was calculated only in these 23 patients, was also comparable (126 ± 44 min for FI and slow reinjection group vs 146 ± 25 min for SI and fast reinjection group). In conclusion, regardless of the speed of injection, there are no differences in anesthetic characteristics of spinal anesthesia using isobaric 0.5% bupivacaine.