Portable nerve stimulators for nerve blocks have been available for more than 40 yr. It is generally accepted that seeking a motor response at low outputs increases the chances of success. It is customary to start the procedure at a higher current with the goal of finding the nerve. After an adequate response is elicited, the current is decreased before the local anesthetic is injected. However, how low is low enough and, for that matter, how high is too high have not been adequately determined. Our experience seems to indicate that, in the supraclavicular block, the type of response obtained is as important as the output at which it is elicited, provided that this current is not higher than 1 mA. In this context, it is theoretically possible that our initial seeking current of 0.9 mA could be an adequate injection current if it is combined with an appropriate response. We designed this study to test the hypothesis that a response of the fingers in flexion or extension, elicited at 0.9 mA, could be followed immediately by the local anesthetic injection. We did not intend to compare 0.5 and 0.9 mA as minimum stimulating currents but rather as currents able to elicit an unmistakable motor twitch. Sixty patients were randomly assigned to one of two groups. Group 1 (n = 30) was injected with a motor twitch in the fingers that was still visible at 0.5 mA. Group 2 (n = 30) was injected after a similar response to that in Group 1 was elicited, but at the initial output of 0.9 mA, without any further decrease. The blocks were injected with 40 mL of local anesthetic solution. One patient was excluded from the study for failing to meet protocol criteria. The success rate in the remaining 59 patients was 100%; success was defined as complete sensory blockade at the median, ulnar, and radial nerve territories of the hand that was accomplished in <or=30 min from the time of injection and that did not require supplementation or general anesthesia. In fact, all blocks became complete within 22 min of the injection. The onset of anesthesia occurred in 10.9 +/- 5.4 min in the 0.5-mA group and 11.4 +/- 4.8 min in the 0.9-mA group; this difference was not statistically different. The onset of analgesia and the duration of anesthesia were also similar in both groups. There were no complications, and the respondents in both groups graded their experience at a similar level of satisfaction. We conclude that during the performance of a supraclavicular block eliciting a clearly visible response of the fingers at 0.9 mA can be immediately followed by the injection of local anesthetic, because decreasing the output to 0.5 mA does not seem to improve the overall quality of the block as measured by the onset and duration of anesthesia or patient satisfaction.
Implications: When nerve blocks are performed with a nerve stimulator, it is customary to reduce the nerve stimulator output to <= 0.5 mA before injecting. Apparently this is not necessary with a supraclavicular block.