Diluent choice affects neuromuscular block times

By Eleanor McDermid

Anesthesiologists can achieve successful axillary brachial plexus block faster by diluting ropivacaine in dextrose rather than saline, results of a randomized trial show.

"Early onset of block is worthwhile in a busy clinical practice because it can allow for proper planning and timing of rescue blocks, if needed," say Shalini Dhir (University of Western Ontario, London, Canada) and team.

The researchers found that diluting 20 mL of 1% ropivacaine in 5% dextrose, rather than saline, halved the sodium content of the resultant 0.5% ropivacaine solution.

They had wished to test the effects of a sodium-free solution, because a high sodium level is thought to interfere with neuromuscular block efficacy. But they note: "Even though the effects of eliminating sodium in the block solution could not be fully evaluated, this methodology would mimic what anesthesiologists can do in clinical practice."

Even this smaller reduction had a significant impact, as assessed in 550 adult patients scheduled for elective surgery of hand, wrist, or forearm under axillary brachial plexus block.

By 30 minutes after block induction, 83.1% of the patients randomly assigned to receive dextrose-diluted block had achieved complete sensory block, compared with 74.6% of those assigned to the saline group.

Complete sensory block was defined as total loss of sensation to a pinprick and was assessed every 5 minutes. The average times to complete sensory block were 18.3 and 22.5 minutes in the dextrose- and saline-dilution groups, respectively, which was also a significant difference.

Motor block was assessed via movements including thumb opposition, finger adduction, and elbow extension. The average time to complete motor block was significantly shorter in the dextrose- than saline-dilution groups, at 19.0 versus 22.3 minutes.

"Based on the results of this study, practice at our site has changed so that all [local anesthetic] drugs for plexus blocks are now diluted with 5% dextrose," the researchers write in Anesthesia and Analgesia.

Adverse events after surgery included symptoms of complex regional pain syndrome in two patients, carpal tunnel syndrome in one, and progressive bilateral sensory deficits thought to be caused by a previously undetected neurologic condition in another. Adverse effects were no more common in the dextrose than saline groups, however.

Notably, most postoperative neurologic symptoms appeared by about 1 to 2 weeks after surgery, which the team says provides support for later follow up than the 1 to 2 days that is common practice.

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