Choyce et al. Reg Anes and Pain Med, 26(2), 2001: 100-4. (Editorial, pp. 97-99)

Reviewed by: R. Prasad, MD

Summary:

• 0.5 mA is "close enough" for axillary brachial plexus blocks in lightly sedated (midaz 1-3 mg) adults when using long-bevel noninsulated needles (77% had paresthesia AND twitch at <=0.5mA)
• 23% had paresthesias (i.e., were close enough to the nerve) but no motor twitch at currents <= 0.5mA. So, to avoid injuring nerves:
• avoid doing blocks under deep sedation
• if get paresthesia before movement at <0.5mA, take it - there is a "lack of complete overlap between the techniques of paresthesia and nerve stimulation"

• Interesting study, but not sure how it applies to our practice, or what to make of the data. Clearly needs more study.
• Does this apply to other blocks?
• Are pediatric nerves different (i.e., why is it probably safe, as per large outcomes studies, to do blocks in sleeping pediatric patients)?
• Why was twitch site different than paresthesia site in 19%? Needle movement? Nerve composition? Type of needle  used?
• Needle type. Results probably do not apply to insulated, short-bevel needles.
• Noninsulated needles. Current density is maximal just proximal to tip, so usually need higher current to elicit twitch. May explain why so many (23%) had no twitch at currents <= 0.5mA.
• Long bevel needles. Easier to elicit paresthesia. What does that mean?
• Paresthesias.
• Is it really the "gold standard" for being close enough to the nerve? Only 88.7% developed complete sensory block at paresthesia site.
• Of remaining 11.3% (incomplete blocks), half (5.7%) needed >0.5mA to elicit twitch.
• How many of the 88.7% needed >0.5mA to elicit twitch? If 0, then only 5.7% of pts had paresthesia requiring >0.5 mA
• If there is any pt movement, would expect it to be away from needle. This would increase the current required to elicit twitch, and strengthen the study's conclusion that 0.5mA is low enough.
• Success rate of single injection technique, used here, is lower. Should we ignore all results except for those looking specifically for block at site of paresthesia?

• Midaz 1-3mg sedation, then 22G 1.5" noninsulated long-bevel needle inserted, looking for paresthesia.
• When paresthesia found, current slowly increased till first visible twitch.
• 0.7 ml/kg lido 1.5% with epi 1:200,000, up to max 60ml
• "Successful block" = surgery within 30min of block without discomfort or any supplementation required.

• 72 blocks: 18 (25%) arterial puncture - excluded. 1 blocked needle (?) ... 53 blocks studied.
• 41 (77%) had twitch <= 0.5mA (median 0.17mA, range 0.03-3.3mA)
• 43 (81%) had twitch in same area as initial paresthesia
• 47 (89%) and 43 (81%) developed complete sensory and motor block, respectively, at site of intial paresthesia. In cases where there was incomplete block at that site, 50% (3/6) had twitch at >0.5mA
• 12/53 required supplementation:
• 7/53 (13%) - supplemental local
• 5/53 (9%) - GA
• 6 of these 12 had surgery in distribution other than initial paresthesia