What is the Relationship Between Paresthesia and Nerve Stimulation for Axillary Brachial Plexus Block?

Abstract BACKGROUND AND OBJECTIVES: To quantify the motor threshold current of a needle following elicitation of paresthesia during axillary brachial plexus block (ABPB). METHODS: This is a prospective, observational study of ABPB in 72 patients.
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   Original Articles What Is the Relationship Between Paresthesiaand Nerve Stimulation for Axillary BrachialPlexus Block? Andrew Choyce, F.R.C.A., Vincent W.S. Chan, F.R.C.P.C.,William J. Middleton, F.R.C.P.C., Paul R. Knight, F.R.C.A.,Philip Peng, F.R.C.P.C., and Colin J.L. McCartney, F.R.C.A., F.E.A.R.C.S.I. Background and Objectives: To quantify the motor threshold current of a needle following elicitation ofparesthesia during axillary brachial plexus block (ABPB). Methods: This is a prospective, observational study of ABPB in 72 patients. Having elicited paresthesia, theminimum current required to produce a motor response was noted. The development and success of the blockwere subsequently followed. Results: Nineteen blocks were excluded (18 because of arterial puncture and 1 blocked needle). Of theremaining 53 blocks, 41 (77%) produced a motor response at 0.5 mA or less. The median current was 0.17 mA(range, 0.03 to 3.3 mA). The site of initial paresthesia and subsequent motor response were related in 43 (81%)of cases. Conclusions: A needle position causing paresthesia produced a motor response at 0.5 mA or less in 77% ofcases studied. This current may, therefore, be a reasonable threshold to aim for when performing an ABPB. Reg Anesth Pain Med 2001;26:100-104. Key Words: Axillary brachial plexus block, Paresthesia, Peripheral nerve stimulator. S uccessful axillary brachial plexus block (ABPB)depends on needle placement near the neuro-vascular compartment. In 1961, De Jong 1 advo-cated paresthesia or aspiration of blood as evidenceof proximity to the plexus. Paresthesia, as a subjec-tive endpoint, is one method for performing ABPB.How close a needle needs to be to a nerve to pro-duce a paresthesia is not known.Some practitioners believe that actively seekingparesthesia may increase the risk of postblockneurological sequelae, 2,3 although this remainsspeculative. 4,5 The peripheral nerve stimulator(PNS) as an alternative technique for nerve local-ization was described in the 1950s 6,7 and its usehas increased with improved technology. ThePNS provides the anesthesiologist with anothermethod of determining the proximity of a needleto a nerve. Accepting a motor response  0.5 mAis often considered the standard for using thisdevice. 8,9 No data have definitively determined the maxi-mum amperage associated with a successful block.Whether 0.5 mA is the optimal threshold current isnot known. This level of current was first estab-lished while studying the accuracy of differentmethods of nerve location. 10 The choice of rheobase(the minimum current required to stimulate anerve with a long pulse width) at  0.5 mA was based on the investigators’ experience stimulatingthe obturator nerve. 10 There are few data on the relationship betweenthe subjective sensory endpoint (paresthesia) and See Editorial page 97 From the Department of Anesthesia, University of Toronto,University Health Network, Toronto Western Hospital, Toronto,Ontario, Canada.Accepted for publication November 13, 2000.Reprint requests: Vincent W.S. Chan, F.R.C.P.C., Departmentof Anesthesia, University of Toronto, University Health Network,Toronto Western Hospital, 399 Bathurst St, Toronto M5T 2S8,Ontario, Canada. E-mail: © 2001 by the American Society of Regional Anesthesia andPain Medicine.1098-7339/01/2602-0002$35.00/0doi:10.1053/rapm.2001.21740 100 Regional Anesthesia and Pain Medicine, Vol 26, No 2 (March–April), 2001: pp 100–104   the objective motor endpoint produced by a PNS. 11 For example, it is not known whether a needleposition associated with a paresthesia will consis-tently produce a muscle twitch at  0.5 mA whenthe needle is stimulated.The aim of this study is to test the hypothesis thata needle position producing paresthesia will consis-tently produce a motor twitch at  0.5 mA whenstimulated. Methods Our institutional ethics committee approved thisstudy protocol. Seventy-two American Society ofAnesthesiologists (ASA) class I-III patients sched-uled to undergo elective upper limb surgery underaxillary block gave written informed consent to par-ticipate. Patients were excluded from the study iftherewasahistoryoflidocaineallergy,neurologicaldeficit in the operative limb, or a communication barrier. Staff, fellows, or supervised residents fromthe regional anesthesia program in our hospital per-formed all blocks. Patients were told to immediatelyreport any sensation below the elbow that was “likean electric shock,” “pins and needles,” or “hittingyour funny-bone,” and then to describe the exactlocation: e.g., dorsal/volar, medial/lateral, or whichdigit.Before performing the axillary block, an intrave-nous catheter was placed in the contralateral limband monitors applied (electrocardiograph, nonin-vasive blood pressure, and pulse oximeter). Withthe patient supine, midazolam 1 to 3 mg was ad-ministered to achieve anxiolysis and mild sedation.On the operative side, the shoulder was abductedand the elbow flexed to 90°. The axilla was thenprepared using an aseptic technique and the axil-lary artery palpated. A subcutaneous injection of 1mL of 2% lidocaine provided dermal anesthesia.A 22-gauge, PrecisionGlide 1.5-in, noninsulated,long-bevel needle (Becton Dickinson, FranklinLakes, NJ) was then introduced through the anes-thetized skin. Attached to the shaft of the needle,via a crocodile clip, was the cathode of a StimuplexNerve Stimulator (Braun Medical Inc, Bethlehem,PA). The anode was attached to the patient via acutaneous electrocardiogram electrode and the de-vice remained off while paresthesia was sought inthe site of the proposed surgery. However, if pares-thesia in a different distribution was reported first,this was accepted and the site recorded. The needlewas then held firmly as the nerve stimulator wasstarted (frequency, 1 Hz; pulse width, 100 ms). Theoutput of the device was slowly increased (to amaximum of 5 mA) until the first visible muscletwitch was seen; the current and site of motor ac-tivity were noted. Thereafter, all patients received astandard dose of 0.7 mL/kg of 1.5% lidocaine with1 in 200,000 epinephrine up to a maximum of 60mL (900 mg lidocaine). Any intensification of par-esthesia or acute pain after injection of 1 to 2 mL ofsolution resulted in the immediate cessation of theinjection. If accidental arterial puncture occurred,the needle was further advanced through the pos-terior wall of the artery. Again, the PNS was at-tached and any motor activity that occurred up to 5mA was noted before completing the block via thetransarterial approach.Before performing the block, baseline motor andsensory (pinprick) function were recorded and,subsequently, the development of the block wasfollowed at 5-minute intervals until the patient en-tered the operating room. A simple 3-point scalewas used to assess sensory and motor block: com-plete, partial, or absent. Sensation to pinprick wastested with a 23-gauge hypodermic needle in thedistribution of median, radial, ulna, and musculo-cutaneous nerves. In the operating room, no fur-ther sedation was given and the need for any sup-plemental anesthesia was noted. We defined asuccessful block as one that allowed the surgery toproceed within a 30-minute time period, withoutdiscomfort to the patient or need for any supple-mental techniques.The patients were encouraged to report any un-usual symptoms throughout the performance of the block and during surgery. All patients were tele-phoned at 2 weeks by a blinded observer and askedstandardized questions regarding complicationsthat might be attributable to the block.Demographic data are expressed as mean  stan-dard deviation. Data not fitting a Gaussian distribu-tion are expressed as median, range, and interquar-tile range. Data were analyzed using SPSS forWindows Version 10 (SPSS Inc, Chicago, IL). Sta-tistical significance was accepted at P   .05. Results Seventy-two patients were recruited to the study.The mean age was 46.2  19.1 years. The meanheight and weight were 169.4  12.2 cm and76.4  16.0 kg, respectively. Of the 72 blocks, 18(25%) resulted in accidental arterial puncture andwere not included in the main analysis, 1 blockcould not be completed because of a blocked nee-dle, and in the remaining 53, paresthesia was elic-ited and the block proceeded as planned. No injec-tion resulted in intense pain suggestive ofintraneural injection.After paresthesia, 77% (41 of 53) of needle place-ments produced motor activity at or below 0.5 mA. Paresthesia v  Nerve Stimulation • Choyce et al. 101  The median value was 0.17 mA and the range 0.03to 3.3 mA. Figure 1 shows the distribution of thismotor threshold current.Table 1 shows the site of paresthesia and whetherthe subsequent motor threshold current was aboveor below 0.5 mA. Four (8%) paresthesias were inmore than a single nerve’s distribution. The site ofinitial paresthesia matched the site of subsequentmotor response in 81% (43 of 53) of cases.Looking at the site of initial paresthesia, 89% (47of 53) developed complete sensory block and 81%(43 of 53) complete motor block, in the same dis-tribution. Interestingly, where the site of paresthe-sia was followed by incomplete block in the samedistribution, 50% (3 of 6) of cases had a motorthreshold current  0.5 mA.Thirteen percent (7 of 53) of blocks requiredsome supplemental local anesthetic (local infiltra-tion or peripheral nerve block) and a further 9% (5of 53) resulted in inadequate block over surgical siterequiring general anesthesia. Of these 12 patientsrequiring some supplementation, 50% (6 of 12)had surgery in a nerve distribution different thanthe initial paresthesia. Also, there was no significantdifference in the number having complete sensory block in 3 or more nerves (radial, median, ulnar, ormusculocutaneous) irrespective of the currentthreshold.There were 18 accidental arterial punctures. Ofthese, 6 of 18 (33%) failed to produce any motorresponse at up to 5 mA. In the remaining 12 (67%),the associated motor threshold current was be-tween 0.5 and 5 mA (median, 3.5 mA).Other untoward effects were minor: 1 episode ofself-limiting tachycardia and 1 of minor dizzinessduring the injection of lidocaine. Two (3%) patientsreported minor persistent symptoms in the hand at2-week follow-up, neither of which was in the dis-tribution of the initial paresthesia. One had a me-dian paresthesia, a motor threshold of 1.6 mA, andsurgery in the ulnar area of the forearm. The blockwas successful and at 2-week follow-up the patientreported numbness in the ulnar area. In the secondpatient, there was radial paresthesia with a motorthreshold of 0.5 mA and surgery on the middlefinger. Again the block was successful, and at 2weeks the patient reported minor tingling in thepalm of the hand. Discussion In the present study we sought to relate elicita-tion of paresthesia with level of motor thresholdcurrent required to produce a positive motor re-sponse. When using an uninsulated needle, wefound that in 77% of cases this was  0.5 mA. Ifparesthesia is the “gold standard” for placing a nee-dle near a nerve before local anesthetic injection,the threshold current of  0.5 mA also appears to bea clinically appropriate endpoint to rely on.An uninsulated needle may perform differentlythan an insulated needle, which is most commonlyused with a PNS. In the latter, the current densitywill be maximal at the tip and be spherically ar-ranged around this point. 12 With an uninsulatedneedle, the current density will be maximal justproximal to the tip and will extend along the shaft Table 1. Site of Paresthesia and Subsequent CurrentRequired to Produce a Motor Response Site of ParesthesiaMotor ThresholdCurrent (mA)  0.5  0.5Median 11 5Radial 13 2Ulna 13 4Musculocutaneous 1 0Median/ulna 1 1Ulna/musculocutaneous 1 0Medial/radial 1 0Total 41 12 Fig 1. Motor threshold current data points (n  53) areplotted, and a box and whisker plot shows the distribu-tion. The bold line and the box represent the median plusthe interquartile range (IQR), respectively. The whiskerrepresents 2 times the IQR. 102 Regional Anesthesia and Pain Medicine Vol. 26 No. 2 March–April 2001  of the needle. 12 What this difference means in prac-tice is that an insulated needle requires slightly lesscurrent to stimulate because of a higher currentdensity at the tip, and stimulation from the shaft ofan uninsulated needle may produce false localiza-tion of nerves. We chose to use an uninsulatedneedle in this investigation because our experienceindicates that paresthesia are difficult to elicit usinga short-beveled, insulated needle. Had we found aninsulated needle practical in this study, we mayhave demonstrated an even lower motor thresholdcurrent.In the remaining 23% (12 of 53) of cases, pares-thesia were associated with a subsequent motorthreshold current  0.5 mA. The reason for this isnot immediately obvious. We can hypothesize thatthisisduetotheneedleabuttingamixedmotorandsensory nerve where the motor component is moredistal to the probing needle than the sensory com-ponent. Further interpatient variability may arisefrom anatomical differences in the epineurium ofthe nerves or, alternatively, paresthesia may resultfrom something other than the proximity of a nee-dle to a nerve. A simpler explanation would beoperator/patient movement dislodging the needle.However, our experiences are not unique. Urmeyet al. 11 found that only 25% of needle positionseliciting paresthesia produce a motor responsewhen stimulated at 1.0 mA. This study used a com- bination of insulated and uninsulated needles in aninterscalene approach to the brachial plexus. In thepresent study, 89% (47 of 53) of paresthesias wereassociated with a motor threshold current in the 0to 1.0 mA range. It is apparent that the functioningof a PNS is not fully understood. Whether suchdevices are safer than a paresthesia technique or ifthey will protect against unintentional intraneuralinjection is not known, and this warrants furtherinvestigation.The safety of placing regional blocks in anesthe-tized patients remains controversial. Reliable evi-dence of paresthesia is only available in the awakeor lightly sedated patient. We have shown that in aminority of patients (23%), motor response at  0.5mA will not be present despite the needle beingnear the nerve. Presumably, if motor response at  0.5 mA was being sought in the anesthetizedpatient under these circumstances, this currentthreshold may not protect against unintentionalintraneuronal injection. We are in agreement withother investigators that placement of blocks with aPNS in heavily sedated or anesthetized adults may be unsafe. 8,11,13 It is our practice to use a single injection of high-volume local anesthetic as recommended by Win-nie 14 and others 8 in the belief that this will reducepostanesthetic neuropathy by avoiding multiple in- jections. We searched for paresthesia in the distri- bution of the intended surgery, but the first pares-thesia elicited was accepted regardless of site and asingle large volume of local anesthetic injected. Aswith any regional anesthetic technique, we expect acertain failure rate. In the present study, 89% (47 of53) of sensory blocks were complete in the samedistribution as the paresthesia, suggesting thatmatching paresthesia with site of surgery is prefer-able; however, in 19% (10 of 53) of cases the site ofparesthesia and of subsequent motor response wereunrelated. In 23% (12 of 53) of our cases someform of supplementation was used. Numbers stud-ied were too small for meaningful statistical analysisof motor threshold current compared with need forsupplementation (when the threshold was  0.5mA the rate was actually higher [11 of 41] thanwhen the threshold was  0.5 mA [1 of 12]). Bara-nowski and Pither 15 reported a success rate of 82%for paresthesia and 72% for nerve stimulationwhen searching for between 1 and 3 nerves in the brachial plexus. Goldberg et al. 16 compared thetechniques of single-nerve paresthesia, PNS, andtransarterial approach and found the success ratefor each to be 79% to 80%.In conclusion, we have shown that paresthesiawas related to a motor response at  0.5 mA in 77%of cases and, therefore, this would appear to be areasonable clinical threshold to choose for brachialplexus blocks in the awake or lightly sedated pa-tient. That paresthesia are coupled with a motorthreshold  0.5 mA in the remainder has led us tocontinue our practice of not performing regional blocks in heavily sedated or anesthetized adults.Finally, lack of complete overlap between the tech-niques of paresthesia and nerve stimulation supportthe practice of accepting whichever endpoint occursfirst rather than striving to achieve only one end-point. Acknowledgment We thank Dr. Hossam El Beheiry for his advice inpreparing this manuscript. References 1. De Jong RH. Axillary block of the brachial plexus.  Anesthesiology 1961;22:215-225.2. Selander D, Edshage S, Wolff T. Paresthesiae or noparesthesiae? Acta Anaesthesiol Scand  1979;23:27-33.3. Plevak DJ, Linstromberg JW, Danielson DR. Pares-thesia vs nonparesthesia—The axillary block [Ab-stract]. Anesthesiology 1983;59:A216.4. Winchell SW, Wolfe R. The incidence of neuropathyfollowing upper extremity nerve blocks. Reg AnesthPain Med  1985;10:12-15. Paresthesia v  Nerve Stimulation • Choyce et al. 103  5. Moore DC, Mulroy MF, Thompson GE. Peripheralnerve damage and regional anaesthesia. Br J Anaesth 1994;73:435-436.6. Bonica JJ. Management of intractable pain with an-algesic blocks. JAMA 1952;150:1581.7. Pearson RB. Nerve block in rehabilitation: A tech-nique of needle localization. Arch Phys Med Rehab 1955;631-633.8. Gentili ME, Wargnier JP. Letter to the Editor. Br J  Anaesth 1993;70:94.9. Cousins MJ, Bridenbaugh PO. Neural Blockade in Clin-ical Anesthesia and Management of Pain, 3rd ed. Phila-delphia, PA: Lippincott-Raven; 1998:188.10. Magora F, Rozin R, Ben-Menacin Y, Magora A. Ob-turator nerve block: An evaluation of technique. Br J  Anaesth 1969;41:695-698.11. Urmey WF, Stanton J, O’Brien S, Tagariello V, Wick-iewicz TL. Inability to consistently elicit a motor re-sponse following sensory paraesthesia during inter-scalene block administration. Reg Anesth 1998(suppl);23:7.12. Pither CE, Raj PP, Ford DJ. The use of peripheralnerve stimulators for regional anesthesia. A review ofexperimental characteristics, technique, and clinicalapplications. Reg Anesth 1985;10:49-58.13. Selander D. Axillary plexus block: Paresthetic orperivascular. Anesthesiology 1987;66:726-728.14. WinnieAP. Plexus Anesthesia: 1. Perivascular Techniques of Brachial Plexus Block. Philadelphia, PA: Saunders; 1983.15. Baranowski AP, Pither CE. A comparison of threemethods of axillary plexus anaesthesia. Anaesthesia 1990;45:362-365.16. Goldberg ME, Gregg C, Larijani GE, Norris MC, MarrAT, Seltzer JL. A comparison of three methods ofaxillary approach to brachial plexus blockade for up-per extremity surgery. Anesthesiology 1987;66:814-816. 104 Regional Anesthesia and Pain Medicine Vol. 26 No. 2 March–April 2001
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