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A Randomized Comparison of Infraclavicular and Supraclavicular Continuous Peripheral Nerve Blocks for Postoperative Analgesia

Mariano, Edward R. MD, MAS*; Sandhu, NavParkash S. MD*; Loland, Vanessa J. MD*; Bishop, Michael L. MD*; Madison, Sarah J. MD*; Abrams, Reid A. MD; Meunier, Matthew J. MD; Ferguson, Eliza J. BS*; Ilfeld, Brian M. MD, MS*

Regional Anesthesia and Pain Medicine: January-February 2011 - Volume 36 - Issue 1 - p 26-31
doi: 10.1097/AAP.0b013e318203069b
ORIGINAL ARTICLES

Background: Although the efficacy of single-injection supraclavicular nerve blocks is well established, no controlled study of continuous supraclavicular blocks is available, and their relative risks and benefits remain unknown. In contrast, the analgesia provided by continuous infraclavicular nerve blocks has been validated in randomized controlled trials. We therefore compared supraclavicular with infraclavicular perineural local anesthetic infusion following distal upper-extremity surgery.

Methods: Preoperatively, subjects were randomly assigned to receive a brachial plexus perineural catheter in either the infraclavicular or supraclavicular location using an ultrasound-guided nonstimulating catheter technique. Postoperatively, subjects were discharged home with a portable pump (400-mL reservoir) infusing 0.2% ropivacaine (basal rate of 8 mL/hr; 4-mL bolus dose; 30-min lockout interval). Subjects were followed up by telephone on an outpatient basis. The primary outcome was the average pain score on the day after surgery.

Results: Sixty subjects were enrolled, with 31 and 29 randomized to receive an infraclavicular and supraclavicular catheter, respectively. All perineural catheters were successfully placed per protocol. Because of protocol violations and missing data, an intention-to-treat analysis was not used; rather, only subjects with catheters in situ and whom we were able to contact were included in the analyses. The day after surgery, subjects in the infraclavicular group reported average pain as median of 2.0 (10th-90th percentiles, 0.5-6.0) compared with 4.0 (10th-90th percentiles, 0.6-7.7) in the supraclavicular group (P = 0.025). Similarly, least pain scores (numeric rating scale) on postoperative day 1 were lower in the infraclavicular group compared with the supraclavicular group (0.5 [10th-90th percentiles, 0.0-3.5] vs 2.0 [10th-90th percentiles, 0.0-4.7], respectively; P = 0.040). Subjects in the infraclavicular group required less rescue oral analgesic (oxycodone, in milligrams) for breakthrough pain in the 18 to 24 hrs after surgery compared with the supraclavicular group (0.0 [10th-90th percentiles, 0.0-5.0] vs 5.0 [10th-90th percentiles, 0.0-15.0], respectively; P = 0.048). There were no statistically significant differences in other secondary outcomes.

Conclusions: A local anesthetic infusion via an infraclavicular perineural catheter provides superior analgesia compared with a supraclavicular perineural catheter.

From the Departments of *Anesthesiology and †Orthopaedic Surgery, University of California, San Diego, CA.

Accepted for publication September 1, 2010.

Address correspondence to: Brian M. Ilfeld, MD, MS, Department of Anesthesiology, UCSD Center for Pain Medicine, 9300 Campus Point Dr, MC 7651, La Jolla, CA 92037-7651 (e-mail: bilfeld@ucsd.edu).

Funding for this project was provided by National Institutes of Health grant GM077026 (Dr. Ilfeld, principal investigator) from the National Institute of General Medical Sciences (Bethesda, MD); and the Department of Anesthesiology, University of California, San Diego Medical Center (San Diego, CA).

Drs. Mariano, Loland, and Ilfeld as well as Ms. Ferguson have received funding for other research investigations from Arrow International (Reading, Pa) and Stryker Instruments (Kalamazoo, Mich). These companies had absolutely no input into any aspect of the present study conceptualization, design, and implementation; data collection, analysis, and interpretation; or manuscript preparation. Drs. Mariano and Loland conduct continuous peripheral nerve block workshops for Stryker Instruments (Kalamazoo, Mich). None of the other authors has any personal financial interest in this research.

Although single-injection supraclavicular brachial plexus nerve blocks have been demonstrated to provide highly reliable and effective surgical anesthesia and postoperative analgesia, continuous supraclavicular nerve blocks remain relatively uninvestigated. No randomized comparison involving this technique is available within the English-language literature. In contrast, continuous infraclavicular brachial plexus nerve blocks have been validated in controlled trials with1-3 and without4,5 ultrasound (US).

The supraclavicular approach may have anatomic advantages over the infraclavicular approach owing to a more compact arrangement of the brachial plexus,6-9 theoretically allowing for more even and complete local anesthetic spread to all components of the plexus. Application of US has resulted in a renewed interest for supraclavicular brachial plexus blocks, in large part because of the belief that this modality decreases the risk of pneumothorax.10-14 Studies comparing single-injection, US-guided supraclavicular and infraclavicular block techniques differ in their results.15-19

Evidence supporting the use of supraclavicular perineural infusions for postoperative analgesia, with or without the aid of US guidance, is limited.7,20,21 No study, to date, has validated the supraclavicular perineural catheter technique; nor has any compared the postoperative analgesia provided by infraclavicular and supraclavicular perineural catheter infusion techniques in a randomized fashion. We therefore designed this study to compare supraclavicular and infraclavicular perineural local anesthetic infusion after distal upper-extremity surgery.

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METHODS

An institutional review board (University of California, San Diego School of Medicine, San Diego, Calif) approved the protocol and oversaw the study through data analysis. Patients offered enrollment included adults (≥18 years) scheduled for at least moderately painful upper-extremity orthopedic surgery at or distal to the elbow who desired, and were approved for, a brachial plexus perineural infusion for postoperative analgesia. Exclusion criteria included known neuropathy of any etiology in the surgical extremity, pregnancy, incarceration, and an inability to communicate with the investigators and hospital staff.

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Protocol

Following written, informed consent, subjects were randomized to 1 of 2 treatment groups-infraclavicular or supraclavicular-using a computer-generated randomization table based in a secure, password-protected, encrypted central server (www.PAINfRE.com, General Clinical Research Center, Gainesville, Fla). All catheters were placed by an attending physician with extensive experience in both placement techniques or by a regional anesthesia fellow under the direct supervision and guidance of the attending physician.

All subjects had a peripheral intravenous catheter inserted and were placed in the supine position with either the arm abducted 90 degrees at the shoulder (infraclavicular) or at the side (supraclavicular). Standard noninvasive monitors were applied, and oxygen was administered via face mask. Midazolam and fentanyl (intravenous) were titrated for patient comfort, while ensuring that patients remained responsive to verbal cues. The area that would be subsequently covered by the catheter dressing was removed of hair, if necessary. Landmarks were drawn for all subjects, the area cleansed with chlorhexidine gluconate and isopropyl alcohol (ChloraPrep One-Step; Medi-Flex Hospital Products, Inc, Overland Park, Kan), and a clear, sterile, fenestrated drape applied.

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Infraclavicular Technique

With a low-frequency curvilinear array transducer (C11; SonoSite M-Turbo, Bothell, Wash) in a sterile sleeve, the brachial plexus and axillary artery were identified in a transverse cross-sectional (short-axis) view. Once the optimal image of the brachial plexus cords was obtained, a local anesthetic skin wheal was raised cephalad to the US transducer, and the tract toward the target nerves infiltrated with 4 mL of 1% lidocaine. Using an in-plane technique, an 8.9-cm, 17-gauge, Tuohy-tip needle (FlexTip; Arrow International, Reading, Pa) was inserted through the skin wheal in-plane beneath the US transducer and directed caudad toward the plexus. Local anesthetic solution (40 mL, mepivacaine 1.5% with epinephrine 2.5-5.0 µg/mL) was injected in divided doses circumferentially around the brachial plexus cords via the needle using a technique similar to one previously described.2,3,22,23

A 19-gauge flexible epidural-type catheter (FlexTip; Arrow International) was then placed through the needle and advanced 1 to 3 cm beyond the needle tip immediately posterior to the axillary artery. The needle was then removed over the catheter, the injection port affixed, and catheter tip position inferred by injecting 1 mL of air via the catheter under US visualization.3 The catheter was not tunneled further but was dressed and secured with sterile liquid adhesive, a clear occlusive dressing, and an anchoring device (StatLock; Venetec International, San Diego, Calif) to affix the catheter hub to the patient.

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Supraclavicular Technique

With a high-frequency linear array transducer (HFL38; SonoSite M-Turbo) in a sterile sleeve, the brachial plexus and subclavian artery were identified in a transverse cross-sectional (short-axis) view. Once the optimal image of the brachial plexus trunks/divisions was obtained, a local anesthetic skin wheal was raised posterolateral to the US transducer, and the tract toward the target nerves infiltrated with 4 to 5 mL of 1% lidocaine. Using an in-plane technique, an 8.9-cm, 17-gauge, Tuohy-tip needle (FlexTip; Arrow International) was inserted through the skin wheal in-plane beneath the US transducer and directed anteromedially toward the plexus. Local anesthetic solution (40 mL, mepivacaine 1.5% with epinephrine 2.5-5.0 µg/mL) was injected in divided doses around the brachial plexus via the needle, with the final position of the needle tip between the subclavian artery, brachial plexus, and first rib as previously described.14

A 19-gauge flexible epidural-type catheter (FlexTip; Arrow International) was then placed through the needle and advanced 1 to 3 cm beyond the needle tip between the brachial plexus and subclavian artery. The needle was removed over the catheter, the injection port affixed, the catheter tip position inferred, and the catheter secured in the manner presented above.

The brachial plexus block was evaluated every 5 mins for the first 15 mins and determined successful when patients demonstrated muscle weakness upon index finger flexion and a decreased sensation to cold of the palmar aspect of the index finger. Subjects with a successful catheter placement per protocol and nerve block onset were retained in the study.

After surgery, the ropivacaine infusion was initiated using a portable, programmable, disposable, electronic infusion pump (Pain Pump 2 BlockAid; Stryker Instruments, Kalamazoo, Mich). The pumps were programmed by investigators with a basal rate of 8 mL/hr and patient-controlled bolus dose volume of 4 mL with lockout interval of 30 mins (400-mL reservoir volume). Although subjects were not specifically informed of their perineural catheter site, subjects should not be considered masked to treatment group.

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Patient Education

Subjects were discharged home with their infusion pump and perineural catheter in situ. Subjects were instructed on care of the perineural catheter, the infusion pump, and signs and symptoms of local anesthetic toxicity; they were also given contact details for a continuously available health care provider. For breakthrough pain, subjects were instructed to depress the bolus button on their infusion pump, wait 15 mins, and then take 5 to 10 mg of their prescribed oral opioid oxycodone, if necessary.

In the event of a completely insensate finger(s)/hand starting after 9:00 am after surgery, subjects were instructed to pause their infusion device until they regained feeling then restart the infusion. "Completely insensate" was defined as being unable to determine with eyes closed that another individual was touching various parts of the hand/fingers. Subjects were instructed to perform this examination during telephone calls in both the morning and afternoon of postoperative days (PODs) 1 to 3. They were also encouraged to perform the examination throughout the infusion period, beginning the morning of POD 1.

Subjects were contacted by a health care provider, per standard institutional routine, beginning the night of surgery, and each afternoon thereafter through POD 3. Patients were questioned about symptoms of local anesthetic toxicity, catheter migration, and leakage; gross sensory and motor function; and the appearance of the catheter site. In the afternoon of POD 2, subjects' caretakers removed the catheters with a physician in telephone contact. The presence of a black catheter tip confirmed complete removal.

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Measurements

Subjects were contacted by telephone on PODs 0 to 3 by a health care provider (unmasked to treatment group). The primary outcome was the average pain numeric rating scale (NRS) reported on POD 1. Pain severity was evaluated using a verbal NRS of 0 to 10, with 0 equal to no pain and 10 being the worst imaginable pain. Satisfaction with postoperative analgesia was as reported on POD 2 using a 0- to 10-point scale (0 = "very unsatisfied" and 10 = "very satisfied").

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Statistical Analysis

Average NRS on POD 1 was the primary outcome variable. Using expected NRS values for infraclavicular catheter subjects of mean = 3.4 and SD = 2.9 and an SD = 2.0 for supraclavicular catheter subjects,2 25 subjects in each treatment arm were required to detect a difference between treatment group means of 2 (2-sided type I error protection of 0.05 and power = 80%), and we included 30 subjects in each treatment arm to account for possible increased SD variability and dropouts.

Normality of distribution was determined using the Kolmogorov-Smirnov test (NCSS-PASS Statistical Software, Kaysville, Utah). Continuous, normally distributed data are reported as mean (SD). Categorical data are reported as median (percentiles or percentages, when appropriate). For normally distributed data, single comparisons were performed using Student t test. For continuous data in distributions other than normal, the Mann-Whitney U test was used. A χ2 test or Fisher exact test (n < 5 in any field) was used for comparisons of categorical data. A 2-sided P < 0.05 was considered statistically significant for the primary outcome. Statistically significant findings in secondary outcomes should be interpreted as suggestive, requiring confirmation in a future trial before considering them as definitive.

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RESULTS

Sixty patients were enrolled and randomized to receive either an infraclavicular (n = 31) or supraclavicular (n = 29) brachial plexus perineural catheter. The groups were comparable in anthropometric and surgical characteristics (Tables 1 and 2). All perineural catheters were successfully placed per protocol. Seven and 5 of the subjects with infraclavicular and supraclavicular catheters, respectively, removed the catheters by the end of POD 1 for various reasons: an insensate extremity (n = 3, 2); discomfort at the catheter site (n = 1, 0); pump not functioning (n = 1, 1); catheter site leakage (n = 1, 1); patient not following provider guidance (n = 1, 1). Because of the large number of protocol violations and missing data due to an inability to contact subjects on various days, we elected not to use intention-to-treat analysis; rather, only subjects with perineural catheters in situ whom we could contact were included in the daily analyses (Table 3).

TABLE 1

TABLE 1

TABLE 2

TABLE 2

TABLE 3

TABLE 3

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Primary Outcome

On POD 1, subjects in the infraclavicular group reported average pain as median of as 2.0 (10th-90th percentiles, 0.5-6.0) compared with 4.0 (10th-90th percentiles, 0.6-7.7) in the supraclavicular group (P = 0.025; Fig. 1).

FIGURE 1

FIGURE 1

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Secondary Outcomes

Least pain scores (NRS) on POD 1 were lower in the infraclavicular group compared with the supraclavicular group (0.5 [10th-90th percentiles, 0.0-3.5] vs 2.0 [10th-90th percentiles, 0.0-4.7], respectively; P = 0.040). Subjects in the infraclavicular group required less oxycodone (in milligrams) for breakthrough pain on the day of surgery compared with the supraclavicular group (0.0 [10th-90th percentiles, 0.0-5.0] vs 5.0 [10th-90th percentiles, 0.0-15.0], respectively; P = 0.048). There were no statistically significant differences in other secondary outcomes (Tables 3 and 4 and Figs. 2 and 3).

FIGURE 2

FIGURE 2

FIGURE 3

FIGURE 3

TABLE 4

TABLE 4

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DISCUSSION

This is the first randomized study to directly compare US-guided infraclavicular and supraclavicular brachial plexus perineural catheters and postoperative local anesthetic infusion. Our results provide evidence that infraclavicular catheters provide superior analgesia compared with supraclavicular catheters when an insertion technique relying exclusively on US guidance is used. Pain scores between treatment groups differed to a statistically significant degree only on POD 1, even though the perineural catheters remained in situ until the afternoon of POD 2. The reason for this is most probably the infusion pump reservoir volume of 400 mL. At a basal rate of 8 mL/hr and patient-controlled boluses of 4 mL (30-min lockout), most subjects probably exhausted their local anesthetic reservoirs early in the morning on POD 2. Therefore, by the time data were collected much later in the day, there was no difference in treatments between the 2 groups.

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Catheter Placement Time

The time required to perform US-guided infraclavicular perineural catheter insertion was shorter in the present study than previously reported.2 The median time savings of 2 mins gained by infraclavicular over supraclavicular-although statistically significant-is probably not clinically-relevant. It is worth noting that all infraclavicular catheters were placed in 8 mins or less, whereas all supraclavicular catheters were placed in less than 11 mins. Setup time was not included in our procedural time measurement because US equipment and regional anesthesia supplies are readily available at all times in our regional anesthesia induction area ("block room"). Because we use the same US and perineural catheter equipment for both techniques, we do not anticipate a difference in equipment preparation between the 2 techniques.

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Catheter Placement Success

Ultrasound-guided infraclavicular catheter placement has been established in large case series3,23 and randomized clinical trials,1,2 and compared with a stimulating catheter technique, US-guided infraclavicular catheters can be placed in less time with high success rates and effective analgesia.2 In contrast, the various methods for supraclavicular perineural catheter insertion have not been previously investigated in controlled studies.

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Onset Time

Although authors have previously theorized about advantages in favor of the supraclavicular approach due to the compact nature of the brachial plexus in this location,6 our study fails to demonstrate faster onset of complete sensory or motor block compared with the infraclavicular technique when using US guidance. Perhaps a 5-min testing interval is too long to differentiate onset time between these 2 techniques, if a difference truly exists. Other studies of single-injection, US-guided supraclavicular and infraclavicular blocks have similarly failed to demonstrate a faster onset time for the supraclavicular approach.15,16

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Study Limitations

Subjects and investigators were not masked to treatment group assignment, although study subjects were unlikely to have developed a preference for 1 insertion site versus another. To minimize potential bias, procedures were performed using a nearly identical protocol with the same perineural catheter equipment. In addition, the initial local anesthetic bolus to provide surgical anesthesia was injected via the needle followed by catheter insertion. Therefore, catheter tip placement immediately adjacent to the brachial plexus was inferred, as opposed to demonstrated with a local anesthetic bolus via the catheter with a resulting nerve block in the expected distribution. Unfortunately, multiple catheters were prematurely removed, and an unusually large number of subjects could not be contacted every day of the infusion period for data collection. Without the use of an intent-to-treat analysis, the risk of bias increases due to the unexpectedly and atypically high amount of missing data. We have therefore provided the actual number of subjects included in each daily analysis for the 2 treatment groups (Table 3).

That surgical duration was prolonged in the infraclavicular group compared with the supraclavicular group mostly likely occurred by chance. Similarly, the supraclavicular group was randomly assigned more subjects who underwent radius or ulna open reduction-internal fixation compared with the infraclavicular group. Although these chance imbalances may have influenced the results of this study, the magnitude of this impact, if any, is unknown.

Our results pertain only to the techniques and equipment used in this investigation. The use of other infraclavicular4,5,24 or supraclavicular7,25 perineural catheter techniques would have undoubtedly influenced the result.

In summary, results of this randomized clinical trial suggest that US-guided infraclavicular perineural catheters provide superior analgesia compared with supraclavicular catheters using the described techniques in this study.

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ACKNOWLEDGMENTS

The authors thank the entire operating and recovery room staff at the University of California San Diego, Hillcrest (San Diego, Calif), for their the invaluable assistance.

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REFERENCES

1. Dhir S, Ganapathy S. Comparative evaluation of ultrasound-guided continuous infraclavicular brachial plexus block with stimulating catheter and traditional technique: a prospective-randomized trial. Acta Anaesthesiol Scand. 2008;52:1158-1166.
2. Mariano ER, Loland VJ, Bellars RH, et al. Ultrasound guidance versus electrical stimulation for infraclavicular brachial plexus perineural catheter insertion. J Ultrasound Med. 2009;28:1211-1218.
3. Sandhu NS, Capan LM. Ultrasound-guided infraclavicular brachial plexus block. Br J Anaesth. 2002;89:254-259.
4. Ilfeld BM, Morey TE, Enneking FK. Continuous infraclavicular brachial plexus block for postoperative pain control at home: a randomized, double-blinded, placebo-controlled study. Anesthesiology. 2002;96:1297-1304.
5. Ilfeld BM, Morey TE, Enneking FK. Infraclavicular perineural local anesthetic infusion: a comparison of three dosing regimens for postoperative analgesia. Anesthesiology. 2004;100:395-402.
6. Brown DL, Cahill DR, Bridenbaugh LD. Supraclavicular nerve block: anatomic analysis of a method to prevent pneumothorax. Anesth Analg. 1993;76:530-534.
7. Cornish PB. Supraclavicular regional anaesthesia revisited-the bent needle technique. Anaesth Intensive Care. 2000;28:676-679.
8. Klaastad O, VadeBoncouer TR, Tillung T, Smedby O. An evaluation of the supraclavicular plumb-bob technique for brachial plexus block by magnetic resonance imaging. Anesth Analg. 2003;96:862-867.
9. Neal JM, Gerancher JC, Hebl JR, et al. Upper extremity regional anesthesia: essentials of our current understanding, 2008. Reg Anesth Pain Med. 2009;34:134-170.
10. Chan VW, Perlas A, Rawson R, Odukoya O. Ultrasound-guided supraclavicular brachial plexus block. Anesth Analg. 2003;97:1514-1517.
11. Perlas A, Lobo G, Lo N, Brull R, Chan VW, Karkhanis R. Ultrasound-guided supraclavicular block: outcome of 510 consecutive cases. Reg Anesth Pain Med. 2009;34:171-176.
12. Renes SH, Spoormans HH, Gielen MJ, Rettig HC, van Geffen GJ. Hemidiaphragmatic paresis can be avoided in ultrasound-guided supraclavicular brachial plexus block. Reg Anesth Pain Med. 2009;34:595-599.
13. Tran de QH, Munoz L, Zaouter C, Russo G, Finlayson RJ. A prospective, randomized comparison between single- and double-injection, ultrasound-guided supraclavicular brachial plexus block. Reg Anesth Pain Med. 2009;34:420-424.
14. Soares LG, Brull R, Lai J, Chan VW. Eight ball, corner pocket: the optimal needle position for ultrasound-guided supraclavicular block. Reg Anesth Pain Med. 2007;32:94-95.
15. Koscielniak-Nielsen ZJ, Frederiksen BS, Rasmussen H, Hesselbjerg L. A comparison of ultrasound-guided supraclavicular and infraclavicular blocks for upper extremity surgery. Acta Anaesthesiol Scand. 2009;53:620-626.
16. Fredrickson MJ, Patel A, Young S, Chinchanwala S. Speed of onset of 'corner pocket supraclavicular' and infraclavicular ultrasound guided brachial plexus block: a randomised observer-blinded comparison. Anaesthesia. 2009;64:738-744.
17. Arcand G, Williams SR, Chouinard P, et al. Ultrasound-guided infraclavicular versus supraclavicular block. Anesth Analg. 2005;101:886-890.
18. De Jose Maria B, Banus E, Navarro Egea M, Serrano S, Perello M, Mabrok M. Ultrasound-guided supraclavicular vs infraclavicular brachial plexus blocks in children. Paediatr Anaesth. 2008;18:838-844.
19. Tran de QH, Russo G, Munoz L, Zaouter C, Finlayson RJ. A prospective, randomized comparison between ultrasound-guided supraclavicular, infraclavicular, and axillary brachial plexus blocks. Reg Anesth Pain Med. 2009;34:366-371.
20. Cornish PB, Leaper CJ, Nelson G, Anstis F, McQuillan C, Stienstra R. Avoidance of phrenic nerve paresis during continuous supraclavicular regional anaesthesia. Anaesthesia. 2007;62:354-358.
21. Plunkett AR, Brown DS, Rogers JM, Buckenmaier CC 3rd. Supraclavicular continuous peripheral nerve block in a wounded soldier: when ultrasound is the only option. Br J Anaesth. 2006;97:715-717.
22. Sandhu NS, Bahniwal CS, Capan LM. Feasibility of an infraclavicular block with a reduced volume of lidocaine with sonographic guidance. J Ultrasound Med. 2006;25:51-56.
23. Sandhu NS, Manne JS, Medabalmi PK, Capan LM. Sonographically guided infraclavicular brachial plexus block in adults: a retrospective analysis of 1146 cases. J Ultrasound Med. 2006;25:1555-1561.
24. Dhir S, Ganapathy S. Use of ultrasound guidance and contrast enhancement: a study of continuous infraclavicular brachial plexus approach. Acta Anaesthesiol Scand. 2008;52:338-342.
25. Pham-Dang C, Gunst JP, Gouin F, et al. A novel supraclavicular approach to brachial plexus block. Anesth Analg. 1997;85:111-116.
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