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The Minimum Effective Anesthetic Volume of 0.75% Ropivacaine in Ultrasound-Guided Interscalene Brachial Plexus Block

Gautier, Philippe MD*; Vandepitte, Catherine MD*; Ramquet, Caroline MD*; DeCoopman, Mieke MD*; Xu, Daquan MD; Hadzic, Admir MD

doi: 10.1213/ANE.0b013e31822b876f
Analgesia: Research Reports
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BACKGROUND: The use of ultrasound to monitor needle placement and spread of local anesthetics (LA) has allowed reductions in the volume of LA required to anesthetize peripheral nerves. In the current study we investigated the minimal volume necessary to accomplish surgical anesthesia with interscalene brachial plexus block.

METHODS: Twenty ASA physical status I–III patients, ages 18 to 75 years and scheduled for shoulder surgery under interscalene brachial plexus block, were enrolled. Using a previously validated step-up/step-down method, we determined the injection volume of 0.75% ropivacaine used for consecutive patients by the outcome of the preceding block. The starting volume was 15 mL (3 injections of 5 mL per each trunk); in the case of block failure, the volume was increased by 1 mL, whereas after successful block, the volume was reduced by 1 mL. The study was stopped upon achieving the secondary stopping rule of 10 consecutive successful interscalane blocks using 5 mL of ropivacaine 0.75%. Successful surgical anesthesia with the brachial plexus block was defined as presence of adequate motor block (motor score of ≤2 on 0 to 4 scale), absent sensation to cold and pinprick sensation within 30 minutes of injection, and absence of the need for general anesthesia for completion of surgery. Duration of sensory blockade was assessed by asking the patient to record the time of first pain sensation.

RESULTS: Under our study conditions, successful surgical anesthesia for arthroscopic shoulder surgery can be achieved with 5 mL of 0.75% ropivacaine, or approximately 1.7 mL per each of the 3 trunks of the brachial plexus (superior, middle, and inferior). The study was stopped after 10 consecutive successful blocks with 5 mL of LA (100%, 95% confidence interval [CI]: 74.1%–100%). For the group as a whole, the median (range) sensory block onset time was 5 (5–20) minutes, the median (range) motor block for the biceps was 7.5 (5–15) minutes, and for abduction 10 (5–15) minutes. The median (range) block duration was 9.9 (5–19) hours, and the mean (SD) block performance time was 8.0 ± 3.2 minutes. Mean duration of analgesia was 9.9 ± 3.7 hours. Duration of analgesia was not associated with volume of LA (r = 0.05, P = 0.83).

CONCLUSIONS: All patients in our study had successful surgical blocks with 5 mL of LA. However, the lower limit of the CI (calculated on the assumption of a single failure) does include the possibility of a 25% failure rate; thus studies using similar stopping rules for doses higher than 5 mL are nonetheless warranted.

Published ahead of print August 4, 2011 Supplemental Digital Content is available in the text.

From the *Department of Anesthesiology, Clinique Ste Anne-St, Remi, Brussels, Belgium; and Department of Anesthesiology, St. Luke's–Roosevelt Hospital Center, University Hospital of Columbia University, College of Physicians and Surgeons, New York, New York.

Funding: The cost of this work was supported by the Department of Anesthesiology of the Clinique Ste Anne-St Rémi.

The authors declare no conflict of interest.

Reprints will not be available from the authors.

Address correspondence to Philippe E. Gautier, MD, Department of Anesthesiology, Clinique Ste Anne-St, Remi, Brussels, Boulevard J. Graindor, 66, 1070-Brussels, Belgium. Address e-mail to P.gautier@skynet.be.

Accepted June 13, 2011

Published ahead of print August 4, 2011

Before the introduction of ultrasound guidance for brachial plexus blockade, larger volumes of local anesthetics (LA) were used to improve the success rate of interscalene brachial plexus block. In his classic publication, Winnie taught that 40 mL of LA was necessary to anesthetize the brachial plexus.1 Other investigators have reported using as much as 70 mL of LA for brachial plexus block.24 Despite continuous efforts to improve block technique, 30 to 50 mL of LA remains in common clinical use.5,6 The use of ultrasound to visualize the spread of the LA has facilitated the use of low-dose axillary block for hand surgery.7 Needle placement and spread of the injectate as guided by ultrasound reduces the amount of LA needed for successful peripheral nerve block (PNB).8,9 We undertook the current study to investigate a clinicially relevant minimal volume of 0.75% ropivacaine required for successful surgical anesthesia with interscalene brachial plexus. We hypothesized that interscalene brachial plexus block can be accomplished with a smaller volume of LA than commonly suggested (<15 mL).

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METHODS

After obtaining institutional ethics committee approval and written informed patient consent, 20 ASA physical status 1 to 3 patients, ages 18 to 75 years and scheduled for shoulder surgery under interscalene brachial plexus block, were prospectively enrolled. Patients who presented with contraindications to interscalene block or were pregnant were excluded. The starting dose of 15 mL of 0.75% ropivacaine was chosen on the basis of our institutional clinical experience. Using a previously validated step-up/step-down method, we determined the injection volume used for consecutive patients by the outcome of the preceding block. In the case of block failure, the injection volume was increased by 1 mL; conversely, for block success, the volume was reduced by 1 mL.10

All blocks were performed in the induction room by 2 anesthesiologists experienced in ultrasound-guided PNBs (PG and CV). Intravenous access was established in the nonoperative upper limb, and standard monitoring was applied (noninvasive arterial blood pressure, electrocardiography, and pulse oximetry). Patients were premedicated with 2 mg of midazolam administered IV. Needle placement and injection of LA was guided by ultrasound and nerve stimulation. For ultrasound, an Aplio XG (Toshiba, Inc.) ultrasound machine was equipped with a 38-mm high-frequency (12 MHz) linear array transducer. Nerve stimulation was accomplished by a commercially available nerve stimulator (HNS 12, B. Braun Melsungen AG, Melsungen, Germany). The superior, middle, and inferior trunks of the brachial plexus were identified approximately 2 cm above the clavicle. A 50-mm 22-gauge needle (Stimuplex®, B. Braun) was introduced percutaneously using an out-of-plane technique. The needle was placed beside each trunk in succession, and the LA was injected in 3 equal aliqots to the predetermined total volume. Motor response was not deliberately sought; however, when obtained, the current was decreased until the motor response was not seen or palpable. Needle insertion and injection were considered intraneural if nerve stimulation at a current of 0.2 mA (0.1 ms) or less resulted in an evoked motor response or if a high resistance (pressure) to injection or nerve swelling was noticed upon initiating the injection. In these instances, the injection was immediately stopped, and patients with suspected intraneural injections were excluded from analyses. Block start time (needle insertion) and procedure duration (end of injection) were recorded. Time of incision, duration of surgical procedure, and time to first analgesic requirement (visual analog scale >3) were also recorded.

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Block Assessment

An examiner blinded to the volume of injectate evaluated the presence of motor and sensory blockade in the territory of C5 to C6. Block assessment was performed at 5-minute intervals up to 30 minutes after completion of the last injection. Simultaneous sensory and motor functions in the contralateral limb were used for comparison purposes. Testing of the sensory block in the deltoid area (axillary nerve territory) was chosen because it was considered the most relevant to the surgical model used. Sensory function was assessed by pinprick (paper clip), and scored as present or absent.

Similarly, motor function was assessed by testing abduction of the arm (axillary nerve) and flexion of the forearm (musculocutaneous nerve). A modified Bromage scale was used7:

  • Score of 4: full power;
  • Score of 3: reduced power but able to lift the arm against resistance;
  • Score of 2: moves relevant muscle group against gravity but unable to lift the arm against resistance;
  • Score of 1: perceptible muscle contraction, but unable to lift the arm purposely;
  • Score of 0: no movement in relevant muscle group.

Assessments were terminated when anesthesia in the deltoid region was deemed complete or after 30 minutes had elapsed, whichever came first. Successful block of the brachial plexus was defined as presence of adequate motor block (motor score of ≤2), absent sensation to cold and pinprick sensation within 30 minutes of injection, and absence of the need for general anesthesia. Block failure was defined as absence of surgical anesthesia at 30 minutes in at least one of the tests or need to convert to general anesthesia for completion of surgery.

Duration of sensory blockade was assessed by asking the patient to record the time of first pain sensation.

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Perioperative Period

Surgery was conducted under interscalene block as a sole technique. Propofol infusion was titrated to light sleep equivalent to the Ramsey scale of 5, patients exhibiting a sluggish response to light glabelar tap or loud auditory stimulus.11

All patients received 1 g of IV paracetamol and 75 mg of diclofenac sodium at the end of surgery. Tramadol 50 mg (IV) was prescribed as rescue analgesia after block regression. Postoperative analgesia consisted of 1 g of oral paracetamol every 6 hours and 75 mg of diclofenac sodium twice daily for 48 hours after surgery.

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Study-Stopping Rules and Sample Size

Two study-stopping rules were used. First, the minimum effective anesthetic volume (MEAV)50 of 0.75% ropivacaine was defined as the midpoint of pairs of volumes from consecutive patients in which a negative response (inadequate block within 30 minutes) is followed by a positive one (adequate block within 30 minutes). On the basis of previous nonprobability sequential dosing used in studies with similar binary outcomes,7 we estimated that a minimum of 10 independent negative–positive up-and-down deflections was required to calculate MEAV50. It was agreed by consensus that reducing the dose below 5 mL was not of clinical importance. The study was stopped on achieving the secondary stopping rule of 10 consecutive successful interscalane blocks using 5 mL of ropivacaine 0.75%.

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Statistical Analyses

Summary data were calculated using the SAS® software version 9.1.3 (SAS Institute Inc., Cary, NC) and presented as median (range) or mean (SD) as appropriate. Single failure had to be assumed to calculate the 95% confidence interval (CI) for rate of overall block success, because no block failures were observed in this study. The modified Wald method as recommended by Agresti and Coull was used.12 Because the Levene's test for equality of variance in duration of blockade between patients who required 5 mL and patients who required >5 mL of LA did not indicate heterogeneity beyond chance (P = 0.524), duration of blockade between these 2 groups was tested by the independent-samples t test on n − 2 df. The corresponding 95% CI for mean difference in blockade between these groups is reported.

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RESULTS

Patient demographics are presented in Table 1. Per protocol, the study was stopped when 10 consecutive successful interscalene blocks were achieved using 5 mL of 0.75% ropivacaine. This protocol was met upon enrollment of 20 consecutive patients. No difficulties were encountered while executing the injection protocol. No intraneural injections occurred. All patients had successful surgical blocks (overall success rate 100%; 95% CI assuming single failure, 74%–100%); thus MEAV50 could not be calculated (Figure 1).

Table 1

Table 1

Figure 1

Figure 1

For the group as a whole, the median (range) sensory block onset time was 5 minutes,520 the median (range) motor block for the biceps was 7.5 minutes,515 and for arm abduction, 10 minutes.515 The median (range) block duration was 9.9 hours,519 and the mean (SD) block performance time was 8 (±3.2) minutes. Mean duration of analgesia was 9.9 ± 3.74 hours. Duration of analgesia was not associated with volume of LA used (Spearman r = 0.05, P = 0.83). The 95% CI for the Spearman rank correlation coefficient, as calculated after Fisher's z transformation, was −0.43 to 0.53; thus 95% of similar trials will likely show no association between dose and duration. Block duration did not differ between patients with successful blocks at 5 mL and those who required >5 mL for successful blocks (9.4 ± 3.4 and 10.4 ± 4.2 hours, respectively; P = 0.55). Moreover, no trend was discernible when a locally weighted scatterplot smoothing procedure (LOWESS, tension α = 0.6) was used to detect changes in duration of analgesia for patients with doses higher than 5 mL (Fig. 2). In our sample of patients, average block duration was 1 hour longer for those who required >5 mL for successful block (95% CI −4.6 to 2.5 hours). It should be noted, however, that the 95% CI is wide and includes the possibility that patients in other trials who require >5 mL for successful blocks could have shorter duration of blockade.

Figure 2

Figure 2

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DISCUSSION

Under the conditions of our study, successful surgical anesthesia for shoulder surgery was obtained in all 20 patients with as little as 5 mL of 0.75% ropivacaine, or 1.7 mL for each of the superior, middle, and inferior trunks of the brachial plexus. This suggests that a substantial decrease in LA volume and dose is possible for surgically successful interscalene block.

By convention, many clinicians use large volumes and doses of LAs to ensure rapid onset and successful interscalene brachial plexus blocks.6,13 However, lower volumes of LA may reduce the risk of toxicity as well as unwanted spread of LAs towards the centroneuraxis and the phrenic nerve (diaphragmatic paralysis).1416 This is because the phrenic nerve (C3 to C5) is typically blocked when high volumes of LA are used in brachial plexus blockade.17 For instance, using an ultrasound-guided technique for interscalene block, Riazi et al. reported that the incidence of diaphragmatic paralysis was significantly lower in the low-volume (5 mL) group than in the standard-volume (20 mL) group (45% vs 100%).

Riazi et al. also reported, similar to our findings, that reduction of the volume of LAs to accomplish analgesia with the brachial plexus at the interscalene location is possible.14 The low volume (5 mL) of 0.5% ropivacaine used in their study did not reduce block quality as assessed by pain scores, sleep quality, and total morphine consumption up to 24 hours after surgery in comparison with 20 mL of 0.5% ropivacaine.18 However, because Riazi et al. used general anesthesia for surgery, they could not determine whether a low volume of LA would be adequate to accomplish surgical anesthesia with interscalene brachial plexus block. In our study, 3 injections of approximately 1.7 mL of 0.75% ropivacaine at the superior, middle, and inferior trunks (totaling 5 mL) resulted in successful surgical anesthesia.

Incremental reduction in the volume of 0.75% ropivacaine in our study was stopped when adequate surgical anesthesia was achieved with 5 mL. This is because our patients were scheduled to have surgery under regional anesthesia as the sole anesthetic. Volumes below 5 mL could lead to higher risk for failures that may necessitate urgent rescue anesthesia with a patient in a suboptimal position for airway management (beach chair position). Moreover, 5 mL of 0.75% ropivacaine (37.5 mg) is an approximate 6-fold reduction in volume from the 30 to 40 mL that is commonly used; thus further reduction would have rapidly diminishing clinical importance. Finally, 5 mL of 0.75% ropivacaine does not appear to be associated with a significant risk for severe systemic toxicity, unless injected intra-arterially. When used as an IV test dose, 25 or 60 mg of IV ropivacaine did not result in systemic toxicity in an adult patient.19,20

In our study, we used a 3-injection technique to accomplish the block. Although using a single-injection technique may be more time efficient and theoretically carry less risk for inadvertent injury to the roots of the brachial plexus, it is not known whether a single injection would be adequate to accomplish surgical anesthesia. Fanelli et al. compared single- and multiple-injection techniques for interscalene block using 20 mL of 0.75% ropivacaine and nerve stimulator guidance and reported that their multiple injection protocol resulted in faster onset of anesthesia.21 Using ultrasound guidance and separate injections for each nerve, O'Donnell et al. were also able to reduce the volume of lidocaine 2% + epinephrine to 1 mL per nerve.7 Using a similar selective drug placement, Eichenberger et al. were able to reduce the ED95 dose of 1% mepivacaine to 0.7 mL to block the ulnar nerve at the proximal forearm.8 However, limited data are available on the ability of low-volume LA to provide surgical brachial plexus anesthesia, particularly with single-injection techniques.

The reports on the correlation among LA dose, concentration, and volume and duration of analgesia after PNBs are conflicting.22,23 Casati et al. found no correlation between the concentration of 20 mL of ropivacaine (0.5%, 0.75%, 1%) and the duration of postoperative analgesia, whereas Klein et al. reported that 30 mL of ropivacaine (0.5% or 0.75%) exhibited similar onset and duration characteristics. In our sample of patients, average block duration was 1 hour longer for those who received >5 mL of LA. It should be noted, however, that the 95% CI is wide; therefore no meaningful statement regarding any difference in the block duration between the patients who received 5 mL or >5 mL of LA can be made. In addition, it is possible that a low volume of LAs may not yield adequate motor relaxation in all patients. Thus, a higher volume of LA may result in a more expeditious onset and more consistent duration of blockade.

In summary, our data suggest that successful surgical anesthesia with ultrasound-guided interscalene nerve block can be accomplished with as little as 5 mL of 0.75% ropivacaine (100%, 95% CI: 74.1%–100%) without clinically apparent deterioration in block onset or duration. Because the lower limit of the CI may be associated with a 25% failure rate, studies using similar stopping rules for doses slightly higher than 5 mL are nonetheless warranted.

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DISCLOSURES

Name: Philippe Gautier, MD.

Contribution: Study design, conducting study, data collection, data analysis, manuscript preparation, and the archival author.

Name: Catherine Vandepitte, MD.

Contribution: Study design, conducting study, data collection and interpretation, manuscript preparation, and revisions.

Name: Caroline Ramquet, MD.

Contribution: Conducting study and data collection.

Name: Mieke DeCoopman, MD.

Contribution: Conducting study and data collection.

Name: Daquan Xu, MD.

Contribution: Manuscript preparation.

Name: Admir Hadzic, MD.

Contribution: Manuscript preparation.

This manuscript was handled by Terese T. Horlocker, MD.

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