For the group as a whole, the median (range) sensory block onset time was 5 minutes,5–20 the median (range) motor block for the biceps was 7.5 minutes,5–15 and for arm abduction, 10 minutes.5–15 The median (range) block duration was 9.9 hours,5–19 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.
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).14–16 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.
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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© 2011 International Anesthesia Research Society
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