LETTERS TO THE EDITOR
Maximum Effective Needle-Nerve Distance What Did We Really Find? Accepted for publication: March 31, 2014. To the Editor: e read with some interest the recent study by Albrecht and colleagues.1 They have demonstrated that local anesthetic can be deposited some distance from the interscalene brachial plexus and yet a successful block may be obtained. The question arises: Why would one want to do this? Spence and colleagues2 have already demonstrated that a periplexus block is as effective as intraplexus interscalene block. In this study,2 this small movement of the needle away from the nerve led to a shorter duration of analgesia. It is likely that if this small increase in needle-nerve distance reduced the duration of analgesia, further increases in nerve-needle distance may lessen the duration of analgesia even more. Although there is evidence to suggest that intraneural injection of local anesthetics may be deleterious,3 local anesthetics have been injected intraneurally without any residual sequelae.4,5 The thrust of Albrecht and colleagues’ study would have import, if it could be established that even extraneural injection of local anesthetic may lead to nerve damage; and that this damage could be lessened by increasing the nerve-needle distance. The authors go on to state that “although a distance of 1.6 mm can achieve a successful block in 95% of patients, the upper limit of our CI suggests that this distance may actually be far greater.” We would like to note that confidence intervals (CIs) extend in 2 directions. The lower limit of the CI from their data (Fig. 3)1 suggests that at a distance of 1 mm the likelihood of block success may be little more than 35%! On the basis of a study of just 20 patients (only 11 of whom had a successful block), the authors were able to compute MED95, MED80, and MED50. The 95% CI for both MED95 and MED80 are reported as 0 (one assumes nerve contact). A careful perusal of Figure 3 in their report1 would suggest a negative value for the lower limit of the 95% CI for both MED80 and MED95. The authors also estimated the sample size based upon an assumed standard deviation (SD) and standard error of the mean
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(SEM). We are unable to glean what measurements these SD and SEM refer to. Both SD and SEM are more meaningful when placed in the context of a mean value. Shashi Bhushan Bhatt, MBBS, MD, FRCA James P. Hofmann, MD Department of Anesthesiology University of Toledo Medical Center Toledo, OH
The authors declare no conflict of interest. REFERENCES 1. Albrecht E, Kirkham KR, Taffe P, et al. The maximum effective needle-to-nerve distance for ultrasound-guided interscalene block. An exploratory study. Reg Anesth Pain Med. 2014; 39:56–60. 2. Spence BC, Beach ML, Gallagher JD, Sites BD. Ultrasound-guided interscalene blocks: understanding where to inject the local anesthetics. Anaesthesia. 2011;66: 509–514. 3. Hadzic A, Dilberovic F, Shah S, et al. Combination of intraneural injection and high injection pressure leads to fascicular injury and neurologic deficits in dogs. Reg Anesth Pain Med. 2004;29:417–423. 4. Bigeleisen PE. Nerve puncture and apparent intraneural injection during ultrasound-guided axillary block does not invariably result in neurologic injury. Anesthesiology. 2006;105: 779–783. 5. Bigeleisen PE, Moayeri N, Groen GJ. Extraneural versus intraneural stimulation thresholds during ultrasound-guided supraclavicular block. Anesthesiology. 2009;110: 1235–1243.
Reply to Drs Bhatt and Hofmann
described, our goal is to question the principle behind most ultrasound-guided regional anesthesia research that asks “How close can we get?” Instead, we ask “How close is close enough?” We agree that Spence and colleagues3 have previously demonstrated the efficacy of a periplexus block. However, the optimal needle position that best balances efficacy and safety has not been defined. Our study is not intended to argue that clinicians should be routinely performing ISB at a distance of 8.4 mm away from the nerve roots. Rather, we are encouraging practitioners to thoughtfully consider how close to the nerve roots they truly need to be. This consideration may be especially poignant if considering ISB in patients who may be more susceptible to nerve injury, such as those with type 1 diabetes mellitus,4 multiple sclerosis,5 or those undergoing chemotherapy.6 Our findings may even influence the way we think about performing ISB in asleep patients. Regarding our statistical analysis, we reiterate that a distance of 0 mm refers to a needle tip position touching the interscalene brachial plexus sheath, as stated in the article. Indeed, the confidence band is wide but asymmetric, which means that values at the lowest part of the band are less probable. In other words, the probability of an effective block at a distance of 1.6 mm is more than 80% even if the inferior limit of the confidence band is 35%. Both standard deviation and standard error of the mean are inappropriate in the absence of a Gaussian distribution. We believe that the 95% confidence interval is more useful as it illustrates the dispersion of the mean. The question raised by Drs Bhatt and Hofmann is “What did we really find?” The answer is that the optimal block location is not necessarily synonymous with that which is closest to the nerve. We endeavor to continue asking and investigating “How close is close enough?” The road is long and the journey fascinating.
Accepted for publication: April 15, 2014. To the Editor: e would like to thank Drs Bhatt and Hofmann for their comments regarding our recent publication exploring the maximum effective needle-to-nerve distance for interscalene nerve block (ISB).1,2 It is precisely to stimulate discussion and comments such as these that we sought to publish our manuscript under the journal’s new Daring Discourse category. As we
W
Regional Anesthesia and Pain Medicine • Volume 39, Number 4, July-August 2014
Eric Albrecht, MD Kyle R. Kirkham, MD Department of Anesthesia and Pain Management, Toronto Western Hospital University of Toronto Toronto, Ontario, Canada
Patrick Taffé, PhD Institute of Social and Preventive Medicine (IUMSP), Centre Hospitalier Universitaire Vaudois and University of Lausanne Lausanne, Switzerland
351
Copyright © 2014 American Society of Regional Anesthesia and Pain Medicine. Unauthorized reproduction of this article is prohibited.
Maximum Effective Needle-Nerve Distance What Did We Really Find? Accepted for publication: March 31, 2014. To the Editor: e read with some interest the recent study by Albrecht and colleagues.1 They have demonstrated that local anesthetic can be deposited some distance from the interscalene brachial plexus and yet a successful block may be obtained. The question arises: Why would one want to do this? Spence and colleagues2 have already demonstrated that a periplexus block is as effective as intraplexus interscalene block. In this study,2 this small movement of the needle away from the nerve led to a shorter duration of analgesia. It is likely that if this small increase in needle-nerve distance reduced the duration of analgesia, further increases in nerve-needle distance may lessen the duration of analgesia even more. Although there is evidence to suggest that intraneural injection of local anesthetics may be deleterious,3 local anesthetics have been injected intraneurally without any residual sequelae.4,5 The thrust of Albrecht and colleagues’ study would have import, if it could be established that even extraneural injection of local anesthetic may lead to nerve damage; and that this damage could be lessened by increasing the nerve-needle distance. The authors go on to state that “although a distance of 1.6 mm can achieve a successful block in 95% of patients, the upper limit of our CI suggests that this distance may actually be far greater.” We would like to note that confidence intervals (CIs) extend in 2 directions. The lower limit of the CI from their data (Fig. 3)1 suggests that at a distance of 1 mm the likelihood of block success may be little more than 35%! On the basis of a study of just 20 patients (only 11 of whom had a successful block), the authors were able to compute MED95, MED80, and MED50. The 95% CI for both MED95 and MED80 are reported as 0 (one assumes nerve contact). A careful perusal of Figure 3 in their report1 would suggest a negative value for the lower limit of the 95% CI for both MED80 and MED95. The authors also estimated the sample size based upon an assumed standard deviation (SD) and standard error of the mean
W
(SEM). We are unable to glean what measurements these SD and SEM refer to. Both SD and SEM are more meaningful when placed in the context of a mean value. Shashi Bhushan Bhatt, MBBS, MD, FRCA James P. Hofmann, MD Department of Anesthesiology University of Toledo Medical Center Toledo, OH
The authors declare no conflict of interest. REFERENCES 1. Albrecht E, Kirkham KR, Taffe P, et al. The maximum effective needle-to-nerve distance for ultrasound-guided interscalene block. An exploratory study. Reg Anesth Pain Med. 2014; 39:56–60. 2. Spence BC, Beach ML, Gallagher JD, Sites BD. Ultrasound-guided interscalene blocks: understanding where to inject the local anesthetics. Anaesthesia. 2011;66: 509–514. 3. Hadzic A, Dilberovic F, Shah S, et al. Combination of intraneural injection and high injection pressure leads to fascicular injury and neurologic deficits in dogs. Reg Anesth Pain Med. 2004;29:417–423. 4. Bigeleisen PE. Nerve puncture and apparent intraneural injection during ultrasound-guided axillary block does not invariably result in neurologic injury. Anesthesiology. 2006;105: 779–783. 5. Bigeleisen PE, Moayeri N, Groen GJ. Extraneural versus intraneural stimulation thresholds during ultrasound-guided supraclavicular block. Anesthesiology. 2009;110: 1235–1243.
Reply to Drs Bhatt and Hofmann
described, our goal is to question the principle behind most ultrasound-guided regional anesthesia research that asks “How close can we get?” Instead, we ask “How close is close enough?” We agree that Spence and colleagues3 have previously demonstrated the efficacy of a periplexus block. However, the optimal needle position that best balances efficacy and safety has not been defined. Our study is not intended to argue that clinicians should be routinely performing ISB at a distance of 8.4 mm away from the nerve roots. Rather, we are encouraging practitioners to thoughtfully consider how close to the nerve roots they truly need to be. This consideration may be especially poignant if considering ISB in patients who may be more susceptible to nerve injury, such as those with type 1 diabetes mellitus,4 multiple sclerosis,5 or those undergoing chemotherapy.6 Our findings may even influence the way we think about performing ISB in asleep patients. Regarding our statistical analysis, we reiterate that a distance of 0 mm refers to a needle tip position touching the interscalene brachial plexus sheath, as stated in the article. Indeed, the confidence band is wide but asymmetric, which means that values at the lowest part of the band are less probable. In other words, the probability of an effective block at a distance of 1.6 mm is more than 80% even if the inferior limit of the confidence band is 35%. Both standard deviation and standard error of the mean are inappropriate in the absence of a Gaussian distribution. We believe that the 95% confidence interval is more useful as it illustrates the dispersion of the mean. The question raised by Drs Bhatt and Hofmann is “What did we really find?” The answer is that the optimal block location is not necessarily synonymous with that which is closest to the nerve. We endeavor to continue asking and investigating “How close is close enough?” The road is long and the journey fascinating.
Accepted for publication: April 15, 2014. To the Editor: e would like to thank Drs Bhatt and Hofmann for their comments regarding our recent publication exploring the maximum effective needle-to-nerve distance for interscalene nerve block (ISB).1,2 It is precisely to stimulate discussion and comments such as these that we sought to publish our manuscript under the journal’s new Daring Discourse category. As we
W
Regional Anesthesia and Pain Medicine • Volume 39, Number 4, July-August 2014
Eric Albrecht, MD Kyle R. Kirkham, MD Department of Anesthesia and Pain Management, Toronto Western Hospital University of Toronto Toronto, Ontario, Canada
Patrick Taffé, PhD Institute of Social and Preventive Medicine (IUMSP), Centre Hospitalier Universitaire Vaudois and University of Lausanne Lausanne, Switzerland
351
Copyright © 2014 American Society of Regional Anesthesia and Pain Medicine. Unauthorized reproduction of this article is prohibited.
