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High- Versus Low-Stimulation Current Threshold for Axillary Plexus Blocks: A Prospective Randomized Triple-Blinded Noninferiority Trial in 205 Patients

Vassiliou, Timon MD*; Müller, Hans-Helge PhD; Ellert, Angela MD*‡; Wallot, Pascal MD*; Kwee, Kuo-Min MD*; Beyerle, Michaela MD; Eberhart, Leopold MD*; Wulf, Hinnerk MD*; Steinfeldt, Thorsten MD*

doi: 10.1213/ANE.0b013e31826fffef
Regional Anesthesia: Research Reports
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BACKGROUND: For nerve stimulator-guided regional anesthesia, one has to compromise between a presumed low success rate (using a high-current threshold) and a presumed increased risk of nerve damage (using a low-current threshold). We hypothesized that high-current thresholds in the range of 0.9 to 1.1 mA are not inferior with respect to the procedural and latency times compared with low threshold currents in the range of 0.3 to 0.5 mA for nerve stimulation in brachial plexus blocks.

METHODS: Two hundred five patients scheduled for elective surgery were randomized to a low (0.3–0.5 mA, n = 103) or a high (0.9–1.1 mA, n = 102) stimulation current threshold for the axillary plexus block with 40 mL local anesthetic mixture (20 mL, each of prilocaine 1% and ropivacaine 0.75%). The primary end point was the time to complete sensory block. The secondary outcome measures were the time to readiness for surgery (defined as the time from the start of block procedure to complete sensory block) and the block performance time. The noninferiority margin was set at 5 minutes and was evaluated using the two-sided 95% bootstrap-confidence intervals ([CIs] 100,000 replications) for differences in means.

RESULTS: The mean times to complete sensory block revealed a significant decrease with the low-current group (17.9 ± 12.1 (mean ± SD) versus 22.8 ± 12.4 minutes; 95% CI, 1.1 to 8.6; p = 0.012). The time to readiness for surgery was 30.3 ± 13.8 minutes in the low-current group and 31.7 ± 12.9 minutes in the high-current group (95% CI, –2.7 to 5.5; p = 0.49). The performance time was significantly shorter in the high-current threshold group (9.5 ± 4.7 versus 11.9 ± 5.7 minutes; 95% CI, –4 to 1.1; p = 0.001).

CONCLUSION: Noninferiority for the high-current threshold technique could neither be confirmed for the primary end point nor for secondary end points. However, we consider a difference in mean times of approximately 8.5 minutes to achieve readiness for surgery acceptable for clinical practice.

Published ahead of print December 7, 2012 Supplemental Digital Content is available in the text.

From the *Department of Anesthesiology and Critical Care, University Hospital Giessen-Marburg, Philipps University Marburg, Marburg, Germany; Institute for Medical Informatics, Biometry and Epidemiology, Ludwig-Maximilians-University, Munich, Germany; Department of Anesthesiology and Intensive Care, Hospital Reinbek St. Adolf-Stift, Reinbek, Germany; and §Department of Anesthesiology and Intensive Care Medicine, Asklepios-Klinik Harburg, Hamburg, Germany.

Accepted for publication July 31, 2012.

Published ahead of print December 7, 2012

See Disclosures at end of article for Author Conflicts of Interest.

Funded by Institutional research budget.

Reprints will not be available from the authors.

Address correspondence to Timon Vassiliou, MD, Department of Anesthesiology and Critical Care, University Hospital Giessen-Marburg, Philipps University Marburg, Baldingerstrasse, 35033 Marburg, Germany. Address e-mail to vassiliou@staff.uni-marburg.de

Nerve stimulation is an established method for the localization of peripheral nerves facilitating nerve blocks. Corresponding to the Coulombs law, eliciting a motor response with a predetermined current depends on the proximity of the noninsulated needletip to the target nerve. Accordingly, it is assumed that specific muscle twitches at a low-current threshold are associated with an increased probability for successful nerve block. However, needle guidance with a very low threshold current (i.e., <0.3 mA) could theoretically result in needle nerve contact with a potential for nerve damage.1,2 In addition, an increased number of needle manipulations is required when low threshold currents are predetermined for eliciting a motor response, thereby increasing the risk of nerve injury.3 Therefore, for nerve stimulator-guided needle placement, there is a compromise in terms of an assumed low success rate (using a high-current threshold) and an assumed increased risk of nerve damage (using a low-current threshold).

A recently published systematic review by Chin and Handoll4 focusing on nerve stimulator-guided techniques for the axillary plexus block revealed that a motor response with a current ≤0.5 mA is deemed appropriate. With this current threshold, complete sensory block was reported in approximately 90% of the subjects for the multiple-injection technique, approximately 80% for the double-injection technique, and approximately 40% for the single-injection technique generally within 30 minutes subsequent to the block procedure.4 The incidence of typical side effects ranged from 3.6 to 6.6% for vascular puncture and from 0.3 to 1.6% for transient nerve dysfunction in the immediate days after the block.4 Permanent nerve damage was described as rare, with 0.4% per year.5 Hence, Perlas et al.6 used ultrasound imaging to evaluate the sensitivity of the motor response after electrical stimulation for the axillary nerve block. The results of this clinical trial indicated a failed motor response after direct needletip to nerve contact in 25% of cases using a current intensity of 0.5 mA or less, but all patients in this trial showed a motor response after direct needle–nerve contact when a current intensity between 0.5 and 1 mA was administered.

The aim of the present study was to investigate whether a high-current threshold (0.9–1.1 mA) was not inferior to the common low-current threshold (0.3–0.5 mA) for nerve stimulation for axillary plexus blocks regarding the procedural and latency times, as well as the quality of anesthesia.

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METHODS

Study Population

After approval by the appropriate IRB Ethics Committee of the Medical Faculty, Philipps-University-Marburg, no. #129/07) and written informed consent, 213 patients of ASA physical status I to III scheduled for elective orthopedic/trauma surgery of the upper extremity were enrolled into the trial between November 2007 and February 2009. Patients who are younger than 18 years, are pregnant, or have a neurological disease or chronic neurological deficits of the involved extremity were excluded, as well as patients with allergies to local anesthetics or patients with inadequate understanding of German or English.

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Study Protocol

The trial was designed as a triple-blind, concealed allocation, randomized noninferiority trial with two groups (high [0.9–1.1 mA] versus low [0.3–0.5 mA] current threshold for nerve stimulation). Each patient was randomized by a computer-generated list using an allocation ratio of 1:1. Patients, anesthesiologists, and observers (including follow-up study personnel) were blinded. Only the assisting person (study nurse) manipulating the nerve stimulator was aware of the assignment to the low- or high-current group. The axillary plexus blocks were performed by two consultants (TV and KMK) and two residents (PW and MB). Before starting each procedure, one of the four anesthesiologists was randomly assigned to perform the axillary plexus block, and the identity of the anesthesiologist was recorded. Monitoring and the approach for the axillary plexus block were performed for each patient according to a standardized protocol.

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Intervention

Hemodynamic monitoring, pulse oximetry, and oxygen supplementation (3 L/min) were applied according to the recommendations of the German Society of Anaesthesiology and Intensive Care Medicine. An insulated short bevel stimulation needle connected to a nerve stimulator (Stimuplex®, HNS11®, B. Braun, Melsungen, Germany) was used for the axillary plexus block. The nerve stimulator was fitted with fresh batteries, and the function was verified before starting the procedure. A modified triple-stimulation technique was applied, addressing the musculocutaneous, median, and radial nerve on the basis of the approach described by Coventry et al.7 The arm was abducted to an angle of 90 degrees after the patient has been placed in supine position. The elbow was bent at 45 degrees, and the forearm was supinated. After disinfection and placement of sterile draping, the pulse of the axillary artery was located by palpation at the level in which the tendon of the pectoralis major muscle passes the arm. Superficial skin anesthesia was achieved by injection of 0.5 mL mepivacaine 1%. This was defined as time “0.” One minute after skin anesthesia, starting with a stimulation current of 2 mA (0.1 millisecond, 2 Hz) the needle was advanced, first addressing the musculocutaneous nerve. After eliciting a motor response, the current was reduced in steps of 0.2 mA (low-current group) or 0.1 mA (high-current group), and the needle position was modified to maintain a motor response. After reaching the target current range (high or low), the first 5 mL of a local anesthetic mixture (equal amounts of prilocaine 1% and ropivacaine 0.75%) was injected. The median (25 mL) and radial nerve (10 mL) were accordingly blocked. The performance time was separately recorded in 1 minute intervals until completion of the block, as well as for the nerves. Thereafter, at the lateral epicondylus (muculocutaneous nerve) of the elbow, at the dorsal basis of digit 1 (radial nerve), and at the tip of digit 3 (median nerve) and digit 5 (ulnar nerve), loss of temperature discrimination was tested at 3 minute intervals, for a maximal duration of 45 minutes, using an 8°C thermal probe. Motor block was immediately assessed after sensory block testing at the same time intervals and maximal duration, respectively. Motor function was tested by checking active flexion (brachialis and biceps muscle; musculocutaneous nerve) and extension (triceps muscle; radial nerve) in the elbow joint and active extension of the wrist and fingers (extensors of the forearm, radial nerve) against gravity. The closing of the fist was evaluated against gravity for testing the median (opposition of digit 1, flexion of digit 2; lumbrical and thenar muscles) and the ulnar nerve (flexion of digit 5; hypothenar muscles).

Only the loss of temperature discrimination and the loss of joint movements against gravity were accepted as successful sensory or motor block, respectively (scoring systems were not used for the evaluation of sensory and motor block). In case of failed sensory block in more than 1 nerve distribution at 45 minutes after the completion of local anesthetics, general anesthesia was administered for surgery. If a failed sensory block was located in only one nerve distribution, the patient was prepared for surgery. In case of insufficient surgical analgesia in the operating field, fentanyl IV was administered in increments of 100 µg for a maximal dose of 200 µg as required. After IV supplementation with the maximal dose of fentanyl (200 µg), local anesthetic infiltration (15 mL prilocaine 1%) was applied inside the operating field by the surgeon. Midazolam (25 µg/kg) was used, if necessary, for reduction of patients’ anxiety.

Neurological dysfunction was assessed using a standardized telephone interview during follow-up 6 weeks (t1) after surgery. If signs of neurological dysfunction were noticed at the follow-up, patients were examined in the trauma surgery outpatient department, the neurology outpatient department and in the pain clinic of our department.

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Outcome Measures

The primary end point was the time to complete sensory block. Key secondary end points were performance time and the time to achieve readiness for surgery. Readiness for surgery was defined as the time from the start of block procedure to complete sensory block (Fig. 1). Secondary end points were the time to complete motor block, the performance times for each individual nerve, times to complete sensory and motor block of each individual nerve, conversion to general anesthesia, supplementary analgesia for surgery (IV fentanyl), supplementation of local anesthetics by the surgeon, the demand for midazolam, and the incidence of paresthesia and vessel puncture.

Figure 1

Figure 1

In addition, we performed an exploratory survey for the incidence of postoperative neural dysfunction 6 weeks after the intervention (t1).

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Statistics

A time to complete sensory block of 15 and 18 minutes with a standard deviation of 5 minutes was estimated for the low- and high-current threshold groups, respectively. A time of approximately 30 minutes with an SD of approximately 33% (10 minutes) was assumed to achieve readiness for surgery. By protocol, the study was designed as a noninferiority trial with a noninferiority margin of 5 minutes increase by use of the high-current threshold compared with the low-current threshold. On the basis of these assumptions and using a type I error level of 5% (2.5% each side), 2 groups of 98 patients each provided a power of 80% to demonstrate noninferiority regarding the primary end point. Assuming a loss to follow-up of 5% at surgical intervention, a minimal initial sample size of 206 participants was chosen.

Expecting at least a 2 minute reduction in the performance time, it was decided to put more emphasis on the time to readiness for surgery by hierarchically pretesting at a type I error level of 5%. Thus, this key secondary end point was analyzed before testing the primary end point. Thereby, also using a noninferiority margin of 5 minutes, the calculated size of 98 patients to be analyzed in each group promised a power of at least 80% for the pretest.

Differences and noninferiority were evaluated using the two-sided 95% confidence intervals (CIs) for differences in the location parameters of the two groups. The t-test CI for the mean difference was prespecified in the protocol, assuming nearly normal distributed end points, as well as the Hodges–Lehmann CI for the shift (corresponding to the Mann–Whitney–Wilcoxon test for the comparison of the two groups) in case of a substantial deviation from normality. To prove the robustness of the results, we provided for the primary and key secondary end points of these two types of 95% CIs and, in addition, the bootstrap-CIs (containing a set of 100,000 replications) for the mean differences. Differences in means for the performance times of each individual nerve, as well as for the times to complete sensory and motor block of each individual nerve, were calculated by 99% CIs. The levels of significance were set at p ≤ 0.05 for the primary end point, as well as for the key secondary end points, and at p ≤ 0.01 for the secondary end points.

An intention-to-treat analysis was acquired in addition to the per-protocol analysis for the key secondary end-point readiness for surgery. In cases of a failed sensory block in only one nerve distribution (incomplete sensory block) readiness for surgery was defined as the sum of performance time and the time for complete sensory block calculated with 45 minutes. In case of general anesthesia as a result of failed sensory block in more than one nerve distribution at 45 minutes, the time to readiness for surgery was defined as the sum of performance time, the time for complete sensory block calculated with 45 minutes, and the time for induction of general anesthesia calculated with an additional time of 6 minutes. Nominal data were analyzed using the Fisher exact test where appropriate. Cleaning of data before the analyses was conducted blind to treatment allocation. Statistical tests and descriptive evaluations were performed using the SPSS software for Windows (IBM® SPSS® Statistics, Release 20.0.0). All data are presented as mean ± SD or frequency.

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RESULTS

Details of patient recruitment, loss to follow-up, and excluded participants are shown in Fig. 2. Two hundred five of the 423 consecutive eligible patients received the allocated intervention. One hundred six patients were randomly allocated to the low-current threshold group. Two patients were lost to follow-up at the day of surgery (t0) because of logistical circumstances. One patient in the low-current threshold group was excluded, because surgery was cancelled after randomization. One hundred seven patients were randomly assigned to the high-current threshold group. Five patients were lost to follow-up on the day of surgery because of the unavailability of the study team. Demographics, anesthesia and surgery-related characteristics for each group are described in Table 1. The mean current threshold in the low-current group was 0.4 mA, and in the high-current group, it was 0.93 mA.

Table 1

Table 1

Figure 2

Figure 2

As indicated by the intention-to-treat analysis (Table 2), the mean time to readiness for surgery was 33.3 ± 15.6 minutes in the low-current group and 38.1 ± 16.8 minutes in the high-current group (P = 0.04). The difference in means was 4.8 minutes with the corresponding 95% t-test-CI (0.4 to 9.2), as well as with the 95% bootstrap-CI (0.4 to 9.2). The Hodges–Lehmann method revealed a shift of 4 minutes (95% CI, 0 to 9). The results of the per-protocol analysis (Table 2) indicated a mean time to readiness for surgery of 30.3 ± 13.8 minutes in the low-current group and 31.7 ± 12.9 minutes in the high-current group (p = 0.49). The differences in means were 1.4 minutes for the t-test (95% CI, –2.7 to 5.6) and for the calculation by bootstrap (95% CI, –2.7 to 5.5). The performance time was significantly shorter in the high-current threshold group (9.5 ± 4.7) versus (11.9 ± 5.7 minutes, p = 0.001). The 95% CI ranged from –4 up to 1.1 minutes, as indicated by t-test and bootstrap calculations, respectively. In contrast with the performance times, the mean times to complete sensory block revealed a significant difference in favor of the low-current group (17.9 ± 12.1 vs 22.8 ± 12.4 minutes, p = 0.012). The 95% CIs for the difference in means ranged from 0 up to 9 minutes. The performance times and the times to complete sensory block contributed to the time to readiness for surgery with 39.2 and 60.8% in the low-current group and with 29.9 and 70.1% in the high-current group, respectively. No significant differences could be noted for the times to complete motor block between the groups (24.9 ± 13.9 vs 24.9 ± 12.3 minutes; p = 0.99), whereas the 95% CIs for the difference in means indicated ranges of approximately 9 minutes.

Table 2

Table 2

The block performance times for each individual nerve and the times to complete sensory and motor block of each individual nerve are listed in Table 3. According to the 99% CIs for the performance times of each individual nerve by bootstrap calculations, the difference in means were from –0.2 minutes (99% CI, –1.3 to 0.9) for the radial nerve up to –1.3 minutes (99% CI, –2.1 to –0.5) for the median nerve. The differences in means for the times to complete sensory block of each individual nerve ranged from 2 minutes (99% bootstrap-CI, –1.7 to 5.8; musculocutaneous nerve) to 4.6 minutes (99% bootstrap-CI, 1.5 to 7.8; median nerve). The differences in means for the times to complete motor block were from 1.4 minutes (99% bootstrap-CI, –4.3 to 6.8) for the ulnar nerve to 3.7 minutes (99% bootstrap-CI, –1.4 to 8.9) for the radial nerve.

Table 3

Table 3

As indicated in Table 4, general anesthesia was administered to 4 of the 103 patients in the low-current group and in 9 of the 102 patients in the high-current group (p = 0.25). In the low-current group, two patients received general anesthesia because of sensory block failure in the distribution of the radial and median nerve. Furthermore, two patients underwent surgical intervention under general anesthesia because of block failure of the radial and ulnar nerve. In the high-current group, general anesthesia was administered to five patients because of sensory block failure of the radial and median nerve. We observed radial and ulnar nerve block failure in three patients. A failed block in the distributions of the radial and musculoctaneous nerve was noted in one patient. IV supplementation of fentanyl was necessary in 10 (low-current) and 8 (high-current) patients (p = 0.81). A single bolus of 100 µg fentanyl was given to two patients in each group (p = 1). A dose of 200 µg fentanyl was for eight patients in the low-current group and six patients in the high-current group (p = 0.78). Local anesthetics were administered by surgeons to 4 patients in the low-current group and 6 patients in the high-current group (p = 0.75). The differences for the frequency of paresthesia (p = 0.37) and the frequency of vessel punctures (p = 0.12) were not significant.

Table 4

Table 4

Transient neural dysfunction was present up to 6 weeks later in 16 patients in the high-current threshold and in 18 patients in the low-current threshold group (Table 5). Consulting pain clinic specialists and neurologists stated that trauma or surgery-related pathology, rather than damage from regional blocks, are the most likely reason in 33 of the 34 patients. In the remaining patient (high-current group), a neurological disorder caused by the axillary plexus block was diagnosed. The patient reported paresthesia on the palm in the area of the median nerve lasting 5 months after the intervention, which was verified at the neurology outpatient department using neurophysiogical diagnostics.

Table 5

Table 5

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DISCUSSION

In our investigation, noninferiority for a high-current threshold technique could neither be confirmed for the primary end point (time to complete sensory block) nor for the key secondary end point (readiness for surgery). Nevertheless, we consider a difference in mean times of approximately 0.5 to 9 minutes to achieve readiness for surgery acceptable in clinical routine with respect to a limited precision of 3-minute intervals for assessment of latency times. In addition, we observed shorter procedural times but somewhat longer times to complete sensory block when high threshold currents were used. Furthermore, there were no differences in terms of the quality of the nerve block using a higher current threshold than the usual threshold.

A motor response obtained at a low output current is regarded as an indicator of close proximity to a target nerve resulting in a successful nerve block.8 The first to support this approach were Magora et al. in 1969.9 They suggested 0.5mA or less as an appropriate threshold current when using Teflon insulated needles. Although optimal threshold currents for a successful block have not been clearly defined, a stimulation current threshold of 0.3 to 0.5 mA is commonly recommended.10,11 However, some clinical studies indicated that higher currents (>0.5 mA) could result in successful blocks as well.6,12,13

Nerve stimulation should result in a motor response and, therefore, in close proximity to motor fibers. We found no difference in the time to complete motor block, but a 25% increase in time to complete sensory block with high-current stimulation. Furthermore, the effects of the time intervals with a precision of at least 3 minutes may be inappropriate to detect small differences in times to complete motor block. This hypothesis is supported by the results of the times to complete motor block of each individual nerve. We observed a tendency of faster times to complete motor block of each individual nerve in the low-current group, but the differences between the groups were not statistically significant.

Previous clinical trials evaluating the effectiveness of multiple-injection techniques for the axillary plexus block selected a current threshold of 0.5 mA or less for nerve location.7,14–16 Volumes of 30 up to 40 mL of mepivacaine or lidocaine were administered for the axillary plexus block. These trials reported success rates for complete sensory block of approximately 90% and 80% for motor block 30 minutes subsequent to injection of local anesthetics.7,14–16 However, we had lower success rates after application of low threshold current (sensory block, 84%; motor block, 71%). This difference can be explained by lower success rates of the radial nerve whereas the success rates for sensory and motor block of the musculocutaneous, median, and ulnar nerve are in line with the findings of current literature. We injected only 10 mL of local anesthetics to the radial nerve instead of 2016 or 15 mL,15 respectively. Furthermore, the use of different local anesthetics and differences in the qualification of the anesthesiologists performing the axillary block may have affected the success rates. In contrast with previous trials, we administered a local anesthetic mixture of prilocaine 1% and ropivacaine 0.75% instead of using only lidocaine 1.5% or mepivacaine 1%. Similar to our results, Rodriguez et al.15 observed conversion to general anesthesia for a triple-stimulation technique in approximately 3% of the patients when low-current intensities were applied. Conversion to general anesthesia was necessary in the present trial in 4% of the patients in the low-current group and in 9% of the high-current group. Nevertheless, although the sample size was acceptable to detect noninferiority in procedural and latency times, this study is underpowered to compare differences regarding the need for general anesthesia (considering a difference of 5%, a sample size of approximately 850 would deemed appropriate). The observed incidence of vascular puncture (20%) corresponds with data from other authors. A meta-analysis by Abrahams et al.17 revealed an incidence of vascular puncture of 18%. Block-related nerve dysfunction was observed in the present trial in 1 of 205 patients. This incidence is in accordance with the data reported by Barrington et al. from Australia.18

The following limitations of our study should be considered. In principle, significant differences of anesthetic quality between the low- and the high-current group would have been observed when smaller local anesthetic volumes were administered. Moreover, the axillary plexus block, performed by a multistimulation technique, may not be the best model for assessing the effectiveness of high- and low-current thresholds for nerve stimulation in terms of anesthetic quality compared with a model evaluating the block quality of a single peripheral nerve (e.g., tibial nerve). The second issue is that the sensitivity of various peripheral nerves to stimulation currents evoking a motor response might differ with nerve diameter and texture. Therefore, our data from the axillary brachial plexus block might not be valid for other nerve blocks. Finally, we assessed neither block duration nor patient satisfaction or postoperative requirements for analgesics.

In conclusion, irrespective of the onset and procedural times, the application of an increased current threshold did not result in significant differences regarding success rate or conversion to general anesthesia. However, we observed prolonged times to complete sensory and motor block with a shorter procedural time when high current is applied. Further studies should focus on the question of whether high output currents (i.e., 0.9–1.1 mA) for successful administration of local anesthetics might help avoid unnecessary and potentially dangerous needle adjustments resulting in nerve injury or vascular puncture.

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DISCLOSURES

Name: Timon Vassiliou, MD.

Contribution: This author helped design the study, conduct the study, and write the manuscript.

Attestation: Timon Vassiliou has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.

Conflicts of Interest: The author has no conflicts of interest to declare.

Name: Hans-Helge Müller, PhD.

Contribution: This author helped analyze the data including statistical design and write the manuscript.

Attestation: Hans-Helge Müller has seen the original study data, reviewed the analysis of the data and approved the final manuscript.

Conflicts of Interest: The author has no conflicts of interest to declare.

Name: Angela Ellert, MD.

Contribution: This author helped conduct the study.

Attestation: Angela Ellert has seen the original study data and approved the final manuscript.

Conflicts of Interest: The author has no conflicts of interest to declare.

Name: Pascal Wallot, MD.

Contribution: This author helped conduct the study.

Attestation: Pascal Wallot approved the final manuscript.

Conflicts of Interest: The author has no conflicts of interest to declare.

Name: Kuo-Min Kwee, MD.

Contribution: This author helped conduct the study.

Attestation: Kuo-Min Kwee approved the final manuscript.

Conflicts of Interest: The author has no conflicts of interest to declare.

Name: Michaela Beyerle, MD.

Contribution: This author helped conduct the study.

Attestation: Michaela Beyerle approved the final manuscript.

Conflicts of Interest: The author has no conflicts of interest to declare.

Name: Leopold Eberhart, MD.

Contribution: This author helped analyze the data and write the manuscript.

Attestation: Leopold Eberhart has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Conflicts of Interest: The author has no conflicts of interest to declare.

Name: Hinnerk Wulf, MD.

Contribution: This author helped analyze the data and write the manuscript.

Attestation: Hinnerk Wulf has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.

Conflicts of Interest: Hinnerk Wulf received honoraria from B. Braun, received honoraria from Sonosite, and received honoraria from Teleflex. Hinnerk Wulf received speaker fees from B. Braun, Sonosite, and Teleflex.

Name: Thorsten Steinfeldt, MD.

Contribution: This author helped conduct the study and write the manuscript.

Attestation: Thorsten Steinfeldt has seen the original study data, reviewed the analysis of the data, reviewed and approved the final manuscript.

Conflicts of Interest: Thorsten Steinfeldt received honoraria from B. Braun and received honoraria from Teleflex. Thorsten Steinfeldt received speaker fees from B. Braun, and Teleflex.

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

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