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Anesth Analg 2007; 105:1605-1609
© 2007 International Anesthesia Research Society
doi: 10.1213/01.ane.0000287642.21534.ed
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PEDIATRIC ANESTHESIOLOGY

The Relationship Between Current Intensity for Nerve Stimulation and Success of Peripheral Nerve Blocks Performed in Pediatric Patients Under General Anesthesia

Harshad Gurnaney, MBBS, Arjunan Ganesh, MBBS, and Giovanni Cucchiaro, MD

From the Department of Anesthesia and Critical Care Medicine, The Children’s Hospital of Philadelphia and University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania.

Address correspondence and reprint requests to Harshad Gurnaney, MBBS, Department of Anesthesiology and Critical Care Medicine, The Children’s Hospital of Philadelphia, 34th St. and Civic Center Blvd., Philadelphia, PA 19104-4399. Address e-mail to gurnaney{at}email.chop.edu.

Abstract

BACKGROUND: We evaluated the relationship between the lowest current amperage used to obtain a motor response, the success rate and the incidence of neurological complications with peripheral nerve blocks (PNB) in pediatric patients under general anesthesia.

METHODS: We reviewed the regional anesthesia database at The Children’s Hospital of Philadelphia and included all pediatric patients who received a single-injection PNB under general anesthesia with the aid of a peripheral nerve stimulator between October 2002 and July 2006. Data analyzed included age, sex, type of block, stimulation threshold, presence of sensory and motor blockade, and neurological complications.

RESULTS: Six-hundred sixty patients received a PNB during the study period. The average age of the patients was 13.8 yr (range = 2–18 yr). All the blocks were performed using a current ranging between 0.2 and 1 (median = 0.5 mA, interquartile range: 0.45–0.55 mA). The overall success rate was 96%. There was no difference in success rate between blocks performed using a stimulation threshold of ≤0.5 or >0.5 mA (96.3% vs 95.9%; P = 0.793). There was no correlation between the success rate and sex, type of block performed or intensity of current used. Two patients reported prolonged nerve blockade of the great toe and dorsum of the foot after a sciatic nerve block, which lasted for 72 h. No long-term sequelae were noted in our patients.

CONCLUSION: In this study, a similar PNB success rate was observed with both a low (*0.5 mA) and a high stimulation threshold (>0.5 mA). Therefore, it may not be necessary to perform needle manipulations to achieve a low stimulation threshold (≤0.5 mA), as this may increase the risk of intraneural injection.

The use of peripheral nerve blocks (PNB) in pediatric patients has significantly increased in the recent years, as shown by the number of articles published in the anesthesia literature (1–4). Despite the introduction of new block needles and a more widespread use of the nerve stimulation technique, anesthesiologists are still concerned about performing PNB under general anesthesia (GA). The occasional reports in the anesthesia literature of children and adults who suffered severe peripheral nerve and spinal cord damage after PNB or epidural catheter placement under GA (5–7) have increased the awareness in the anesthesia community about the risk of this practice. The subsequent publication of two relatively large series of patients who underwent epidural catheter or PNB placement under GA with minimal (<1%) or no consequences (8,9) has done little to convince practitioners that it is safe to routinely perform nerve blocks under GA (10).

Pediatric anesthesiologists, because of the lack of cooperation from their patients and families, are obliged most of the times to perform regional anesthesia in anesthetized patients. The placement of a successful PNB depends on the precise localization of the nerve and delivery of the local anesthetic in close proximity to the nerve, while avoiding an intraneural injection. Before the introduction of ultrasound in regional anesthesia practice, the most reliable technique available to identify peripheral nerves was nerve stimulation (3,11), which is based on the motor response to electrical nerve stimulation. A current of 0.5 mA or less with a pulse width of 0.1 s is considered by most adult regional anesthesia practitioners as an acceptable indicator that the needle is close enough to a nerve for a successful nerve block (12). Although there is some evidence in adult regional anesthesia literature to support this recommendation (13–15), no study has determined the maximum current intensity associated with a successful block. Similarly, no study has determined a safe current intensity to use when performing PNB under GA to prevent the intraneural injection of local anesthetics.

The primary aim of this study was to find a correlation between the stimulation threshold used to perform PNB and success rate of the PNB in pediatric patients. The secondary aim was to determine the incidence of neurological complications after PNB conducted under GA in pediatric patients and its correlation with the stimulation threshold.

METHODS

After obtaining approval from our IRB, we reviewed the data of pediatric patients who received a single-injection PNB under GA with the aid of a peripheral nerve stimulator between October 2002 and July 2006 at The Children’s Hospital of Philadelphia by accessing the departmental regional anesthesia database and hospital records.

GA was induced either with inhaled anesthetics via mask or IV with propofol (3–5 mg/kg) and patients were either tracheally intubated or had a laryngeal mask airway placed without neuromuscular blockade. Anesthesia was maintained with desflurane or sevoflurane in 50% nitrous oxide in oxygen. PNB were performed by 1 of 11 pediatric anesthesia attendings or by a pediatric anesthesia fellow under the supervision of these attendings. PNB was performed with the aid of an insulated needle (Stimuplex, Braun Medical, Bethlehem, PA) of the appropriate gauge and length depending on the type of PNB performed (24 g 25 mm for interscalene nerve blocks, 22 g 50 mm for femoral nerve blocks and 21 g 100 mm for sciatic, lumbar plexus and infraclavicular nerve blocks). The needle was connected to the negative lead of a constant current nerve stimulator (Stimuplex HNS-11, B-Braun/McGaw Medical). The stimulation frequency used was 2 Hz with a pulse width of 0.1 ms. The initial current was set at 1–1.5 mA and, on observing the appropriate motor response, the stimulating current was decreased until loss of stimulation was obtained. The needle was manipulated to obtain a stimulation threshold at which the motor response was lost. This threshold was based on the preference of the individual anesthesiologist performing the block and was documented in every case. Two different local anesthetics, bupivacaine with epinephrine (1:200,000) (concentrations ranging between 0.1% and 0.25%) and plain ropivacaine (concentrations ranging between 0.1% and 0.2%), were used, depending on the anesthesiologists’ preferences.

All data collected was entered on a PNB datasheet the same day and then transferred to the departmental regional anesthesia database. Data collected included demographic information (age, sex); description of the PNB (type of nerve block, mA and local anesthetic used); postoperative and long-term follow-up data (presence, extent, and duration of sensory and motor block; administration of IV and oral opioids in the intra and postoperative period; length of hospital stay; complications from the PNB including prolonged numbness, dysesthesia, paresthesia; episodes of vomiting; use of ultrasound and fluoroscopy for placement of PNB; clinical data including vital signs and pain scores). The initial physical examination was conducted in the recovery room by nurse practitioners on the pain service or an anesthesiologist. The sensory examination was performed by evaluating patient response to either pinprick or cold sensation and comparing it with the same dermatome on the unblocked extremity. Clinical data were recorded by nurses every 4 h for the duration of the hospital stay. Patients discharged home were followed by the anesthesiologists or nurse practitioners in the pain management service with daily or twice daily phone calls until resolution of sensory block or possible side effects. Parents were asked to record the duration of sensory block and intake of oral opioids during the follow-up period and to report these data to the interviewer. Patients who experienced a motor block in the immediate postoperative period were discharged from the hospital only after complete resolution of the motor block.

A PNB was deemed to be unsuccessful when there was an incomplete or absent sensory block in the distribution of the nerve on physical examination in the postoperative period.

For an analysis, patients were divided into two groups based on the recommendations in adult literature of using a current intensity of ≤0.5 mA with a pulse width of 0.1 ms. The patients were further subdivided into subgroups based on the current intensity used for placing the PNB: 1) 0.2–0.3 mA and 2) 0.6–1 mA to evaluate differences in the success rate between these subgroups. The data were analyzed using Stata for Windows version 9 (Statacorp, College Station, TX). Outcomes between the groups were compared with {chi}2 or Fisher’s exact test for proportions. A logistic regression analysis was conducted to identify the relationship among mA, age, sex, type of block, and incidence of failed blocks. A logistic regression was also conducted to verify the relationship between mA used and nerve injury in our population.

RESULTS

Six-hundred sixty patients (344 men, 316 women) received a PNB at The Children’s Hospital of Philadelphia during the study period. The median age of the patients was 15 yr (IQR: 12–16 yr). The types of block performed are listed in Table 1. All the blocks were performed using a current ranging between 0.2 and 1 mA (median = 0.5 mA, IQR: 0.45–0.55 mA) (Fig. 1).


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  		Table 1. Type of Block

 

Figure 118
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  		Figure 1. Number of blocks by lowest current amplitude in mA at which a motor response could be obtained.

 

The overall success rate was 96%. There was no difference in success rate between blocks performed using ≤0.5 or >0.5 mA (96.3% vs 95.9%; P = 0.793) (Table 2, Fig. 2). We did not observe a difference in success rate after stratifying by type of block. The success rate was 100% when a current ranging between 0.2 and 0.3 mA was used. There was no statistically significant difference in success rate between blocks placed at 0.2–0.3 mA and those placed at 0.6–1.0 mA (100% vs 98%, P = 0.726). The logistic regression analysis showed that, among the different variables considered in the analysis (sex, age, type of block performed, mA used), the only predictive factor for a successful block was age (P = 0.02) (Table 3), with the failure rate decreasing in older patients (Table 4).


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  		Table 2. Number and Percentage of Success and Failure of Single Injection Peripheral Nerve Block Placed With Motor Response to Electrical Nerve Stimulation by the Current Used and P Value for the {chi}2 Test for Correlation (Fisher’s Exact Test for Correlation)

 

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  		Figure 2. Graph of the success rate of peripheral nerve block by lowest stimulation current at which motor response was obtained.

 

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  		Table 3. Logistic Regression Analysis

 

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  		Table 4. Age Distribution and Correspondent Failure Rate of the Peripheral Nerve Blocks

 

Paresthesia or dysesthesia was not reported by any patient. Two patients reported a prolonged sensory blockade of the great toe and dorsum of the foot after a sciatic nerve block, which lasted for 72 h. These blocks were done using threshold stimulations of 0.4 and 0.5 mA, respectively, and no correlation was found between the mA used and the presence of prolonged numbness.

DISCUSSION

This study failed to show a significant difference between success rate of a PNB and the stimulation threshold used in ranges between 0.2 and 1.0 mA. The issue of minimum current intensity used when placing a PNB with a nerve stimulator is important because it is a surrogate measurement of the proximity of the block needle to the nerve. This information is essential to place a successful block and at the same time prevent nerve injury, particularly when performing PNB in pediatric patients. However, there are no clear data on the ideal stimulation threshold to be used when performing PNB.

Current recommendations are to choose a stimulation level under a threshold of 0.5 mA (16,17). However, in a study on the optimal stimulation threshold for blocking the brachial plexus, the authors reported a success rate of 95% in their patients when using a stimulation threshold between 0.6 and 0.8 mA (14).

The use of ultrasound, which allows for a direct visualization of the distance between the block needle and the nerve, has further complicated the interpretation of clinical findings. In a prospective study, there was only a 74% correlation between motor responses with electrical stimulation at <0.5 mA and needle contact with the nerve on ultrasound examination (18), showing that, in many patients, contact with the nerve is not sufficient to elicit a motor response at current levels <0.5 mA. The authors also reported that it was necessary to increase the stimulation current up to 1.0 mA to obtain a motor response despite ultrasound images showing contact between the block needle and the nerve. They concluded that this finding has potential implication for safety. This imaging study seems to confirm clinical data, which have shown that the elicitation of paresthesia with a block needle is associated to a motor response in 30% of adult patients, despite using a current intensity of 1 mA (19). At the same time, it was not possible to elicit a motor response at <0.5 mA in 23% of the patients, despite the presence of paresthesia (20).

A possible explanation of these findings is an unequal distribution of motor and sensory nerve fibers within the nerve bundle, with the sensory fibers located on the outer mantle. This seems to contradict Winnie’s description of the distribution of motor and sensory nerve fibers within the peripheral nerves, with the motor fibers located on the outer mantle (21).

Inhaled anesthetics have been shown to enhance the neuromuscular blockade produced by nondepolarizing muscle relaxants (22–24). It is therefore possible that a higher stimulation threshold (>0.5 mA, 0.1 s) in patients under inhaled GA, compared with those awake or sedated, may be necessary to elicit a similar motor response. This could explain the high success rate (similar to adult studies) observed in our study with stimulation threshold (>0.5 mA, 0.1 s). Future prospective studies will be needed to clarify this issue. It should be emphasized that in all these studies, the reported clinical success rate has been more than 95%, independently from the intensity of the stimulating current used.

Clinical data suggest that reliance on a nerve stimulator for performance of a PNB does not eliminate the potential for nerve injury (4,25,26), and the presence of heavy sedation or GA may be a significant risk to adult patients (27). However, contrary to a significant amount of data on the relationship between current intensity, location of stimulating needle and success rate of PNB, there are no data in the literature on the relationship between current intensity, location of stimulating needle, and nerve injuries.

The incidence of nerve injury after PNB varies, depending on the type of nerve blocked and the technique used to place the block, and it ranges from 1% to 4.5%, with the highest reported incidence of paresthesia after interscalene nerve blocks (2,28–31).

The issue of identifying the precise relationship between the nerve and block needle is essential when performing a PNB in anesthetized patients. The key clinical signs that indicate a possible intraneural injection, like paresthesia from contact with the sensory fibers of the nerve and worsening of the paresthesia or pain during injection of the local anesthetic, may not be evident to the anesthesiologist when the patient is under GA.

The prolonged sensory blockade seen in two patients occurred after sciatic nerve blocks. The stimulation threshold in both cases was ≤0.5 mA. No long-term sequelae were noted in any of our patients. In a large prospective study conducted in France, no complications related to PNB were observed in more than 1200 pediatric patients (9). The PNB were performed with patients under GA in 89% of the cases. In another prospective survey that evaluated the incidence of complications from PNB done in awake adults, the authors found 12 cases of peripheral neuropathy (0.0002%) after PNB. Two cases were associated with a motor response with electrical stimulation at <0.5 mA, and three with a paresthesia at the time of injection (32). The exact cause of the peripheral neuropathy could not be determined.

In our regression model, age had a positive correlation with PNB success, i.e., with increasing age there was an increase in success rate of PNB. The actual success rate by age groups was about 93% in the 2–10 yr, 96.6% in the 11–15 yr, and 97.26% in 16–18 yr. A possible explanation for this could be the relative difficulty in locating a small nerve using a nerve stimulator in younger pediatric patients while performing PNB.

The limitations of this retrospective study are the lack of a control group and the absence of pain scores during rest and movement.

In conclusion, this study finds a similar success rate for PNB in anesthetized pediatric patients when using a lower stimulation threshold (*0.5 mA) or higher stimulation threshold (>0.5 mA). Therefore it may not be necessary to perform needle manipulation in close proximity to the nerve to achieve a low stimulation threshold (<0.5 mA), as it may increase the risk of intraneural injection. However, our data failed to clarify the safe stimulation threshold at which intraneural injection of local anesthetic can be avoided. Further, large prospective multicenter studies will be needed to evaluate the risk of complications from PNB performed under GA in pediatric patients.

Footnotes

Accepted for publication August 8, 2007.

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Lippincott, Williams & Wilkins Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins and Stanford University Libraries' HighWire Press®. Copyright 2007 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999 HighWire Press