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REGIONAL ANESTHESIA AND PAIN MANAGEMENT

Concentration of Lidocaine Affects Intensity of Sensory Block During Lumbar Epidural Anesthesia

Department of AnesthesiologyShimane Medical University, Izumo City, Japan

Address correspondence to Shinichi Sakura, MD, Department of Anesthesiology, Shimane Medical University, 89-1 Enya-cho, Izumo City, 693-8501 Japan. Address e-mail to ssakura{at}shimane-med.ac.jp

	Abstract

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We investigated the effects of a twofold difference in concentration and volume of lidocaine on lumbar epidural block using a cutaneous current perception threshold (CPT) quantitative sensory testing device. Twenty ASA I patients scheduled for elective gynecological surgery were randomly divided into two equal groups to receive either 20 mL of 1% lidocaine or 10 mL of 2% lidocaine through an epidural catheter inserted at the L1-2 interspace. CPTs at 2000-, 250-, and 5-Hz stimulation and sensation to light touch, temperature, and pinprick at ipsilateral dermatomes V, T9, and L2 were measured before and every 5 min until 60 min after the epidural lidocaine. Epidural anesthesia with both solutions produced a significant increase in all CPTs at dermatomes T9 and L2. Alterations in CPTs were similar for both groups at T9 but were significantly greater in patients given 2% lidocaine than in those given 1% lidocaine at L2. There were no differences in the upper level of sensory block to cold, pinprick, and touch between the two groups. We conclude that lumbar epidural anesthesia with 10 mL of 2% lidocaine produces more intense blockade of both large- and small-diameter sensory nerve fibers than that with 20 mL of 1% lidocaine.

Implications: The effects of local anesthetic concentration and volume on the quality of epidural anesthesia have not been adequately investigated. The results of the present study suggest that the concentration affects the intensity of sensory block during epidural anesthesia with lidocaine.

	Introduction

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Despite a long history of using epidural anesthesia in clinical practice, the relative effects of the mass, volume, and concentration of local anesthetic solutions on the development of this anesthesia are still unclear. Although increasing the concentration of local anesthetics has been believed to improve the quality of an epidural block (e.g., rate of onset, degree of motor block, duration), clinical studies have shown inconsistent results, probably because of minimal consideration of total dose and lack of appropriate measurements (1–3). It is unclear whether concentration has any influence on the intensity of sensory block during epidural anesthesia when the total dose is held constant. In addition, a recent clinical study used 2-chloroprocaine (4), whose physicochemical properties are different from other commonly used local anesthetics.

Recent technological advances allow quantitative measurement of the functional integrity of both large- and small-diameter sensory nerve fibers using a cutaneous current perception threshold (CPT) quantitative sensory testing device (5,6). CPTs are defined as the minimal amount of current applied transcutaneously that an individual consciously perceives. Using this device, we previously demonstrated that the dose of lidocaine administered for lumbar epidural anesthesia affected the intensity of sensory blockade of different types of nerve fibers (7). Accordingly, we performed the present study to examine effects of a twofold difference in concentration and volume of a fixed mass of lidocaine on the quality of lumbar epidural block selectively and quantitatively.

	Methods

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With institutional review board approval and written, informed consent, we studied 20 unpremedicated ASA physical status I patients presenting for elective gynecological surgery. None was taking regular medications or had evidence of neurological disease.

The study was performed in a quiet room on the day of surgery. Noninvasive arterial blood pressure, heart rate, and pulse oximetry were monitored continuously throughout the study. After local anesthesia with 0.1 mL of 1% lidocaine, an IV infusion of acetated Ringer’s solution was started in a hand or forearm vein at a rate of 10 mL · kg^–1 · h^–1. Patients were then placed in the left lateral decubitus position, and the skin, subcutaneous tissue, and supraspinous ligament were anesthetized with 2 mL of 1% lidocaine. The epidural space was identified with a 18-gauge Tuohy needle with the bevel directed cephalad via the midline approach at the L1-2 vertebral interspace, and a catheter was advanced 5 cm into the epidural space. The catheter was aspirated to exclude intrathecal or IV placement and then secured. The patient was then returned to the supine position.

Patients were trained to perceive perception thresholds to transcutaneous constant sine wave electrical currents at 2000, 250, and 5 Hz with a commercially available device (Neurometer CPT; Neurotron, Inc., Baltimore, MD) before the baseline measurements began. The current was delivered to the skin by a pair of 1-cm diameter gold surface electrodes that were separated by 1.7 cm with a clear Mylar spreader (Neurotron, Inc.). The stimulating surface of each electrode was covered by a thin layer of electroconductive gel. CPT testing was performed at V (the trigeminal area; in front of the ear), T9 (above umbilicus on the anterior axillary line), and L2 (above knee) dermatomes, on the left side with the rapid screening procedure. Perception thresholds at each frequency were determined by stepwise increases in the current (controlled by the device) to a maximum of 9.99 mA until the patient reported sensation. The current was terminated, then repeated. The procedure for all the three frequencies at all the test sites required approximately 4–5 min.

Patients were randomly divided into two equal groups to receive 20 mL of 1% lidocaine (Group I) or 10 mL of 2% lidocaine (Group II). After baseline measurements had been taken, patients were given the local anesthetic through the epidural catheter, and the CPTs and the dermatomal levels of blockade of light touch, temperature, and pinprick discrimination were measured every 5 min until 60 min after the epidural lidocaine by an investigator blinded to the local anesthetic solution injected. The loss of each sensory modality was determined by the patient’s verbal response to the stimulus applied to the left anterior axillary line starting caudad and moving cephalad. The dull, hinged end of a sterile safety pin was used to examine light touch; an alcohol-soaked swab was used for temperature; and the sharp tip of a safety pin was used for pinprick.

Results are expressed as mean ± SD unless otherwise stated. CPT data were converted to percent maximal possible effect (%MPE), calculated as [(CPT value – baseline)/(cutoff – baseline)] x 100. Continuous variables including CPT data were compared using repeated-measures analysis of variance and Dunnet’s test or Student’s t-tests for post hoc testing. The Mann-Whitney U-test was used to determine differences in the maximal level of sensory block between the groups, and the Friedman test and Wilcoxon signed rank test with Bonferroni’s adjustment were used for comparisons among the three modalities in each group. P < 0.05 was considered significant.

	Results

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Patient characteristics and baseline CPTs did not differ between the two study groups (Table 1). All the patients had CPT values within the normal range before epidural lidocaine was given.

View larger version (21K): [in this window] [in a new window]   Figure 1. Time course of effect of epidural block with 20 mL of 1% lidocaine (Group I) or 10 mL of 2% lidocaine (Group II) on current perception thresholds (CPTs) at T9 and L2. The CPT neurometer selectively stimulates Aβ, A, and C fibers using a 2000-, 250-, and 5-Hz stimulus, respectively. CPT values are expressed as percent maximum possible effect (%MPE), where %MPE = [(CPT value – baseline)/(cutoff – baseline)] x 100. Data reflect the mean ± SE. *P < 0.05 versus baseline. P < 0.05 versus Group I.

View larger version (19K): [in this window] [in a new window]   Figure 2. Maximal cephalad spread of epidural block with 20 mL of 1% lidocaine or 10 mL of 2% lidocaine as determined by loss of sensation to cold, pinprick, and touch (n = 10). Boxplot displays median and 10th, 25th, 75th, and 90th percentiles for values. *P < 0.05 versus cold and pinprick sensation.

After completion of the study, all the patients were given 10–15 mL of 2% mepivacaine, which produced satisfactory anesthesia for the surgery proposed.

	Discussion

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The results of the present study demonstrate that altering the concentration and volume of lidocaine solutions resulted in different intensities of sensory block after lumbar epidural lidocaine even when the total dose was administered identically. Despite no differences in the extent of sensory block to cold, pinprick, and touch, 10 mL of 2% lidocaine produced a more intense sensory block than 20 mL of 1% lidocaine, with all three CPT values at L2 increasing to a greater degree in patients given the former solution.

Increasing the total dose of local anesthetic provides greater quality of blockade during epidural anesthesia. This is based on the data of previous clinical studies investigating the effects of increasing concentrations of local anesthetics on epidural anesthesia with the volume held constant. For example, Scott et al. (1) found that an increase in the concentration of bupivacaine from 0.5% to 0.75% and etidocaine from 1.0% to 1.5% for epidural block resulted in a more rapid onset of sensory analgesia and motor blockade, greater frequency of adequate analgesia, greater depth of motor block, and longer duration of sensory analgesia and motor blockade. An animal study performed by Cusick et al. (8) confirmed these clinical findings by demonstrating that varying masses of epidurally administered bupivacaine induced alterations in the sites and magnitude of evoked potential response using electrodes positioned along the conducting pathways of the monkey.

In contrast, the effects of increasing the concentration without changing the total dose of local anesthetics on the quality of epidural anesthesia did not seem to be as evident in the literature. Although Cusick et al. (8) demonstrated that an increased concentration influences the local anesthetic’s penetration at the dorsal root entry zone at the lower levels of total drug mass, previous clinical studies have revealed little importance of concentration. For example, Duggan et al. (2) showed that onset and recovery intervals were similar with 0.75% and 0.5% bupivacaine solutions when administered at the same dose. However, they only used the qualitative examination of pinprick sensation, which may have made their results less conclusive. Liu et al. (4) compared the effects of a threefold difference in concentration and volume of 2-chloroprocaine using volunteers. They assessed the quality of epidural block using tolerance to transcutaneous electrical nerve stimulation and found a similar intensity of sensory block to electric stimulation and motor block with the two different concentrations. However, because 2-chloroprocaine has unique physicochemical properties (9), their results may not hold true for all commonly used local anesthetics.

Quantitative and direct measurement of the functional integrity of sensory nerve fibers is now possible with the use of CPT testing (5,6). The electrical stimulus selectively excites distinct subpopulations of nerve fibers as a function of the sinusoid frequency. Correlation between low-frequency stimulation and activation of small nerve fibers and that between higher frequency stimulation and activation of large nerve fibers has been demonstrated with thermal perception threshold testing and quantitative vibration testing, respectively (10). This technology has been validated as a useful tool in the clinical evaluation and management of a variety of neurologic disorders (11). For example, Liu et al. (12) observed a sensory block after lidocaine spinal anesthesia and demonstrated that recovery of CPTs at 5, 250, and 2000 Hz was correlated with regression of sensory block to cold, pinprick, and touch sensation.

The results of the present study indicate that the concentration of lidocaine affects the intensity of sensory blockade of both small- and large-diameter nerve fibers during lumbar epidural anesthesia. Using this technique, we previously demonstrated that the dose of lidocaine was also a determinant of intensity of epidural anesthesia (7). Because we did not examine the effect of the volume of local anesthetic solutions in the present study, in which a group of subjects receiving 10 mL of 1% lidocaine was not included, the question of whether the dose or the concentration of lidocaine has more impact on the quality of resultant epidural anesthesia remains to be determined.

Although there was a suggestion that the two solutions produced different alterations in CPTs at T9 and L2, statistical significance was achieved only at the latter site. There may have been a difference in the concentration of lidocaine in a place closer to the site of epidural injection, producing the difference in %MPE between the two solutions. In contrast, at T9, which was remote from the site of injection, the difference in the concentration of lidocaine may have been small, resulting in similar neural blockade for the two solutions. However, the small number of study subjects may have been responsible for the lack of statistical significance at the latter site.

The findings that no differences were found between the two solutions in the spread of loss of three sensory modalities in the present study were somewhat surprising. Liu et al. (4) demonstrated that a high volume of low-concentration solution of 2-chloroprocaine produced a greater extent of sensory block. The following may account for the difference between the results of their study and those of the present study. First, we used a different anesthetic and dose for epidural anesthesia. Second, there may be limitations to these qualitative measurements, which were also used in a different way between the two studies. Loss of sensation was defined as no sensation elicited with the three different stimuli in the present study, whereas a dermatome was considered free of sensory block if sensation was reported to be the same as the reference sensation in their study. Finally, statistical power may have been insufficient to detect a significant difference in the extent of sensory block, in which a wide individual variability is commonly seen.

Differential sensory block was observed with 1% lidocaine. The level of touch sensation was significantly lower than that of other sensations. This finding is consistent with that of other studies (7,13,14), as well as current theories of differential sensory block during epidural anesthesia (15).

In clinical practice, in which repeated and/or long-time administrations may be required, the total dose of local anesthetics should be held low because of the potential for systemic toxicity. Thus, the results of the present study lead us to recommend using a high concentration and low volume of local anesthetic solution when profound neural blockade is required.

In summary, using a new technology involving electrical stimulation, we found that lumbar epidural anesthesia with 10 mL of 2% lidocaine produced more intense blockade of both large- and small-diameter sensory nerve fibers than that with 20 mL of 1% lidocaine. These data suggest that the concentration affects the intensity of sensory block during epidural anesthesia with lidocaine.

	Acknowledgments

 
This study was supported by Grant 10770754 from the Ministry of Education, Science and Culture, Japan.

	References

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a colleague Similar articles in this journal Similar articles in Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Web of Science (14) Citing Articles via Google Scholar Google Scholar Articles by Sakura, S. Articles by Kosaka, Y. Search for Related Content PubMed PubMed Citation Articles by Sakura, S. Articles by Kosaka, Y.