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REGIONAL ANESTHESIA

The Effect of Age on Sciatic Nerve Block Duration

R. Kyle Hanks, BS^*, Ricardo Pietrobon, MD, PhD, Karen C. Nielsen, MD, Susan M. Steele, MD, Marcy Tucker, MD, PhD, David S. Warner, MD, Kathryn P. King, MD, and Stephen M. Klein, MD

*School of Medicine, and Departments of Anesthesiology and Surgery, Duke University Medical Center, Durham, North Carolina

Address correspondence and reprint requests to Stephen M. Klein, MD, Box 3094, Department of Anesthesiology, Duke University Medical Center, Durham, NC 27710. Address e-mail to klein006{at}mc.duke.edu.

	Abstract

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The physiologic changes that occur with advancing age and their effect on the duration of peripheral nerve blocks have yet to be defined. We prospectively studied the duration of sciatic nerve block using mepivacaine in younger and older patients. Eighty ASA physical status I-III patients, aged 18–35 (n = 40) or 55–80 (n = 40) yr, having outpatient knee arthroscopy with a femoral block and a standardized sciatic nerve block were enrolled; 37 in each group completed the study. All patients received a Labat sciatic nerve block using 20 mL of 1.0% mepivacaine with 0.1 mEq/mL sodium bicarbonate and 1:400,000 (2.5 µg/mL) epinephrine and a femoral nerve block. The duration of sensory block (sensation of pinprick, temperature, and vibration), motor block (plantar and dorsi flexion), and complete sensory and motor block in the sciatic nerve distribution of the operative extremity were measured. The time for complete return of both sensory and motor function was longer in the older group, 329 ± 47 min compared with 306 ± 46 min (mean ± sd) in the younger group (P = 0.04). The difference was small under the conditions of this study and would not be perceived clinically. Age also increased the time to return of vibratory sensation (younger = 292 ± 58 min, older = 257 ± 50 min; P = 0.007). The other measurements did not differ between groups. We conclude that age may affect peripheral nerve blocks and that more investigation is needed to determine the pharmacologic, physiologic, and chronologic factors behind these findings.

	Introduction

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Age-related physiologic changes in the central and peripheral nervous systems may have a direct effect on the clinical duration of peripheral nerve blocks. For example, there is a decrease in the number and density of nerve fibers (1), a degeneration of axons (2), and an increase in motor unit action potentials with increased age (3). The effect of these changes was demonstrated by Dorfman and Bosley (4), who measured sensory and motor conduction velocities of peripheral (median) nerves in elderly individuals and found an age-dependent decrease in conduction velocity of both motor (–0.15 m/s per yr) and sensory (–0.16 m/s per yr) fibers. Clinically, this was supported by Paqueron et al. (5) who compared the duration of a brachial plexus block using a small volume of 0.75% ropivacaine in older (mean age, 77 yr) and younger (mean age, 39 yr) populations and reported that the duration of complete motor and sensory block was approximately 2.5 times longer in the older group. If there were a similar prolongation after a sciatic block with mepivacaine, this could potentially impede older patients’ ability to walk or place them at an increased risk of falling. These data in the elderly are also essential not only to tailor the appropriate dose and duration to an individual but also to differentiate a prolonged block from an unexpected adverse outcome that requires further investigation.

Currently, there is limited additional information about the duration of peripheral nerve blocks in older patients (5), and data about the effect of aging on the duration of a sciatic nerve block are unavailable. We present a prospective study investigating the duration of sciatic nerve blockade using 1% mepivacaine in younger and older groups of patients undergoing outpatient knee arthroscopy.

	Methods

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The Duke University Medical Center IRB approved the study protocol and written informed consent was obtained from all participants. Eighty ASA physical status I–III subjects 18–35 or 55–80 yr old scheduled for unilateral knee arthroscopy performed under a femoral nerve block and a standardized sciatic nerve block were recruited. Subjects were excluded if they had diabetes mellitus or peripheral neuropathies that would contraindicate peripheral nerve block or if they had a central nervous system disorder affecting their ability to detect peripheral stimuli (e.g., multiple sclerosis, Charcot-Marie Tooth Disease). Patients were divided into two age groups that could be readily studied in our outpatient facility. An upper age limit for the younger group was arbitrarily chosen to be 35 yr. The lower age limit of the older group (55 yr) was influenced by the available population at our center. Patients of advanced age (>70 yr) require arthroscopic knee surgery less often. The two groups were as follows: younger group: 18–35 yr old; older group: 55–80 yr old.

Immediately before neural blockade, baseline motor function and sensation along the L4 – S1 dermatomes of the operative extremity were assessed. Sensation to pinprick (2 = sensation to sharpness, 1 = sensation to touch only, 0 = no sensation at all) using a sharp piece of wood, vibration (1 = vibration sensation, 0 = no vibration sensation) perception using a 128 Hz weighted aluminum tuning fork placed onto the bony prominence of the lateral malleolus, and temperature (2 = feels cold, 1 = feels touch only, 0 = no sensation) using an alcohol wipe were assessed. The strengths of plantar flexion and dorsiflexion were assessed using a standardized scale (0/5 = no visible muscle movement, 1/5 = visible muscle movement but no movement at the joint, 2/5 = movement of the joint but not against gravity, 3/5 = movement against gravity but not against added resistance, 4/5 = movement against resistance but less than normal, 5/5 = normal strength).

All patients were brought to a preoperative holding area, and standard ASA monitors were used during block placement. Premedication consisted of titrated doses of IV midazolam (1–5 mg) and fentanyl (1–5 µg/kg) for moderate sedation (arousable on command) and comfort. Using a nerve simulator (2 Hz, 0.1 µs) initially set at 1.0 mA and a 21-gauge, 100-mm insulated blunt needle (B. Braun Medical, Bethlehem, PA), sciatic nerve block was performed by a faculty member experienced with the procedure using the technique initially described by Labat (6). This type of block was chosen because it could readily be studied in our outpatient facility using mepivacaine and was expected to only moderately increase length of stay. After an appropriate motor response was localized (plantar flexion or dorsiflexion with a current <0.5 mA), 20 mL of 1.0% mepivacaine with 0.1 mEq/mL sodium bicarbonate and 1:400,000 (2.5 µg/mL) epinephrine was injected incrementally. The completion of local anesthetic injection for the sciatic nerve block was recorded as Time 0 for clinical assessments. To provide adequate anesthesia for the knee arthroscopy a femoral nerve block, not involved in the assessments, was also placed. Sensory and motor evaluation of this block was not performed because different local anesthetics (mepivacaine and ropivacaine) are frequently used in our practice, making standardization difficult.

Patients were taken to the operating room after evidence of a successful sciatic nerve motor block. Block failure was defined as residual plantar flexion or dorsiflexion after 30 min and these patients were excluded from analysis. During surgery, no additional local anesthetics were administered. Intraoperatively, an IV propofol infusion of 10–50 µg · kg^-1 · min^–1 titrated to moderate sedation (arousable to command) was administered as appropriate. On completion of surgery, patients were transferred to the postanesthesia care unit. The neurological examination, similar to that performed before surgery, was repeated with serial testing every 15 min until the patient returned completely to their baseline neurological status. Both pre-block and serial postoperative neurologic assessments were performed by the same individual.

Sample size estimation was based on the assumption that a duration difference of 20 min in complete return of sensory and motor function would be detectable with the study design. Based on a standard deviation of 40 min, a two-tailed was set at 0.05 with a power of 80%. The 40-min value was consistent with prior studies using mepivacaine (7,8) and our previous clinical experience. Based on these assumptions, a total requirement of 64 patients (32 per group) was estimated for the study. Eight patients were added to each group to account for block failures or patient dropouts. Values are reported as mean ± sd. Normally distributed parametric data (age, height, weight, and ASA physical status) were analyzed with Student’s t-test. The ² test was used to evaluate the difference in gender between groups. After testing for normal distribution, duration of complete sensory block, duration of sensory/pinprick block, duration of sensory/vibration block, duration of sensory/temperature block, duration of complete motor block, and duration of complete sensory and motor block having a normal distribution were compared using an unpaired two-sample Student’s t-test with equal variances. Linear regression analysis was used to measure the association between the subject’s age and total duration of block. A P value 0.05 was considered significant.

	Results

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Eighty subjects were enrolled in the study. Seventy-four subjects completed the study protocol: 37 in each group. One subject in the older group was excluded from analysis because of a failed block. Five subjects, three from the younger group and two from the older group, chose to withdraw from the study before the complete resolution of the sciatic block was achieved; their data were excluded from analysis. The mean age of the younger group was 27 ± 5 yr and the mean of the older group was 63 ± 6 yr. There were no statistically significant differences in ASA classification, gender, height, or weight between groups (younger/older: gender [m:f] 15:22/17:20, height [cm] 173 ± 9/166 ± 17, weight [kg] 84 ± 29/84 ± 18).

Complete return of all sensory and motor function, was significantly longer in the older group versus younger group (P = 0.04) (Table 1) (Fig. 1). Return of vibratory sensation was also significantly longer in the older group compared with the younger group (P = 0.007) (Table 1) (Fig. 2).

View larger version (19K): [in this window] [in a new window]   Figure 1. Scatter plots of the total duration of sciatic block (return of both motor and sensory function) in younger (18–35 yr of age) and older (55–80 yr of age) subjects performed with 20 mL of 1% mepivacaine with 0.1 mEq/mL sodium bicarbonate and 1:400,000 (2.5 µg/mL) epinephrine. The horizontal line at the middle indicates the mean. P = 0.04

View larger version (19K): [in this window] [in a new window]   Figure 2. Scatter plots of the return of vibration perception in younger (18–35 yr of age) and older (55–80 yr of age) subjects performed with 20 mL of 1% mepivacaine with 0.1 mEq/mL sodium bicarbonate and 1:400,000 (2.5 µg/mL) epinephrine. The horizontal line at the middle indicates the mean. P = 0.007

The additional measurements, recovery of pinprick, temperature perception, complete sensory function, plantar flexion, dorsiflexion, and complete motor function (in both components) all demonstrated a trend in duration toward the older group but none was statistically significant (Table 1). Linear regression analysis also did not demonstrate a significant association between individual chronologic age and individual block duration (P = 0.27).

	Discussion

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This study demonstrated that the overall duration of a sciatic nerve block with 1.0% mepivacaine was prolonged in older people by an average of 23 minutes. The greatest difference was in the delay in the perception of vibration. This effect was present even with a small milligram dose of 1% mepivacaine, an intermediate-acting local anesthetic. Nevertheless, the difference was small under the conditions of this study and would be difficult to perceive in daily practice.

There are well documented age-related physiologic changes that occur in the central and peripheral nervous systems. Studies have demonstrated that aging affects the anatomy, pharmacokinetics, and pharmacodynamics of local anesthetics used in central neuraxial blockade (9–12), as well as the structural and functional properties of the peripheral nervous system. There is loss of both myelinated and unmyelinated nerve fibers (13,14), potentially rendering peripheral nerves more sensitive to the effects of local anesthetics. Decreased expression of myelin proteins may lead to a deterioration of myelin sheaths (15). A reduction in the expression and axonal transport of cytoskeletal proteins may lead to axonal atropy, often seen in older nerves (16). Age-dependent changes in sodium and potassium channel function are poorly characterized (13). Aging has well-documented effects on the functional and electrophysiologic properties of peripheral nerves, all of which may impact on the response to local anesthetics. These include a decline in nerve conduction velocity, muscle strength, sensory discrimination, autonomic responses, and endoneurial blood flow (15). In a clinical model, Lafratta and Canestrari (17) studied sensory and motor action potentials in the median nerve in 23- to 91-year-old males. They demonstrated a greater diminution of sensory nerve conduction velocity compared with motor conduction velocity and a higher magnitude of sensory latency compared with motor latency with advancing age.

The results of our study do not demonstrate the same magnitude of difference in duration detected by Paqueron et al. (5), who reported that complete motor and sensory block lasted approximately 2.5-times longer in older patients after a brachial plexus block in which a small volume of ropivacaine was used. Several differences in the study design may account for this discrepancy. The most likely difference was the use of mepivacaine instead of ropivacaine. Mepivacaine is an intermediate-acting local anesthetic, chosen here to avoid a prolonged motor block in this series of patients scheduled for same-day discharge. In general, mepivacaine has a predictable and rapid regression in contrast to long-acting drugs, where offset is longer and more variable and theoretically could amplify the physiologic impacts of increasing age

Another reason that the results in our study may have differed from Paqueron et al. (5) may have been our inclusion of fewer patients of advanced age (>70 years). Conceptually, some of the physiologic changes that may play a role in anesthetic duration, such as an increased susceptibility to block or local anesthetic molecules persisting longer at their binding sites, may not manifest directly with chronologic age. These constraints may have limited the ability to demonstrate a correlation between individual ages and block duration. Finally, each individual injection site may be affected differently by age. Hence, the response of individual nerves from the brachial plexus may differ from that of the sciatic nerve. At the same time, the shared findings that both upper and lower extremity block duration is dependant on age indicate a common physiologic effect of aging in these two components of the peripheral nervous system that is likely also common to different local anesthetics.

Although the use of a single evaluator for both the baseline assessments and the subsequent examinations diminished the effect of inter-examiner interpretation of function, the inability to realistically mask the observer from the age groups prohibited blinding and may have introduced bias. Ultimately, the duration differences between the groups were small and the majority of variables did not differ significantly. It is possible that age does not affect the duration of other sensory or motor variables or that the effects are only manifested in an older population. A larger sample size may have increased the ability to confirm whether these differences exist.

A growing body of evidence indicates that normal and pathologic changes in the central and peripheral nervous system may impact the duration and effects of local anesthetics in elderly patients. The data in our study substantiate this and demonstrate that the duration of sciatic nerve block is prolonged in older compared with similar younger patients. However, the difference is too small to be detected in daily practice, suggesting that clinical management based on duration should not be altered in this group of older patients. Further study is warranted to investigate the underlying biologic mechanisms and to determine whether these changes are manifested with different local anesthetic solutions at different anatomic sites.

	Footnotes

 
Presented, in part, as an abstract at the American Society of Anesthesiologists Annual Meeting, San Francisco, California, October, 2003.

Accepted for publication September 2, 2005.

	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 Google Scholar Google Scholar Articles by Hanks, R. K. Articles by Klein, S. M. Search for Related Content PubMed PubMed Citation Articles by Hanks, R. K. Articles by Klein, S. M.