Anesth Analg 2004;99:140-145
© 2004 International Anesthesia Research Society
doi: 10.1213/01.ANE.0000114181.69204.72
TECHNOLOGY, COMPUTING, AND SIMULATION
Preoperative Sciatic Nerve Block Decreases Mechanical Allodynia More in Young Rats: Is Preemptive Analgesia Developmentally Modulated?
Douglas G. Ririe, MD,
David Barclay, MD,
Heather Prout, BA,
Chuanyo Tong, MD,
Joseph R. Tobin, MD, and
James C. Eisenach, MD
Department of Anesthesiology and Center for the Study of Pharmacologic Plasticity in the Presence of Pain, Wake Forest University School of Medicine, Winston-Salem, North Carolina
Address correspondence and reprint requests to Douglas G. Ririe, MD, Department of Anesthesiology, Wake Forest University School of Medicine, Medical Center Blvd., Winston-Salem, NC 27157-1009. Address e-mail to dririe{at}wfubmc.edu
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Abstract
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Postoperative sensitivity to tactile stimuli differs as a function of age. In this study, we hypothesized that preoperative sciatic nerve block (SNB), by providing preemptive analgesia, would result in better analgesia than postoperative SNB in the young rat. With the paw incision model of postoperative pain, male Sprague-Dawley rats, aged 2 or 4 wk, underwent general anesthesia and then received a left SNB with 5 µL/g of 0.5% bupivacaine or normal saline. SNB was performed either before or after surgery. Mechanical allodynia was assessed by using von Frey filaments before and at various times after SNB and surgery. In the 2-wk-old rats, preoperative SNB produced a significant reduction in mechanical allodynia, as reflected by a higher threshold at 2, 5, and 24 h when compared with saline control (P < 0.03). At 24 h, the threshold was 4.0 ± 0.7 g in the preoperative SNB group compared with 1.6 ± 0.3 g in the postoperative SNB group (P = 0.004). There was no difference at any time point between the preoperative and the postoperative SNB in the 4-wk-old animals. These results suggest that preoperative SNB in young animals provides a preemptive analgesic effect on mechanical allodynia that is age or developmentally dependent.
IMPLICATIONS: With a model of postoperative pain, younger animals demonstrated preemptive analgesic effects from sciatic nerve block, whereas older animals did not. These data in rats suggest that preemptive analgesia in the postoperative model may be more effective in the young.
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Introduction
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Incision in the hindpaw of the rat results in reproducible spontaneous and evoked pain behaviors and is a reliable model for acute postoperative pain (1). This model has been used to characterize the physiology of postoperative pain and the pharmacology of postoperative analgesia (1–3). Using this model, we have established that rats respond to incision differently as a function of age (4). In particular, 2-wk-old animals recover more rapidly than older animals after paw incision.
The literature is replete with evidence both for and against a preemptive analgesic effect. Although many studies that have examined preemptive analgesia in animals have suggested that there is a preemptive effect, the human clinical data are less conclusive (5,6). This is most likely related to the varied definitions of preemptive analgesia and the end-points used to determine its efficacy (7). Of clinically used treatments, regional anesthesia seems to be the most consistent for providing preemptive analgesia (5).
Few studies have investigated preemptive analgesia with regional techniques in children. Langer et al. (8), using ilioinguinal/hypogastric nerve block in infants for inguinal hernia repair, demonstrated a beneficial effect of preoperative local anesthetic nerve blockade. Caudal local anesthesia has also been shown to have a preemptive analgesic effect (9). However, axillary block failed to demonstrate a similar efficacy (10). The effects of reducing pain in a preemptive fashion may be even more important early in development (11,12). However, the role of development in preemptive analgesia and in modulation of nociceptive input from neural blockade has received little attention.
In this study, we hypothesized that preoperative sciatic nerve block (SNB) would provide better analgesia than postoperative SNB in the young rat with hindpaw incision surgery. We further hypothesized that this effect would be developmentally modulated, with the younger animals demonstrating a greater reduction in pain thresholds after regional nerve block than the older animals.
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Methods
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After review and approval from the Animal Care and Use Committee, male Sprague-Dawley rats were studied. Two ages of animals were studied: 2 wk (n = 55) and 4 wk (n = 44), representing preweanling and postweanling animals.
After baseline testing, 2- and 4-wk-old animals were anesthetized with 2% halothane and oxygen under spontaneous ventilation through a nose cone. The protocols were as follows for the SNB: 1) preoperative group—general anesthesia (GA) with halothane, SNB, 20-min recovery, GA with halothane, paw incision, recovery, and testing; 2) postoperative group—GA with halothane, 20-min recovery, GA halothane, paw incision, SNB, recovery, and testing. Animals underwent SNB as previously described (13,14). Briefly, the area was prepared with alcohol, and the index finger was placed between the iliac crest and the greater trochanter with the thumb on the knee. A 30-gauge needle was placed at the midpoint between the fingers and toward the femur until bone was contacted. The needle was then withdrawn just slightly from the bone, and after negative aspiration, 5 µL/g of either 0.5% bupivacaine (BUP) or normal saline (NS) was injected. The animals were placed in cages under a warming light. As previously described (1,4), the plantar aspect of one hindpaw was prepared in a sterile manner with a 10% povidone-iodine solution. With a sterile technique, a No. 11 scalpel was used to make a midline linear incision from the heel to the base of the toes. This was done rather than a fixed-length incision, thereby allowing the incision to be relatively comparable as a function of the size of the foot for the different sizes and ages of animals (4). A small forceps was used to elevate the flexor tendon from the heel to the toes. The incision was then closed with 2 inverted mattress stitches by using 5-0 nylon sutures and an FS2 needle.
Behavior was also assessed in 2-wk-old animals that received preoperative SNB BUP (n = 8) in the leg opposite from the operated leg. This was done to eliminate the possibility that the observed effect was related to systemic effects of BUP.
For the postoperative SNB groups, after the surgical procedure but during the same anesthetic, SNB with the same amount of BUP or NS was performed as described for the preoperative SNB group. All animals were allowed to recover from GA for 2 h before testing. Preweanling animals recovered in the cage with their mothers. Block with BUP was assessed by a blinded observer on the basis of dragging of the paw on the blocked side 30 min after recovery during ambulation. All animals that received SNB with BUP developed block based on motor block, whereas no control animal that received SNB with NS dragged the foot. No animal in the study had a wound dehiscence or infection during the study, and, therefore, all animals were included in the data analysis.
Animals were placed in a Plexiglas cage with a mesh floor. They were allowed to acclimate to the environment for 20 min before testing. von Frey filaments were used to determine withdrawal threshold by placing the filament on the foot pad just anterior and lateral to the incision until the filament bent. The von Frey filaments used were 3.84, 4.08, 4.31, 4.56, 4.74, 4.93, 5.18, 5.46, and 5.88, corresponding to 0.5, 0.9, 1.7, 3.7, 5.5, 8.0, 12.4, 21.5, and 53.0 g. This was done three times; a positive response was determined by brisk withdrawal of the foot from the filament. The threshold to withdrawal with a 50% probability to testing with calibrated von Frey monofilaments was determined with the up-down method as previously described (15). Mechanical withdrawal thresholds were determined before surgery and then at 2 h, 5h, and 1, 2, 3, and 7 days. All sensory testing was performed by a person blinded to the treatment of the animal. It was obvious which animals had a block at the 2-h time point, but blinding remained as to whether the block was preoperative or postoperative. Withdrawal thresholds to mechanical stimulation are presented as mean ± SE. The withdrawal thresholds for the paw ipsilateral to the operated paw for the preoperative and postoperative BUP and NS SNB groups were compared with each other and with those for control animals that did not receive surgery on either paw.
Withdraw thresholds were analyzed with repeated-measures analysis of variance between groups of similar ages for time and treatment effects. Multiple comparisons were accounted for by using Fisher’s protected least significant difference where appropriate. Significance was considered at P < 0.05.
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Results
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The control 2-wk-old animals demonstrated a threshold at baseline of 4.5 ± 0.2 g (n = 12). There was no difference between the right and left hindpaw threshold at baseline. This was not different between groups. All animals receiving BUP had an effective block as measured by motor block, which was defined by dragging the paw after emerging from anesthesia that then resolved over time (within 2–3 h). Preoperative SNB BUP (n = 9) in the 2-wk-old animals produced a significant reduction in mechanical allodynia as measured by an increase in threshold at 2, 5, and 24 h when compared with NS SNB (n = 10) (P < 0.05) (Fig. 1). Postoperative SNB BUP (n = 8) produced a reduction in mechanical allodynia at 2 and 5 h (thresholds of 3.7 ± 0.4 g and 2.0 ± 0.4 g, respectively), but this was not statistically significant when compared with saline controls (n = 8). When comparing the effects of preoperative SNB BUP with postoperative SNB BUP, the most significant difference was seen at 24 h after surgery (Fig. 2). At 24 h, the threshold was 4.0 ± 0.7 g in the preoperative SNB BUP group compared with 1.6 ± 0.3 g in the postoperative SNB BUP group (P < 0.05). In 2-wk-old animals receiving an SNB BUP (n = 8) before surgery in the contralateral leg from the surgery, no difference in withdrawal threshold from the BUP NS was found (data not shown).
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Figure 1. Bupivacaine (BUP) sciatic nerve block (SNB) (n = 9) before surgery versus normal saline (NS) SNB (n = 10) before surgery as control before paw incision in 2-wk-old animals. There was a significant reduction in threshold from baseline for the NS group (*P < 0.05) until Day 7. There was no significant reduction from baseline in the SNB BUP group until Day 3 (+), which then resolved by Day 7.
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Figure 2. Bupivacaine (BUP) sciatic nerve block (SNB) (n = 9) before surgery versus SNB BUP (n = 8) after surgery in the paw incision model in 2-wk-old animals. There was a significant difference between the preoperative SNB and the postoperative SNB groups at 24 h. At 24 h, the threshold was 4.0 ± 0.7 g in the preoperative SNB group compared with 1.6 ± 0.3 g in the postoperative SNB group (*P < 0.05).
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The 4-wk-old animals demonstrated a threshold at baseline of 19.2 ± 0.8 g (n = 12). There was no difference between the right and left hindpaw threshold at baseline. Preoperative SNB BUP (n = 8) in 4-wk-old animals demonstrated that block was present in the 4-wk-old animals at 2 h after surgery as measured by a withdrawal threshold indistinguishable from that of controls (no surgery). However, the block resolved by 5 h as measured by a decrease in withdrawal thresholds from no-surgery controls (P < 0.05) and was indistinguishable from NS SNB (n = 8) surgery controls that remained throughout the 7-day study period (Fig. 3). The preoperative SNB BUP was not different from the postoperative SNB BUP (n = 8) in the 4-wk-old animals (Fig. 4).
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Figure 3. Bupivacaine (BUP) sciatic nerve block (SNB) before surgery (n = 8) versus normal saline (NS) SNB (n = 8) before surgery as control before paw incision in 4-wk-old animals. There was a significant reduction in threshold from baseline for the NS group through 7 days. There was no significant reduction in withdrawal threshold from control withdrawal thresholds at 2 h. This 2-h point was different from animals with surgery and NS SNB (*P < 0.05). The block resolved by 5 h as measured by a decrease in withdrawal thresholds, which remained through 7 days and was no different from NS SNB. This was different from the preoperative SNB in the 2-wk-old animals.
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Figure 4. Bupivacaine (BUP) sciatic nerve block (SNB) (n = 8) before surgery versus SNB BUP (n = 8) after surgery in the paw incision model in 4-wk-old animals. There was no difference between preoperative SNB BUP and postoperative SNB BUP at any time point.
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Discussion
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In this study, using the paw incision model of postoperative pain in the rat, preoperative SNB BUP provided analgesia as measured by an increase in the mechanical threshold when compared with NS SNB. This is the first study to demonstrate the efficacy of SNB at providing analgesia in this model of acute surgical pain.
The duration of the analgesic effect of preoperative SNB BUP is of interest. In all animals in this study, motor block was present after emergence from GA, as measured by dragging of the limb while walking about the cage. This effect is limited in duration, because the motor block was absent in all animals by the five-hour time point. BUP could still have been blocking the sensory nerves because it has been shown to have differential nerve-blocking abilities (16). However, others have shown, in similar animals, that the duration of sensory nerve block from SNB does not exceed four hours (13). In addition, a preemptive effect was likely because the postoperative block, which was placed approximately 30 minutes after the preoperative block, did not demonstrate any significant effect at five hours after surgery. This suggests that the effect of the SNB with preoperative BUP lasts well beyond the expected nerve block itself. Thus, the preoperative SNB with BUP in this model of postoperative pain appears to provide a preemptive analgesic effect. This effect is limited, however, to the younger animals. In addition, the overall duration of alleviation of the tactile hyperalgesia was significantly longer with the preoperative than with the postoperative SNB. Although the influence of the timing of analgesia has been somewhat controversial, a greater effect of timing in our study may be related to differences in development (6).
In this study, the preemptive analgesic effect of SNB was developmentally dependent; it exceeded the duration of the block itself only in the two-week-old animals. This observation is consistent with previous studies demonstrating a developmental shift that alters the pain response between two weeks and three to four weeks of age (4,17). There are numerous changes occurring during this time period in development that could contribute to the differing responses. For example, during this period, A-ß fibers withdraw from the superficial lamina in the dorsal horn, and descending inhibitory pathways are rapidly maturing (18,19). Additionally, inhibitory transmitters also become more prominently expressed during this period and may play a role in the observed effects (20).
In addition to the central nervous system changes, the peripheral nervous system is also rapidly developing during this time period. There are numerous possible targets for modulation of the nociceptive processing that could be occurring both peripherally and centrally (21,22). Recently, BUP SNB has been shown to have little or no effect on the hyperinnervation response to injury in immature animals (23). This suggests that the preemptive analgesic effects observed in this study are probably not related to alterations in the hyperinnervation response to injury in the periphery. However, the preemptive effect of the SNB in the younger animals may still be altering peripheral processing.
SNB has been used in other models of pain. The acute pain model is related to tissue trauma and resulting inflammation. This is different from the true inflammatory model and is distinguished by differences in behavior and by central and peripheral changes. There are similar differences for the neuropathic pain model, which is nerve injury with resulting inflammation. Despite these differences, the effects of similar interventions in these different models are valuable in gaining an understanding of what might be occurring in a given model.
Previous studies in adult animals have shown that local injection of BUP provides analgesia in a model of inflammatory pain (24). This effect is limited to the time of the presence of the local anesthetic, and there was no preemptive effect of the administration before the insult. However, local paw edema was reduced only with the locally administered BUP before the insult. We were unable to demonstrate any analgesic effect of locally administered BUP in this model (unpublished data). However, the decrease in edema reported from the locally administered BUP before the insult is of interest (24). Others have shown an effect of systemically-administered local anesthetic that may play a role through peripheral reduction in inflammation or through modulation of central processing (25,26). Although the local effect on edema cannot be excluded, we do not believe that the systemic effect played a role in our model because we injected the same volume of BUP into the opposite paw before surgery and found that no significant analgesia was present in the operative side at any time in the two-week-old animals.
We previously demonstrated that two- and four-week-old animals exhibit a similar reduction in tactile threshold after paw incision but that the duration of the mechanical allodynia is much shorter in the young animals. The shorter duration of allodynia may play a role in the preemptive effect. The shorter duration may be a manifestation of less insult, less reaction to the insult, or greater plasticity and capacity for repair. It has been suggested previously that less of a surgical effect may be a permissive factor for preemptive analgesia to be demonstrated (27). The block duration relative to the duration of the pain may also play a role in the age effect (28,29). Prolonged duration of SNB decreased allodynia in carrageenan-injected adult rats for 24 hours, whereas a short block did not (28). This implies that a critical duration of inhibition of neural traffic is required to prevent sensitization after injury. It is conceivable that the age difference observed in this study with SNB is related to this critical time effect. That is, because allodynia is shorter in the younger animals, the critical time for block duration may be achieved with the single shot only in the younger animals. Further studies examining SNB of differing durations in both age groups could address this hypothesis.
The more immature nervous system seems to be more sensitive to local anesthetic actions (13,14,30,31). Epidural doses of local anesthetic that achieve a reduction in hypersensitivity in very young animals do not necessarily produce a change in threshold in the unaffected limb. It is therefore possible that minor prolonged sensory blockade could have occurred in the very young animals (30). However, if this were the only effect and not a preemptive effect, the postoperative block should have been as effective as the preoperative block in reducing hypersensitivity. Thus, although the small dose of local anesthetic may play a role after the block apparently resolves (no motor block and sensory block evident in the absence of surgery), this does not explain the efficacy of the preoperative block in reducing hypersensitivity and certainly would not appear to play a role at 24 hours because the effects reported seem to be of approximately 90 minutes’ duration (30).
Although preoperative SNB reduced allodynia at 24 hours, this effect was gone at 3 days after surgery, suggesting that the process of sensitization—peripherally, centrally, or both—is not permanently blocked. Whether more prolonged neural blockade or the use of adjuncts to the local anesthetic effect that may alter sensitization via other mechanisms could totally block sensitization is unknown (21,22). Further study may help to define the role of block duration and adjuncts in prolonging the preemptive effect in the young animals and in establishing the effect in the older animals.
In conclusion, preemptive analgesia from preoperative SNB occurs in the two-week-old rat after a surgical insult in the foot. This effect appears to be developmentally modulated because the four-week-old animals did not demonstrate any significant analgesic effect beyond what would be expected from the effect of the local anesthetic nerve block alone. Further studies to examine the effect of the SNB in modulating the processing of the signal in the central and peripheral nervous systems will be important in understanding both preemptive analgesia and the modulation of pain as a function of age and development during this critical time.
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Acknowledgments
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Supported in part by the Foundation for Anesthesia Education and Research, Rochester, MN.
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References
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Accepted for publication December 8, 2003.
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Abstract
Introduction
