Local anesthetics are used to block the transmission of noxious stimuli before incision or to potentiate analgesia postoperatively in veterinary and human dentistry and oral surgery (17,20). Lidocaine and bupivacaine are the most commonly used drugs, and opioids, and α-2 agonists are used in combination as adjuvants (4). While the use of adjuvant has been proven safe for over a century, there is only a modest understanding of its intended effects, including prolonging block duration, and intensifying anesthesia and analgesia (3,16).

Butorphanol, a kappa-opioid receptor agonist and mu-opioid receptor antagonist, is an adjuvant for local analgesics and sedative properties and ready availability (8,21,33). However, its use as a sole analgesic has been questioned because of its limited efficacy in the presence of higher pain levels and short duration of analgesia (6,8,32). Tramadol, a synthetic codeine analog, is an analgesic with mixed mu (μ)-opioid and non-opioid activities. It inhibits the reuptake of norepinephrine and serotonin from the nerve endings and potentiates the effect of LA when mixed in peripheral nerve blocks (6,28,32). Recent studies have reported that systemic administration of buprenorphine and butorphanol were combined with a local anesthetic to increase the duration of analgesia without complications in human brachial plexus block (19) and in cats undergoing orchiectomy (13).

Acceleromyography (AMG) is a technique that is clinically to objectively monitor neuromuscular transmission. It is based on the assumption that the peak acceleration of an extremity in response to nerve stimulation is directly proportional to the force applied to the extremity by muscle contraction (20,30). In several recent studies, a neuromuscular monitoring device has been used to reduce the risk of residual paralysis in the early postoperative period by confirming the onset and depth of neuromuscular blockage when injecting neuromuscular blocking agents (2,14,29). This device not only provides qualitative information in the form of visible muscle twitch and quantitative real-time measurements of neuromuscular function, but also provides several patterns of nerve stimulation, including train-of-four (TOF), tetanic, and post-tetanic count (PTC).

We measured the onset time, duration of action, and depth of local anesthetics by using a quantitative method through AMG-based neuromuscular monitoring (#NB-STIM). This study aimed to compare the efficacy of lidocaine alone with that of a combination of lidocaine and opioids. We hypothesized that the combination of lidocaine-butorphanol or lidocaine-tramadol would provide better analgesia than lidocaine alone in mandibular nerve block. A secondary hypothesis was that dogs receiving opioids with lidocaine block would have shorter onset times and longer duration than dogs receiving lidocaine alone.

Animals

Fiftten beagle dogs (all female), 12 months old and weighing 6.5-8.0 kg that were demonstrated to be clinically healthy on general examination were used. This study was approved by the Institutional Animal Care and Use Committee of Gyeongsang National University. (Approval no. GNU-210201-D0008). The dogs were fasted for 12 h before anesthesia. Complete blood count and serum biochemistry were also performed to ensure that they did not have any underlying disease that may affect the maintenance of general anesthesia.

The dogs were randomly allocated to the following three groups: L (lidocaine alone), LB (lidocaine + butorphanol), and LT (lidocaine + tramadol). The dogs were received 2% lidocaine (4 mg/kg), and LB (2% lidocaine; 4.0 mg/kg + of butorphanol; 0.1 mg/kg), and LT, 2% lidocaine (4.0 mg/kg) + tramadol (2.5 mg/kg) in the same method. All the local anesthetic drugs used in this study had the same volume of 0.365 mL.

General anesthetic procedure

The dogs were sedated with medetomidine (0.02 mg/kg, SC, Domitor^®; Pfizer, NY, USA), acepromazine (0.05 mg/kg, SC, Sedaject^®; Samu Median, Seoul, Korea) and atropine (0.04 mg/kg, SC, Atropine^®, Jeil Pharmaceutical Co., Daegu, Korea); anesthetized with etomidate (2 mg/kg, IV, Etomidate-^®Lipuro, B. Braun Melsungen AG, Germany); and intubated. Cefazolin (25 mg/kg, IV, Hankook Korus Pharm Co., Seoul, Korea) was used as a prophylactic antibiotic. Anesthesia was maintained using a mixture of 2.0-3.0% isoflurane (Terrell^TM, Piramal Critical Care, USA) and oxygen. The heart rate was monitored, and the respiratory rate was maintained between 10 and 20 breaths/min. The end-tidal partial pressure of carbon dioxide was maintained between 35 mmHg and 45 mmHg. A circulating water blanket (Medi-Therm^®, Gaymar, NY, USA) was used at a temperature of 38-39°C to maintain the body temperature.

They were instrumented with a lead II electrocardiogram monitor, a pulse oximeter, and an oscillometer blood pressure cuff placed at the metacarpal region for indirect measurements of arterial blood pressure (B40 Patient monitor, GE medical systems Co. Ltd. USA). Physiological parameters during anesthesia were maintained in all dogs as follows for objective study results: heart rate, 90-110 beats/min; systolic blood pressure, 80-120 mmHg; minimum alveolar concentration: 1.1-1.4.

Local anesthesia procedure and equipment

A neuromuscular monitor (#NB-STIM, Stimpod NMS450X: Xavant Technology, South Africa) that stimulates the peripheral nerve while also recording, quantifying (TOF and PTC), and numerically displaying the evoked responses were used for quantitative monitoring.

Under general anesthesia, the dogs were placed in the right or left lateral recumbency. The area where the AMG was to be attached and the area where the nerve was to be blocked was clipped and cleanly sterilized. The tri-axial accelerometer (#NBCA) was immobilized in the intermandibular space before the nerve was blocked and measured (Fig. 1). This provides real-time feedback on the strength of contraction of the affected region.

Figure 1. Neuromuscular monitoring equipment.

A nerve-mapping probe was used to locate the nerve (Fig. 2A). A stimulating nerve blocking needle (Stimuplex^®Ultra 360^®, B. Braun Melsungen AG, Germany) was used to increase the overall nerve blocking success rate while simultaneously injecting the anesthetic (Fig. 2B). All dogs underwent extraoral inferior alveolar nerve block in each treatment group. The needle electrode (electrosurgical electrode – length 70 mm, Union Medical, Gyeonggi-do, Korea) (cathode) was placed on the nerve for stimulation, and the other needle electrode (anode) was placed beneath the skin at 2.5 cm for hyperpolarization (Fig. 3). The current intensity was set to 5 mA during the local blockage. The TOF count (TOFC) and PTC results were recorded at 1-min intervals for the initial 30 min, and then at 10-min intervals until recovery. The previously reported descriptions of nerve stimulation patterns and levels of neuromuscular block (21) used in this study are presented in Table 1. The graph was subdivided into the following six depths of block zones according to a previous study (15).

Table 1 . Description of nerve stimulation patterns and levels of neuromuscular block.

Stimulation pattern	Frequency, Hz	Levels of neuromuscular block
Train-of-four stimulation	4 stimuli every 0.5 sec (2 Hz)	Acceptable recovery TOFC = 4
Tetanic stimulation	50 Hz for 5 sec	Minimal block: TOFC = 3-4 Shallow block: TOFC = 1-2 Moderate block: PTC ≥ 4, TOFC = 0
Post-tetanic twitch count	An initial tetanic stimulation followed by a pause for 3 sec; then response to 1-Hz single twitch stimulation	Deep block: PTC = 1-3 Profound block: PTC = 0

TOFC, train-of-four count; PTC, post tetanic count..

Figure 2. The nerve mapping probe is used to locate the nerve (A), and the stimulating nerve blocking needle is used to inject drugs while increasing the accuracy of the nerve block (B).

Figure 3. The needle electrode (cathode, gray) is placed on the nerve for stimulation, and the other needle electrode (anode, red) is placed beneath the skin at a distance of 2.5 cm for hyperpolarization.

Statistical analysis

SPSS (version 25.0; SPSS, Inc., Chicago, IL, USA) was used for the analysis. The onset time and the duration of anesthetic effect were tabulated and subjected to the Kruskal-Wallis test to compare between the three groups; if statistical significance was detected, Dunn’s post hoc test was performed to compare each group. The scores obtained on the local anesthetics’ depth of anesthesia rating scale were analyzed using the Friedman test to see possible differences between the three groups over time, and Dunn’s post hoc test was performed to determine which groups had significant differences. The level of statistical significance for the statistical tests was set at p < 0.05.

In all dogs, nerve blocks were performed to evaluate the efficacy of local anesthetics, and all dogs tolerated well despite several nerve stimulations and recovered without any problems from anesthesia.

Comparison of the onset time of motor nerve block

Comparison of the onset time of anesthesia showed 4.60 ± 2.06 min for the lidocaine (L) group, 2.00 ± 0.00 min for the lidocaine with butorphanol (LB) group, and lidocaine with tramadol (LT) group was found to be 2.60 ± 1.62 min, respectively (Table 2). The onset time tended to be faster in the LB and LT groups than in the L group. However, it was statistically confirmed that there was no significant difference between the three groups (p < 0.05) (Fig. 4A, Table 2).

Table 2 . Baseline parameters of the block in the three groups and statistical analysis between the groups.

Characteristics of the block	Group L (n = 5)	Group LB (n = 5)	Group LT (n = 5)	Kruskal-Wallis p-value	Dunn’s multiple comparisons test between group, p-value
Onset time to motor block (min)	4.6 ± 2.06	2 ± 0.0	2.6 ± 1.62	0.09	L – LB, 0.14 L – LT, 0.25 LB-LT, >0.99
Duration of motor block (min)	111.8 ± 34.78	302 ± 76.72	260 ± 49.88	<0.01	L – LB, <0.001 L – LT, 0.058 LB-LT, >0.99

Data are expressed as means ± SD..

p < 0.05 considered statistically significant..

L, lidocaine only group; LB, lidocaine + butorphanol; LT, lidocaine + tramadol..

Figure 4. Comparison of the onset time and duration of the motor nerve block in the three study groups. Dunn’s multiple comparisons test, Duration between L and LB, **p < 0.001; L, lidocaine only group; LB, lidocaine + butorphanol; LT, lidocaine + tramadol.

Comparison of the duration of motor nerve block

The mean duration of motor nerve blockade in the L group was 111.80 ± 34.78 min, and in the LB group was 302.00 ± 76.72 min, and in the LT group was 260.00 ± 49.88 min, respectively (Table 2). The LB and LT groups showed a tendency to increase the duration of action compared to the L group. In the Kruskal-Wallis test, there was a statistically significant difference between the groups. However, in Dunn’s multiple comparison test, the LE group had a shorter duration of action than the (B) group, but longer than the (L) group. In the duration of action, a significant difference was confirmed between the L and LB groups (p < 0.001) (Fig. 4B), and no statistically significant difference was found between the L and LT groups (p < 0.01) (Table 2).

In the comparison of onset time, there was no significant difference between the (B) and (LE) groups, and in the duration of action, a significant difference was observed in the L and LB groups.

Comparison of the depth of motor nerve block with profile graph

The profile graph displays all the stimulation results of the current study, indicating the result, the depth-of-block zone, and the relative time from the start of the study. Only auto, TOF, and PTC mode are supported. Thus, it is possible to confirm the distribution rate of the depth of anesthesia according to the drug.

Since the LT group showed minimal block within 1-10 mins, it showed a tendency to start earlier than the L and LB groups. After 20-40 mins, the LB group demonstrated deep block, showing the deepest depth of anesthesia among all three groups. The L and LT groups showed only a moderate block from the start of drug action to recovery time.

In the L group, acceptable recovery status was shown in the initial 1-10 mins and start 160-170 mins, and between 10 to 150 mins, TOFC was 0, and PTC was 4 or more moderate blocks. There was an onset time of lidocaine’s action during the initial 10 min, and recovery was achieved over 10 min between 160 and 170 min (Fig. 5A).

Figure 5. Profile graph of the depth of anesthesia in (A) L, (B) LB, and (C) LT group by using quantitative method through neuromuscular blockade monitoring. (Numbers 0 to 4 refer to train-of-four count (TOFC) 0-4; Numbers –21 to –1 refer to post-tetanic count (PTC) 0-20).

In group LB, acceptable recovery was observed in the initial 1-10 mins and start 380-390 mins, and deep (PTC = 1-3) blocks were shown between 20 to 40 mins. Moderate (PTC ≥ 4, TOFC = 0) blocks were shown between 50 to 350 mins. A minimal block was observed between 350 and 370 min. There was an onset time of the drug action of lidocaine and butorphanol during the initial 10 min, and recovery was achieved gradually over 40 min between 350 and 390 min (Fig. 5B).

In the LT group, minimal block (TOFC = 3-4) was observed in the initial 1-10 mins, and moderate (PTC ≥ 4, TOFC = 0) block was observed at 10-290 mins after injection. Between 300 to 320 min, acceptable recovery from shallow blocks was observed in stages.

The depth of motor nerve block at different time intervals in all three groups

In all groups, the depth of the motor nerve block was measured at different time intervals due to their local anesthetic properties, which have various and many changes in the initial stage.

In the items that evaluated the depth of nerve block up to the first 30 mins, statistically, the Friedman test and Dunn’s multiple comparison test showed that there was a difference between groups with a p-value <0.001 (Table 3). The LB and LT groups demonstrated faster onset times than the L group. Unlike the other two groups, the BT group showed a moderate block from the initial 3 min and a deep block between 20 and 30 min (Fig. 6).

Table 3 . The statistical difference between the depth of nerve blocks up to the first 30 minutes and the total duration of drug action.

	Until first 30 min (1 minute interval) p-valve	Total duration (10 minutes interval) p-valve
Friedman test
Three groups	<0.0001	<0.0001
Dunn’s multiple comparison test
L – LB	0.0002	<0.0001
L – LT	0.0072	0.1599
LB – LT	<0.0001	<0.0001

L, lidocaine only group; LB, lidocaine + butorphanol; LT, lidocaine + tramadol..

Figure 6. The graph of depth of motor nerve block and statistical graph with standard error are expressed up to 30 mins at 1-min intervals in three groups. (Numbers 0 to 4 refer to train-of-four count (TOFC) 0-4; Numbers –21 to –1 refer to post-tetanic count (PTC) 0-20) (Friedman test, Between three groups; p < 0.001); (Dunn’s multiple comparisons test, Between groups; L and B, ns p > 0.05 ; L and LE, ***p < 0.001 ; B and LE, ***p < 0.001).

Unlike in the LB group, the L and LT, moderate block status was maintained for the first 30 min. In the L group (lidocaine single injection) during the first 30 min, moderate block showed a tendency to show more profound block, which was observed between 25 and 30 min.

Statistically, there was a significant difference between the three groups (p < 0.001) (Fig. 6B). When comparing the differences between groups, there were significant differences in the L, LB, and LT groups (Friedman test, p < 0.001; Dunn’s multiple comparison test, p < 0.001) (Table 3).

Deep block appeared only for a short time (20-40 mins) in the LB group and was maintained as a moderate block after that. Other groups showed only moderate blockade of anesthesia over time, no more profound blockade, and maintenance until the recovery phase. The recovery phase of the local nerve block appeared between 160-170 mins, 350-390 mins, and 300-320 mins in the L, LB, and LT groups, respectively. In other words, the L, LB, and LT groups had recovery times of 10, 40, and 20 min, respectively (Fig. 7).

Figure 7. The graph of depth of motor nerve block and statistical graph with standard error are expressed up to recovery at 10-mins intervals in three groups. (Numbers 0 to 4 refer to train-of-four count (TOFC) 0-4; Numbers –21 to –1 refer to post-tetanic count (PTC) 0-20).

There was a significant difference between the three groups (p < 0.001) in the depth of motor nerve block at 10-min intervals until recovery (Fig. 7, Table 3). When comparing the differences between the groups, no significant differences were confirmed between the groups in the L and LT groups (p = 0.159, Table 3).

The present study found that lidocaine, lidocaine with butorphanol, and lidocaine with tramadol all provided benefits in dogs undergoing mandibular nerve block. The efficacy and effectiveness of these local anesthetics were evaluated using a quantitative method through neuromuscular blockade monitoring without complications.

In a previous study of nerve block, it was common to determine the onset and duration of the drug through subjective evaluation (for example, visual analog scale (19), Lovett’s rating scale (27), and neurological examination (9,24). movement generates a voltage in the piezoelectric crystal that is proportional to the force of contraction, and the signal is analyzed and displayed on the monitor. The two common sites for neuromuscular monitoring are the ulnar nerve, which affects the adductor pollicis muscle, and the facial nerve affecting the orbicularis muscle in a human study (14). AMG-based measurements correlated well with mechanomyography (MMG)-based measurements and showed reliable results (29). AMG can be used to measure the amount of blockade by quantifying the number of twitches via the acceleration of muscle tissue in response to nerve stimulation.

The average onset time of lidocaine has been reported to be within 5 min (7,12), which is similar to the results of this study. The onset time of blockage was reduced by using the drug in combination with lidocaine alone. Butorphanol requires an onset time of approximately 20 min after IM. After tramadol injection, the minimal effective plasma concentration was reached after a few minutes (1.1 ± 0.2 min) and maintained for approximately 6-7 h (2,26).

Opiates have been shown to exert peripheral analgesic action in addition to their well-known central effects, although clear-cut discrimination between peripheral and central analgesics is debatable (18). The analgesia produced by both peripheral and central mechanisms may be additive or even synergistic (1). In this study, there was a tendency to decrease the time when lidocaine and butorphanol or tramadol were combined, but the difference was not statistically significant. Wakhlo et al. (32), it was reported that butorphanol and tramadol were co-administered as adjuvants for local anesthesia in the brachial plexus. This difference in onset time is thought to be due to the difference due to the anesthesia of the supraclavicular plexus block (i.e., the plexus) in this study, whereas a single nerve branch of the mandibular nerve was blocked in this study.

A butorphanol dose of 0.2-0.8 mg/kg BW, SC, is effective for visceral analgesia in the dog duration of analgesia was found to be 23-53 min (25). Tramadol in dogs has a very short half-life (1.7 h) and negligible amounts of the opioid M1 metabolite are produced (22). In the present study, when an opioid was used as an adjuvant to lidocaine, butorphanol was able to significantly increase the duration of action. The action time increased to 260.4 min in the group that received tramadol as an adjuvant to lidocaine compared to 111.8 min in lidocaine alone, but it was not statistically significant in this study. However, since the statistical significance is the p-value of 0.058%, it is judged that if the number of individuals is increased, it can appear significant.

In the present study, no surgical pain was induced, and the analgesic effect through VAS was not evaluated. When evaluated by the values of TOF and PTC, the LB group showed a higher neuro-blocking effect than the L and LT groups. In one human study of postoperative analgesia in patients undergoing lower extremity surgery, comparing the VAS scores for different time intervals in the two groups showed that butorphanol was able to relieve pain in a similarly much better manner than tramadol, with pain relief up to 8 hours (5). Duration of analgesia was 5.35 ± 0.29 h and 6.25 ± 1.58 h in butorphanol and tramadol groups respectively and the difference was found to be statistically significant (5). In the present study, the L group, the LB group, and the LT group showed recovery between 160-170 mins, 350-390 min, and 300-320 min, respectively, and the analgesic effect was reduced for 3 h, 6 h, and 5 h, respectively. In the present study, we showed that the LB group is better than LT in preventing postoperative pain after local blockage. However, the pain after complex dentoalveolar procedures was most severe between 6 and 8 h (26). When the nerve was blocked with lidocaine alone, it recovered in 160-170 min. Therefore, the addition of butorphanol as an adjuvant has a better analgesic effect, but additional analgesic treatment may be required.

In recent human studies, many reports have evaluated AMG-based neuromuscular monitoring when administering neuromuscular blocking agents, but there are few veterinary studies (10,11,23). Therefore, one of the limitations of this study is that few studies can be directly compared in dogs as an adjuvant to local anesthetic in mandibular nerve block. More studies in dogs are needed to evaluate the efficacy of local anesthetics through neuromuscular monitoring in veterinary practice. Another limitation is that although the previous study (31) recommends that tetanus stimulation not be performed more frequently than every 6 minutes due to interference between PTC stimulation and actual neuromuscular block in the monitored hand, it was performed at shorter intervals in this study. Further studies are needed on the effect of this on the measured outcomes.

In conclusion, our study was conducted as an objective method to determine the duration of action and the depth of motor nerve blocks. According to our study, local anesthetics with butorphanol or tramadol used in dental and oral surgery have shown excellent neuromuscular blockade in dogs. Thus, we conclude that both butorphanol and tramadol are effective for postoperative analgesia when used in veterinary dentistry patients undergoing surgical procedures. Lidocaine with butorphanol has a longer duration of analgesia but better pain relief than lidocaine alone. Lidocaine with tramadol showed a tendency for a longer duration of analgesia than lidocaine alone; however, the difference was not statistically significant. More prospective studies are required to recommend any drug as a useful adjunct for enhancing postoperative analgesia.

This report summarizes work contained within a thesis submitted by S.S. Jang to fulfill the requirements for MSc degree. The authors declare that there were no third party funding and support for this research or manuscript and no conflicts of interest.

The authors have no conflicting interests.