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Does mixing local anaesthetics confer an advantage?



Abstract

Objective:  

  1. To audit factors which influence nociception in dogs and cats undergoing dental extractions.  

  1. To assess the combination of lidocaine & bupivacaine compared to plain bupivacaine.  

 

Background: Theory suggests the onset time of bupivacaine is slow. Our current practice is to use bupivacaine alone. Some authors suggest a bupivacaine-lidocaine combination may be more effective at preventing nociception.  

 

Methods: Observational data were collected in three phases. Phase I identified factors requiring standardisation. Phase II used a standard premedication and standard volume of bupivacaine 0.5% (0.03ml/kg/site). Phase III assessed a combination of lidocaine 2% (0.015ml/kg) and bupivacaine 0.5% (0.015ml/kg). Nerve blocks performed were the maxillary tuberosity approach to the maxillary nerve and the intra-oral approach to the mandibular nerve.  

 

Results:  

Phase I: Nociception was detected in 5/16 (30%) dogs and 0/9 (0%) cats. In cats the mean volume of bupivacaine used was 0.04ml/kg per block. Current practice was deemed effective in cats. In dogs, variables affecting nociception were volume of local anaesthetic and premedication drugs.  

Phase II: Nociception was detected in 15/31 dogs (48%). With dogs receiving a standard premedication nociception was 4/15 (26%).  

Phase III: Nociception was detected in 6/23 dogs (26%). With dogs receiving a standard premedication nociception was 1/12 (8%).  


Conclusion: When premedicants and volumes of bupivacaine are standardised nociception is reduced in dogs & cats undergoing dental extractions. Nociception is further reduced when a combination of lidocaine & bupivacaine is used.  

 

Application: This work is applicable to veterinary surgeons undertaking dental extractions in dogs and cats.  


For details on dental blocks, please watch our free videos or our webinar.


Background

Theory suggests that if we perform a bupivacaine nerve block and start the dental extraction at once, we see nociception because the onset time of bupivacaine is slow (Dugdale et al 2020). Our current practice is to use bupivacaine alone. Some authors suggest a bupivacaine-lidocaine combination may be more effective at preventing nociception. Pascoe et al (2016) consider a lidocaine and bupivacaine combination to provide a rapid onset of anaesthesia with a good duration of action. These authors did not examine the benefits of a lidocaine bupivacaine combination over bupivacaine alone.  

 

Studies in cats have demonstrated the combination of lidocaine and bupivacaine to be effective in reducing volatile agent use and improving post-operative pain scores (Aguiar et al 2016), however these authors did not examine the benefits of either agent alone. What this study does inform us is that the combination was effective in preventing intra-operative nociception and produced acceptable patient comfort post-operatively.  

Any further investigation into the efficacy of a combination would first require consideration of other factors which could affect nociception and confound results.  

 

The aim of this clinical audit was to evaluate our current practice by assessing the factors that affect nociception in dogs and cats undergoing dental extractions following a bupivacaine local anaesthetic technique. The second aim was to assess whether a combination of lidocaine & bupivacaine is superior to bupivacaine alone for dental local anaesthesia. 


Materials & methods

Data were collected in an observational manner in a clinical audit exercise, divided into three phases. Client consent included the use of data for clinical audit purposes.  

 

Phase I 

All dogs and cats undergoing dental procedures at Anderson Moores Vet Specialists were included. Parameters recorded included patient demographics, body weight, body condition score, premedicants drugs used, teeth (number of and location) extracted, local anaesthetic technique performed, and volume of local anaesthetic used. The time from performing the nerve block to starting the extraction were recorded.  

 

In phase I dogs and cats were anaesthetized with a range of premedicants. Sedatives included dexmedetomidine or acepromazine. Methadone was used for opioid analgesia. Efficacy of premedication was scored using a subjective scale. Anaesthesia was induced with propofol to effect to allow tracheal intubation with a cuffed endotracheal tube. Anaesthesia was maintained with sevoflurane delivered in an air: oxygen mix via a suitably sized breathing system. Hartmann’s solution was administered IV at 3-5 ml/kg/hr. Vital parameters (heart rate, respiratory rate, ECG, oscillometric blood pressure, haemoglobin saturation with oxygen and temperature were monitored with a multiparameter device and recorded every 5 minutes by the same registered veterinary nurse for every case.  

 

Local anaesthetic techniques were performed by the same experienced veterinary dentist each time using bupivacaine 0.5%. The maxillary nerve block was performed using the maxillary tuberosity approach whereby the needle is directed towards the maxillary foramen with the needle inserted behind the last molar tooth (110 or 210). The mandibular nerve block was performed by palpating the mandibular foramen and directing the needle towards this point. These techniques are described in Dugdale et al (2020) with videos available at www.zeropainphilosophy.com

 

Prior to commencement of the procedure a baseline heart rate, respiratory rate and blood pressure was recorded. If at any point in the procedure these values increased above 20% of this baseline, this was recorded as nociception and further analgesia was provided. Further analgesia consisted of either fentanyl 0.001 mg/kg IV or dexmedetomidine 0.001-0.004 mg/kg IV. If further analgesia was required a second time, methadone 0.1 mg/kg was administered IV. 

 

Phase II 

In phase II some adjustments to the protocol were made based on the findings of phase I. The premedication in dogs was standardised to dexmedetomidine 0.004 mg/kg IV and methadone 0.1 mg/kg (standard premed, SP). Given the observational nature of the work, data for dogs where this premedication was deemed not suitable (ie: cardiac disease) were also collected. These dogs were premedicated with methadone alone or methadone-acepromazine combination and referred to as a non-standard premed (NSP). No cats were included in phase II due to the efficacy of bupivacaine in phase I cases. For dogs in phase II, volume of local anaesthetic was set at 0.03ml/kg per nerve block based on the average volume used from phase I. Induction and maintenance of anaesthesia was consistent with phase I.  

 

Phase III 

In phase III the only change made was to the local anaesthetic used. A combination of lidocaine 2% (0.015ml/kg) and bupivacaine 0.5% (0.015ml/kg) was used.  


Results

Phase I 

Nine cats were included in phase I. The mean volume of 0.5% bupivacaine used was 0.04 ml/kg per nerve block. Nerve blocks performed were the caudal maxillary block via the maxillary tuberosity approach and the caudal mandibular nerve block with an intra-oral approach. The mean time from performing the block to starting the procedure was 6.7 minutes (0-15 minutes). The number of cats in which nociception was detected was 0/9. No interventional analgesia was required during the procedures.  

 

Sixteen dogs were included in phase I. The mean volume of 0.5% bupivacaine used was 0.03 ml/kg per nerve block. Nerve blocks performed were the caudal maxillary block and the caudal mandibular nerve block. The mean time in all dogs from performing the nerve block to starting procedure was 6.6 minutes (0-20 minutes). The number of dogs in which nociception was detected was 5/16 = 31%. Of the 5/16 dogs where nociception was detected the mean time from nerve block to procedure starting was 10 minutes (0-25). Of the 5/16 dogs where nociception was detected, 3 had dexmedetomidine doses less than 0.004 mg/kg.  

 

Phase II 

No cats were included in phase II. 

 

Data were obtained from 31 dogs. Fifteen dogs received a standard premed. The volume of 0.5% bupivacaine used was set at 0.03ml/kg per nerve block. Nerve blocks performed were the caudal maxillary block and the caudal mandibular nerve block. The mean time from performing block to starting procedure was 8.9 minutes (0-25 minutes).  

The number of dogs in which nociception was detected was 15/31 = 48%. In the 15 dogs that experienced nociception, 4 received the standard premedication and 11 did not. In 7 cases the dose of dexmedetomidine was less than 0.004 mg/kg and in 4 cases methadone alone was used for premedication. In dogs that received the standard premedication the incidence of nociception was 26%.  

 

Phase III 

Data were obtained from 22 dogs. Eleven dogs received a standard premed. A combination of lidocaine 2% (0.015ml/kg) and bupivacaine 0.5% (0.015ml/kg) was used for each nerve block. Nerve blocks performed were the caudal maxillary block and the caudal mandibular nerve block. The mean time from performing block to starting procedure was 7.0 minutes (0-30 minutes).  

The number of dogs in which nociception was detected was 6/22 = 27%. In the 6 dogs that experienced nociception, only one dog received a standard premedication. In dogs that received the standard premedication the incidence of nociception was 9%. 


Discussion

In dogs receiving a standardised premedication, with a set volume of local anaesthetic the combination of lidocaine and bupivacaine resulted in a lower incidence of nociception (9%) compared to bupivacaine alone (26%). In cats, when a volume of 0.04ml/kg bupivacaine was used, no nociception was detected.  


Various authors report that the advantage of lidocaine combined with bupivacaine is that lidocaine has a short onset of action and bupivacaine has a long duration, thus giving the best of both worlds. Whilst this is often written in the literature it is difficult to find studies to support this combination and there are no such studies in the veterinary literature.


A preliminary study in humans used local anaesthetic injected into the foot with onset and duration assessed with monofilament wire. This work suggests that there is no clinical advantage, with respect to onset and duration of local blockade, to using a 50/50 mixture of plain lidocaine and plain bupivacaine in place of their independent use (Ribotsky et al 1996). 


In the clinical audit reported herein we documented a lower incidence of nociception (9%) when the combination was used compared to plain bupivacaine (26%). These figures were obtained from dogs where the premedication combination was optimised. A lesser reduction in nociception was seen without an optimised premedication combination with 48% of dogs exhibiting nociception following plain bupivacaine compared to 26% nociception with lidocaine bupivacaine combination.  


The sedative and analgesic effects of dexmedetomidine are documented in the literature. Our work suggests there is a dose-dependent and clinically relevant analgesic effect with dexmedetomidine. One study (Van Oostrom et al 2011) reports a dose dependent analgesic effect whereby 0.003 mg/kg/hr produced sedation and antinociception but 0.001 mg/kg/hr did not. Our decision to optimise the dexmedetomidine dose to 0.004 mg/kg was based on this work. It is clear that below this dose the incidence of nociception is higher. It also evident that in the absence of dexmedetomidine there is a greater risk of nociception. Therefore it is advisable to include dexmedetomidine at a dose equal to or greater than 0.004 mg/kg in dogs undergoing painful procedures where cardiovascular stability permits. It should be noted that methadone was used at 0.1mg/kg in combination with dexmedetomidine. A synergistic effect of alpha-2-agonists and opioids has been documented in dogs and it is not clear from the present work whether dexmedetomidine would have a similar effect if used alone. It is therefore recommended to use dexmedetomidine combined with methadone.  


The design of this clinical audit centered around step wise quality improvement, starting with a clinical question – does the combination of lidocaine and bupivacaine result in less nociception compared to plain bupivacaine? In order to answer this question, we first considered factors which would affect this from the results of phase I. This highlighted that certain areas required standardisation before moving to the next audit phase. The volume of local anaesthetic used was standardised to 0.03 ml/kg. Where possible the premedication combination was standardised. Where this was not possible it was due to the dog’s clinical condition and this was at the discretion of the attending anaesthetist. An advantage of this study was that one experienced veterinary dentist performed all nerve blocks and one experienced RVN monitored each anaesthetic.  


Using descriptive statistics it is difficult to draw conclusions on whether the nerve block was more effective if more time was allowed between performing the block and starting the extraction. In some cases the block was performed and the extraction started immediately. In cases requiring extractions from multiple arcades the blocks were performed at the same time but the extractions occurred at different time points, hence the reason for some blocks being in place for 30 minutes prior to the extraction starting. To fully answer this question we do not have enough cases in the required groups. Further work is required to compare cases with a standard premedication, a lidocaine-bupivacaine block where a short onset time (such as 5 minutes) is compared to a longer onset time (such as 30 minutes).  


It is possible that the findings from the cats included in this study may not be fully representative. In phase I we were unable to detect nociception in these nine cats and concluded that our current practice was effective. This result could be related to the volume of local anaesthetic used or to the size of feline nerves compared to canine nerves as this is a factor in efficacy of nerve blocks (Dugdale 2020).  


Clinical audit answers a clinical question, which is what the work reported here has done. Clinical audit is a lower level in the evidence hierarchy and to answer this question with a greater degree of certainty a prospective randomised blinded clinical study is required. The data produced here can be used to perform a sample size calculation to ensure results from such a study are robust.  


Based on this clinical audit the following recommendations can be made; 

Where cardiovascular stability permits, include dexmedetomidine (>0.004mg/kg) plus methadone in premedication. 

Use 0.03ml/kg total volume of local anaesthetic, split 0.015ml/kg lidocaine 2% and 0.015ml/kg bupivacaine 0.5%.  

Monitor for nociception and be prepared to intervene with further analgesia.  


For details on dental blocks, please watch our free videos or our webinar.


References

Aguiar, J., Chebroux, A., Martinez-Taboada, F., & Leece, E. A. (2015). Analgesic effects of maxillary and inferior alveolar nerve blocks in cats undergoing dental extractions. Journal of Feline Medicine and Surgery, 17(2), 110–116. https://doi.org/10.1177/1098612X14533551 

 

Best CA, Best AA, Best TJ, Hamilton DA. (2015) Buffered lidocaine and bupivacaine mixture - the ideal local anesthetic solution?. Plast Surg (Oakv) 23(2):87-90. doi:10.4172/plastic-surgery.1000913 

 

Dugdale AHA. (2020) Local Anaesthetics. In: Veterinary Anaesthesia Principles to Practice Second Edition Eds Dugdale AHA, Beaumont G, Bradbrook C, Gurney M. Wiley Blackwell. 

 

Pascoe PJ. (2016) The effects of lidocaine or lidocaine-bupivacaine  mixture administered into the infraorbital canal in dogs. Am J Vet Res 77, 682-687  

 

Ribotsky BM, Berkowitz KD, Montague JR. (1996) Local anesthetics. Is there an advantage to mixing            solutions? J Am Podiatr Med Assoc. 86(10):487-91. doi: 10.7547/87507315-86-10-487.  

 

van Oostrom H, Doornenbal A, Schot A, Stienen PJ, Hellebrekers LJ. (2011) Neurophysiological assessment of the sedative and analgesic effects of a constant rate infusion of dexmedetomidine in the dog. Vet J. 190(3):338-44. doi: 10.1016/j.tvjl.2010.11.025.  


This clinical audit was conducted at Anderson Moores Veterinary Specialists by Dr Matt Gurney, Dr Matthew Oxford and Sue Vranch RVN.

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