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Edition 2 - pelvic limb blocks for stifle surgery

In our latest research roundup we provide an easy-to-read digest on three items of research - with this edition focusing on nerve blocks for pelvic limb surgery.


This research roundups provide a single point of reference for the reader. Despite this paper not being open-access, we have provided a digest and also the link to the abstract. https://pubmed.ncbi.nlm.nih.gov/35550342/



Subject

Pelvic limb nerve blocks for stifle surgery in dogs.


Title of the paper

Femoral and sciatic nerve blockade of the pelvic limb with and without obturator nerve block for tibial plateau levelling osteotomy surgery in dogs.


Aim of the paper

This paper questions the previously held understanding that it is necessary to block the obturator nerve as well as the femoral and sciatic nerves in order to provide anaesthesia to the stifle.


Review of methodology

In this work, 100 dogs were split into two groups. Group FSO received a nerve block of the femoral, sciatic and obturator nerves prior to surgery for tibial plateau levelling osteotomy (TPLO). In the second group (FSP) the syringe used for the obturator nerve block contained saline – to which the operator was blinded.


Intra-operative cardiopulmonary variables and rescue opioid requirement were assessed. If heart rate or mean arterial pressure increased >20% of baseline, 2mcg/kg fentanyl was administered IV.


The anaesthesia protocol used was a completely reasonable one for TPLO surgery – including dexmedetomidine 2mcg/kg and methadone 0.2mg/kg as premedication. We always want to check whether any other analgesics used could have confounded the results – and we feel this is a very sensible drug choice for this study.


The local anaesthetic used was ropivacaine (0.75%) using a volume of 0.1 ml/kg per block – which is an accurate volume for the nerve blocks described.


Results

This paper did not find statistically significant differences between the two groups, and concluded that anaesthesia of the obturator nerve in addition to the femoral & sciatic nerves does not confer an additional clinical benefit.


In each group, 7 dogs required rescue fentanyl. Of the 14 dogs that required rescue analgesia, two needed a second injection of fentanyl before the end of surgery, both in the FSP group.


Key points to aid reflection

To date, our understanding has been based on a paper from 1982 by O’Connor & Woodbury which found that the obturator nerve contributes to innervation of the medial stifle in 27% of dogs. This knowledge always created a degree of question over approaches to the femoral nerve that do not block the obturator nerve.

It still remains possible that the obturator nerve does require blocking to provide total antinociception - this study was a clinical assessment and not a study of nerve conduction so perhaps one could argue that it simply wasn’t designed to assess nerve activity vs clinical pain. The authors acknowledge this with the following statement;

‘The study does not provide evidence for the role of obturator nerve involvement in stifle nociception per se.’

This study used a standard methodology for documenting intra-operative efficacy of the nerve block, using physiological parameters to assess sympathetic stimulation.


Conclusion of the paper

This study does not support the routine use of an obturator nerve block for TPLO surgery in dogs.


Our assessment of the paper

Our view is that this is a useful piece of work – we like it when a paper answers clinical questions that we have always asked ourselves!


In this section you will find our ‘everyone should read this’ recommendation and a review that we have chosen to support your next steps in integrating this with your clinical practice.


Everyone should read this

Comparison of ultrasound-guided lumbar plexus and sciatic nerve blocks with ropivacaine and sham blocks with saline on perianesthetic analgesia and recovery in dogs undergoing tibial plateau leveling osteotomy surgery


I like the fact that this study clearly documents a benefit to nerve blocks vs no nerve block for TPLO surgery. The anaesthetic protocol is complicated to me. Hydromorphone premedication (0.2mg/kg) was given subcutaneously. The complicated titration of the isoflurane is the reason I think a significant number of dogs became hypotensive – contrast this anaesthetic protocol to the study detailed above and you ask yourself why take this approach? This is really an aside to the main take home of the paper, however I feel it is important to point out that I feel the anaesthetic protocol used here plays a key role in the hypotension seen, not the fact that local anaesthesia was used and led to hypotension.


Recommended review article

Taylor et al (2020) Basic pharmacology of local anaesthetics


Where do we see the value in this work?

· Reviews the mechanism of action of local anaesthetics

· Helps with a revision of pharmacology of these drugs

· Details additional mechanisms of actions of local anaesthetics – adding some interesting detail not found in many other reviews.


Reflections

Do the studies reviewed here help settle that question over the importance of the obturator nerve in stifle anaesthesia?

How has the review of local anaesthetics refreshed your knowledge, or what are the main new learnings for you?

What are the unanswered questions that you need further help with? Please contact us with these questions.



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