Local anaesthetic blocks

RE: Local anaesthetic blocks

Rachel Perry — Sun, 31 Mar 2019 11:41:59 GMT

Hi everyone,

I think regional nerve blocks are great, and use them in all my patients having anything painful done, and do routinely use bupivacaine for the additional post-operative analgesia. My impression.understanding from human dentistry was that mepivacaine lasts about 2 hours, so you don't get significant post-operative analgesia. It's worth also considering local infiltration- a bleb under the mucosa on top of the periosteum overlying the roots of interest- which is used a lot in people, especially in the maxilla. They may not work so well in our patients as the cortical bone plate is perhaps thicker therefore diffusion harder. You won't get the prolonged action with local infiltration like you will with a regional block however.

It's worth reviewing the pertinent anatomy, even if you perform blocks regularly, ideally on skulls just to remind yourself where the foramina are, and how close other important anatomy is: i.e. globe!

Infraorbital In dogs: Beware the length of the canal is very short in brachycephalic breeds, so extreme caution regarding puncturing the eye. Foramen above P3, I insert just into the canal, aspirate, then inject a small volume very slowly. If I want a deeper block I may use the technique published by Viscasillas et al in 2013 when they advanced an IV cannula deeper into the canal (find foramen with stylet, then remove to advance silicone cannula caudally) and again a small volume- it's an enclosed space so a large volume/pressure could cause neuropraxia.

The maxillary block can be performed intraorally behind the last molar, under the rostral zygomatic arch through the skin (care- a case report of chihuahua with globe penetration after this block) or transorbitally ventrally through the conjunctiva (!). All approaches are with the needle perpendicular to the nerve so perhaps risk of nerve damage higher than the infraorbital where the needle runs parallel to nerve fibres, and you also need to be very careful about not perforating the eye as essentially you're passing the needle close to it but blindly (no pun intended). So, approach this block with extreme care.

Mental- don't do it. The neuromuscular bundle pretty much fills the whole foramen, so i think nerve damage is highly likely and you'd never know.

Inferior alveolar nerve block ('caudal mandibular')- I use an intraoral approach. And like Alex, I use the notch in the caudal mandible as my landmark for the angular process just caudal, then keep my index finger of non-dominant hand on this process, with my thumb on molar three. The foramen lies half way along this imaginary line on the medial aspect. In order to avoid the lingual nerve you need to ensure your needle stays very close to the medial surface of the mandible, and you should feel it running down the bone, aiming for your finger tip. To do this, approach from the opposite side you're blocking. Try not to prick the periosteum however, as this can cause pain, so keep the bevel of the needle facing the bone. Aspirate, aspirate, aspirate! The lingual nerve becomes blocked if you stray too far caudally and medially. I don't like the extra oral approach as you have to guess how far dorsally to place the needle, plus again you are approaching the nerve perpendicularly so perhaps nerve damage more likely?

Cats - the infraorbital canal is literally only a 2-3 mm long, and the globe is proportionally much larger than dogs, so globe penetration is a real risk with this block or the maxillary block. I published a case report on a cat that had globe perforation after an intro-oral maxillary block in JSFM a few years ago. So, to avoid this, either just enter the foramen with your needle and stop, or pre-measure the needle, so you know where the needle hub needs to stop for the needle tip not to go any more caudally than the medial canthus of eye- it might be the needle hub is on rostral aspect of maxillary canine for instance. By doing the infraorbital you're essentially doing a maxillary block as the canal is so short. If either block scares you, stick to local infiltration- a small bleb under the mucosa, on top of the periosteum. Do be are in cats if you use bupivacaine that they are very sensitive. I calculate 0.75mg/kg as a MAX dose and on't go over that.

IAN/Mandibular, landmarks essentially same as dog, but there isn't a distinct notch in caudal mandible, so open mouth to feel angular process, then the foramen is half way along an imaginary line between this and the molar.

Mental in the cat- don't even try.

If you're comfortable with the blocks you're doing, I wouldn't necessarily switch, but talking a moment to refresh anatomy is a good idea. Splash blocks upon closure also a good idea as long as you don't go over your total calculated dose for the patient.

Hope that helps!

RE: Local anaesthetic blocks

Alexander Geddes — Thu, 28 Mar 2019 18:12:32 GMT

I'm not aware adding opiates increases your chance of blocking the tongue, would of course leave your tongue blocked for longer.

Between the team here its been seen twice in 15 years - one managed with prolonged sedation the other with maxillo-mandibular fixation for 3 days. We use relatively low volumes for the mandible to minimise dispersion that may cover the lingual branch.

Students are placing these blocks under direction and the two times it has happened the block was actually performed by faculty or a resident. I think there a many risks in veterinary medicine that are higher than this happening so the extended cover is worth the risk, just my thoughts.

As for tips I woulds suggest palpate the angular process of the mandible and then place your thumb at the 3nd molar. If you look at a skull model the inferior alveolar foramen is right between these two landmarks. So then measure half that distance and insert your needle distolingual to the 3nd molar. Go through the lingual periosteum and advance it towards your finger placed at the angular process. You should then instil right over the foramen fairly consistently with low volumes.

Hope that helps!

RE: Local anaesthetic blocks

Norman Johnston — Thu, 28 Mar 2019 17:28:30 GMT

Not happened to me Liz because I have never added opiates

one if the other correspondents can maybe expand though - a presentstion regarding this was given 18 months ago in Nashville

RE: Local anaesthetic blocks

Liz Barton — Thu, 28 Mar 2019 16:48:04 GMT

[quote user="Norman Johnston"]One point about adding opiates to the local block, especially mandibular. If you inadvertently anaesthetise the motor nerves to the tongue you have a 72 hours “situation” on your hands and not just 4-6 hours. The dog can’t control the tongue.[/quote]

Have you ever seen this in practice?

[quote user="Norman Johnston"]Mandibular block very effective if done properly[/quote]

Any tips? I palpate the divot in the medial angle of the mandible and inject there.

RE: Local anaesthetic blocks

Norman Johnston — Thu, 28 Mar 2019 08:47:36 GMT

There are 4 pages in the new BSAVA manual 4^th edition. (2018) – chapter 6

One point about adding opiates to the local block, especially mandibular. If you inadvertently anaesthetise the motor nerves to the tongue you have a 72 hours “situation” on your hands and not just 4-6 hours. The dog can’t control the tongue.

Maxillary block I think more effective and less likely to damage infraorbital canal contents

Mandibular block very effective if done properly

Needle size needs to be no larger than 27g. Damage to soft tissue was 100% in a study by Anthony in 2007 as yet unpublished

Norman Johnston DipAVDC DipEVDC FRCVS

RE: Local anaesthetic blocks

Alexander Geddes — Thu, 28 Mar 2019 01:51:32 GMT

We never use the mental block I don't find it that easy and variable effects. Predominantly use maxillary, infra-orbital probably a bit easier landmark wise if you're just doing some rostral work. If those blocks are working I don't see any additional advantage to splash blocking, its not going to work great in comparison either I don't think.

We use bupivacaine and buprenorphine combinations for up to 72hrs of coverage and I think thats a great option. The less systemic analgesia you need the better and I feel good sending the patients home knowing they're blocked for at least the night. The cascade argument is a good one, theres definitely not literature looking at mepivacaine and buprenorphine, and have to admit we don't have cascade limitations here so its a non-issue. But as I said above I think prolonged block is very beneficial, is that enough of an argument? - mitigate unacceptable suffering - is that the definition for straying from the cascade?

RE: Local anaesthetic blocks

Michael Woodhouse — Thu, 28 Mar 2019 00:09:46 GMT

[quote user="F Brookes"]I think there's quite a good argument for bupivicaine on cascade as duration of action is fair bit longer. [/quote]

Genuinely interested. Sources seem to suggest that mepivacaine will last 3-5 hours and bupivicaine 4-12. So some overlap. Is there any good evidence that local analgesia for 10 hours is better than local for 5 hours in our veterinary patients?

I can see why we would use local but I'm not sure why it's better if the dog starts feeling things at 3pm or 8pm. To be honest I think I'd rather the local wore off whilst we still had the dog at the surgery and then we can always top up analgesia, rather than at night when the animal is home. I'm not really seeing the benefit once it's outlasted our GA and we've let our other analgesia get on board. That, and we should use licensed products wherever possible. Does bupivicaine really "mitigate unacceptable suffering" in a way that mepivacaine does not?

RE: Local anaesthetic blocks

Freya . — Wed, 27 Mar 2019 23:52:51 GMT

I think there's quite a good argument for bupivicaine on cascade as duration of action is fair bit longer.

RE: Local anaesthetic blocks

Michael Woodhouse — Wed, 27 Mar 2019 23:43:39 GMT

Where do you stand with the cascade? There are vet licensed procaine, lidocaine (=lignocaine) and mepivacaine, should we not be using those products?

RE: Local anaesthetic blocks

Evelyn Barbour-Hill — Wed, 27 Mar 2019 23:32:19 GMT

Cecilia Gorrel's chapter in the BSAVA Guide to Pain Management makes good reading.