HOW TO STOP PAIN IN ITS TRACKS: LOCAL/REGIONAL BLOCKS!

Tamara Grubb, DVM, PhD, CVMA, CVPP, Diplomate ACVAA

President, International Veterinary Academy of Pain Management (2023-2025)

Local anesthetic drugs are extremely effective, inexpensive and easy to use. When local anesthetic drugs are administered, pain impulses originating in the periphery are blocked and prevented from reaching the central nervous system. This blockade has several positive consequences:

The sensation of pain is alleviated or even eliminated for the duration of the block. Local anesthetic

drugs work by blocking sodium channels in nerve membranes so that the threshold potential is not

achieved and an action potential is not propagated, thus the pain impulse is not propagated. Local

anesthetics bind more readily to ‘open’ channels, thus rapidly firing nerves are more susceptible to

blockade.

• Analgesia allows the patient to be maintained under a lighter plane of anesthesia and this makes the anesthetic episode safer for the patient. In fact, local anesthetic drugs decrease the minimum alveolar concentration (MAC) of all anesthetic gases.

• The likelihood that ‘wind-up’ or hypersensitization will occur is greatly decreased because the portion of the pain pathway called ‘transmission’ is blocked. Transmission involves the conductance of pain impulses from the peripheral nociceptors to the dorsal horn neurons in the spinal cord. The neurons in the dorsal horn are responsible for central sensitization. By blocking input to these neurons, central sensitization (or ‘wind up’) is less likely to occur.

• And blocks are very cost effective!!

Commonly used local anesthetic drugs in veterinary medicine include

Lidocaine

• Onset of action: rapid; approximately 1-2 mins (less than 5 minutes)
• DOSE: 4-6 mg/kg in the dog and 2-4 mg/kg in the cat
• Duration of action: 60-120 minutes
• Convulsive dose in dogs: 11-20 mg/kg; Lethal dose in dogs: 16-28 mg/kg
• ‘Toxic dose’ in cats reported as 6-10 mg/kg

Bupivacaine

• Onset of action: approximately 5-10 minutes (up to 20 minutes)
• Duration of action: 4 to 6 hours
• DOSE: 1-2 mg/kg in the dog and 1 mg/kg in the cat
• Convulsive dose in dogs: 3.5-4.5 mg/kg IV
• Lethal dose in dogs: 5-11 mg/kg IV
• Data is mostly anecdotal in the cat but the general feeling is that 3 mg/kg IV is the toxic dose
• NOCITA®: (not available in all countries) liposome-encapsulated bupivacaine that provides
• analgesia for 72-hours.
• FDA-approved in both dogs (5.3 mg/kg) and cats (10.6 mg/kg)
• FDA-approved for tissue infiltration (dog) and peripheral nerve block (cat) but used for both tissue infiltration and nerve blocks in both species.
• Injection technique important – put the liposomes where you want them – ie, deliberate, thorough injection into the tissues/around the nerves.

Ropivacaine

• Onset of action: approximately 5-10 minutes (up to 20 minutes)
• Duration of action: 4 to 6 hours
• IMPORTANT POINT: Less cardiotoxic than bupivacaine
• DOSE: 1-3 mg/kg in the dog and 1-2 mg/kg in the cat
• International Veterinary Academy of Pain Management IVAPM.org
• Convulsive dose in dogs: 20 mg/kg IV
• Lethal dose in dogs: 42 mg/kg IV
• No data in cats but predicted to have same wider safety range than bupivacaine

Mepivacaine

• Onset of action: 2-5 minutes (up to 10 minutes)
• Duration of action: 2-3 hours
• DOSE: 3-5 mg/kg in the dog and 2-3 mg/kg in the cat
• Used primarily for diagnostics in equine lameness but effective for all blocks
• Lethal and convulsive dose in dogs: 29 mg/kg
• No toxic dose published for cat

Additions to local blocks to prolong duration of action (potentially up to 24 hrs), minimum dosages:

• Buprenorphine: 0.003–0.004 mg/kg (Snyder & Snyder, 2016; Grubb & Lobprise 2020)
• Dexmedetomidine: 0.0001 mg/kg (Bartel et al., 2016; Grubb & Lobprise 2020)

Adverse effects of local anesthetic drugs

Adverse events are extremely rare but can include any of the following:

• Local tissue effects – swelling, bleeding, inflammation, ‘tingling’? (unknown if this occurs in animals)
• Central nervous system – muscle tremors, seizure, coma
• At lower concentrations, depression of inhibitory neurons occurs and can cause cerebral excitation, which may lead to seizures. At higher concentrations, profound CNS depression with subsequent coma, respiratory arrest and death can occur. The latter is more likely following IV boluses of large doses.
• Cardiovascular system – the myocardial conduction system is sensitive to local anesthetics and IV boluses, especially of bupivacaine (but not NOCITA – high safety margin) can result in cardiovascular collapse. ONLY LIDOCAINE CAN BE ADMINISTERED IV.
• Anaphylaxis – rare, more common with esters (but still rare).
• Methemoglobinemia – rare, but can occur in cats.

Commonly used local anesthetic blocks in veterinary medicine (starting with the author’s favorite – and most commonly used - blocks)

For many of the blocks listed below, a suggested volume of drug is listed based on the amount of drug that can physically be injected into the site. However, with all blocks, the total dose that the patient can receive should be calculated and the cumulative dose (add up the dose or volume injected for each block) should not exceed this total dose.

• Advanced techniques: All of the blocks in this manuscript can be completed with appropriate knowledge of anatomy, good palpation skills, a needle and syringe. However, more advanced techniques (nerve locators, ultrasound guidance) can also be used and may be an option for some practices. Desensitization for some blocks (e.g., brachial plexus) may be more consistent with these techniques, whereas for some blocks (e.g., intraperitoneal lavage) these techniques are not useful.

A. General blocks

• ‘Field’ block (also called incisional block or line block)
• Blocking the ‘field’ of surgery. Local anesthetic drugs can be administered around the incision or directly into the incision. It is NOT true that lidocaine in an incision causes a delay in healing.
• NOCITA is a good choice for long duration ‘field’ block in both dogs and cats. Duration = 72 hrs.
• Indwelling catheter block (long duration field block)
• Indwelling, or ‘soaker’, catheters should be considered for large wounds or incisions that may be difficult to block or that may require continuous or intermittent delivery of drug for several days.
• The catheters can be buried in or near incisions and local anesthetic infused through the catheter to provide longer analgesia.
• Very useful for surgeries with large incisions, e.g.: amputations, mastectomies, etc.
• Local anesthetic drugs can be infused via a pump or administered by intermittent injection (eg, q 6–8-hour injections of bupivacaine at 1-2 mg/kg).
• The catheter is generally removed in 48-96 hours but can be left in longer.

B. Blocks on the Head

 Maxillo-facial (‘dental’ or ‘oral’) blocks (Figure 1)

Blocks listed below will cause unilateral desensitization from the site of injection rostrally to

midline.

o Maxillary or infraorbital nerve block – cranial approach

 The infraorbital nerve exits the infraorbital foramen, which can be palpated as a

depression in the buccal mucosa dorsal to the root of the maxillary 3rd premolar (just

cranial to the root of the 4th premolar or carnassial tooth in the area where the gingiva

on the maxillary bone and the gingiva on the lip join).

 Block the nerve by injecting local anesthetic under the gingiva just rostral to the

foramen or insert the tip of the needle into the infraorbital canal and inject. Injecting

into the foramen insures more caudal spread of the block but is not necessary if the oral

surgery site is rostral to the foramen. Also, the foramen can be difficult to locate or to

enter in small dogs and cats & infusion rostral to the canal is still useful as there will be

some caudal migration of the local anesthetic into the canal.

 A vessel runs with this nerve so aspirate, then slowly infuse drug

o The volume that can be injected is approximately 0.1 to 1.0 ml, depending on patient size.

o Caudal Maxillary

 The caudal maxillary approach is often preferred over the infraorbital approach because

the field of desensitization is much larger. This block will desensitize all ipsilateral tissue

from the caudal molars rostrally and from the skin to midline. Use this approach if

working on the caudal molars or doing surgeries on the nares, nasal passages, sinuses,

soft palate or any other structures of the maxilla. In cats and brachycephalic dogs, the

distance from the infraorbital foramen to the pterygopalatine fossa (where the maxillary

nerve and its branches enter the skull) is very short and caudal diffusion of drug injected

at the infraorbital foramen may be adequate for blocking the caudal structures of the

skull.

 Extraoral approach 1 (extraoral, from zygomatic arch): Insert the needle percutaneously

along the ventral border of the zygomatic arch approximately 0.5 cm caudal to the

lateral canthus of the eye. The needle is kept horizontal and directed medially and

slightly cranially (in an angle that would draw an imaginary line with the premolars on

the opposite side of the head) until it hits bone. At this site, the maxillary nerve enters

the pterygopalatine fossa.

 Extraoral approach 2 (extraoral, from bony orbit): Approach the pterygopalatine fossa

from the bony orbit. The needle is placed at the midpoint of the ventral rim of the bony

orbit and inserted straight down between the globe and the bone.

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 Intraoral approach: Open the mouth as wide as possible. From inside the mouth, insert

a short needle of no more than 2-4 mm (to avoid being close to the globe) inside the

mouth just caudal and medial to the last molar.

 For all 3 techniques, aspirate and inject. The volume that can be injected is

approximately 0.1 to 1.0 ml, depending on the patient’s size.

o Mandibular nerve block

 The mandibular foramen or the mandibular nerve can often be palpated on the lingual

side of the mandible just rostral to the angle of the mandible and just caudal to the last

molar in approximately the middle 1/3rd of the mandible (as measured from top to

bottom).

 Regardless of whether the nerve or foramen can be palpated (often difficult to palpate

in very small patients), the landmarks described above will be utilized for deposition of

local anesthetic drug.

 The nerve ENTERS the mandible at the mandibular foramen and cannot be blocked

between the mandibular foramen and the mental foramen.

 Intraoral technique:

• With the patient’s mouth supported in the open position (ie, use a mouth gag, roll

of tape or some other method to ensure that the patient doesn’t close its mouth

while your hand is in the oral cavity), direct the tip of the needle to the site

described above.

o REMEMBER: Rigid mouth gags should NOT be used in cats. They can cause

occlusion of the maxillary artery with resultant blindness and/or neurologic

complications.

• Aspirate, then slowly infiltrate (0.2 -2.0 mls). The foramen cannot be entered so the

drug is merely infused under the gingiva at the site of the nerve.

 Extraoral technique:

• Landmarks are the same as those described above but the approach is from the

outside, through the skin at the angle of the mandible. This technique is easier than

the intraoral technique in cats and in some small dogs.

• Pass the needle through the skin along the medial aspect of the mandible to a point

where the tip of the needle is at the site of the foramen (again, aiming for a site

just caudal to the last molar on the lingual side of the mandible).

• With a finger in the oral cavity the needle can be felt under the gingiva.

• When the site near the mandibular foramen is reached, aspirate and inject the local

anesthetic drug (0.2-2.0 mls).

o Mental nerve block

 The mandibular nerve EXITS the mandible at the middle mental foramen which can be

palpated just ventral to the root of the 2nd premolar, immediately caudal to the labial

frenulum.

 Insert the needle tip just cranial to the foramen, aspirate and slowly infuse 0.1-0.5 mls

local anesthetic. Apply digital pressure over injection site for 30-60 seconds to ensure

maximum caudal/distal diffusion of the drug into mandibular canal.