Have you got the nerve? (Proceedings)

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. Decreased permeability to sodium slows the rate of depolarization 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.

     • 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.

     • The 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.

Furthermore, local anesthetic blocks are extremely cost effective and can increase profits to the clinic.

Commonly used local anesthetic drugs in veterinary medicine include

     • Lidocaine

          o Onset of action: rapid (less than 5 minutes)

          o Duration of action: 60-120 minutes

          o Dose 2-6 mg/kg (use the lower end of the dose in cats)

          o Convulsive dose in dogs: 11-20 mg/kg

          o Lethal dose in dogs: 16-28 mg/kg

          o 'Toxic dose' in cats reported as 6-10 mg/kg

          o The general recommendation for clinical use is ≤ 6 mg/kg in the dog and ≤ 3-4 mg/kg in the cat.

     • Bupivacaine

          o Onset of action: approximately 5-10 minutes after injection (up to 20 minutes)

          o Duration of action: 4 to 6 hours

          o Dose 1-2 mg/kg (use the lower end of the dose in cats)

          o Convulsive dose in dogs: 3.5-4.5 mg/kg

          o Lethal dose in dogs: 5-11 mg/kg

          o Data is mostly anecdotal in the cat but the general feeling is that 3 mg/kg is the toxic dose.

          o The general recommendation for clinical use is ≤ 2 mg/kg in the dog and ≤ 1 mg/kg in the cat.

Adverse events caused by 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)

     • Anaphylaxis – rare, more common with esters (but still rare)

     • Central nervous system – muscle tremors, seizure, coma

          o 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.

          o Cardiovascular system – the myocardial conduction system is sensitive to local anesthetics and IV boluses can result in cardiovascular collapse. ONLY LIDOCAINE CAN BE ADMINISTERED IV.

          o Methemoglobinemia – rare, but can occur in cats.

Commonly used local anesthetic blocks in veterinary medicine

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 orvolume injected for each block) should not exceed this total dose.

     • 'Field' block

          o 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.

     • Oral blocks

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 distal 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 together).

               • 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 formamen. 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 (0.1 to 1.0 ml).

          o Maxillary or infraorbital nerve block – caudal approach

               • Insert the needle percutaneously along the ventral border of the zygomatic process approximately 0.5 cm caudal to the lateral canthus of the eye. The needle is 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. Aspirate and slowly inject 0.1-1.0 mls local anesthetic.

               • An alternate technique is to approach the pterygopalatine fossa from the ventral margin of the orbit.

          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 ⅓rd of the mandible (as measured from top to bottom).

               • Regardless of whether or not 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 contraption to insure that the patient doesn't close its mouth while your hand is in the oral cavity), direct a needle to the site described above.

                    √ 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 with the needle perpendicular to the mandibular cortical bone, to the level 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 labial frenulum.

               • Insert needle tip just cranial to the foramen, aspirate and slowly infuse Apply digital pressure over injection site for 60 seconds in order to ensure maximum caudal/distal diffusion of agent into mandibular canal.

     • Onychectomy blocks

          o Three point block

               • Locate the carpus and the accessory carpal pad

               • Inject 0.1-0.3 mls subcutaneously at each of three sites: 1) medial to the accessory carpal pad (blocks median nerve and palmar branch of the ulnar nerve); 2) lateral and proximal to the accessory carpal pad (blocks dorsal branch of the ulnar nerve); and 3) on the dorsal-medial portion of the carpus (blocks superficial branches of the radial nerve).

          o Ring block

               • Insert a 22 or 25 G needle subcutaneously across the dorsum of the paw just proximal to the accessory carpal pad.

               • Inject ¼ of your calculated dose (eg, ¼ of 1 mg/kg bupivacaine in a cat) of local anesthesia in a 'line' as you withdraw the needle.

               • Repeat this process on the palmar aspect of the paw, in essence making a 'ring' by utilizing the line blocks across both the dorsal and palmar surfaces of the paw.

     • Intercostal block

          o Inject local anesthetic in the tissues caudal to the proximal portion of the ribs. Inject local anesthetic in 2-3 rib spaces in front of and 2-3 rib spaces behind the area that needs to be desensitized.

     • Brachial plexus block

          o Locate the point of the shoulder, the first rib and the transverse processes of the cervical vertebrae.

          o Insert a 2-3 inch needle (an epidural needle will work) at the point of the shoulder to the point where the tip of the needle is even with the first rib. Keep the needle horizontal during placement so that the tip does not enter the thoracic cavity.

          o Aspirate, then inject ⅓ of the local anesthetic (2 mg/kg bupivicaine diluted with saline to a total 1 ml solution per 4.5 kg body weight) at this site, slowly withdraw the needle to the middle of the area to be blocked, aspirate and inject ⅓ of the local anesthetic. Withdraw the needle to a site just before it exits the skin, aspirate and inject the remaining ⅓ of the local anesthetic.

     • Intra-articular block

          o This block is used for analgesia following intraarticular anesthesia. Inject 1-5 ml local anesthetic into the joint prior to surgery and repeat the injection after the joint has been sutured closed.

     • Epidural block

          o Opioids are most commonly used but local anesthetic drugs can be used in conjunction with opioids.

               • mg/kg morphine (preservative-free is gold standard but morphine with preservative is commonly used in veterinary medicine)

               • Dilute to 1 ml/4.5 kg with bupivacaine, sterile saline or sterile water

          o Provides up to 24 hours of analgesia with little to no systemic effects. The opioids will cause sensory blockade but will not cause motor blockade. The local anesthetics can cause motor blockade, however, the motor effects are generally minimal or absent by the time the patient recovers from anesthesia to the point that it is ambulatory.

          o Consider for any pain in caudal half of patient. Examples include, rear limb soft tissue or orthopedic surgery, abdominal exploratory and bladder surgeries, surgeries on the tail or perineal region, etc...

          o Technique:

               • Place the anesthetized patient in dorsal or ventral recumbency

                    √ Legs can be placed forward or to the back. I prefer forward in cats and small dogs.

               • Locate the wings of the ilium and palpate the lumbo-sacral (LS) space (almost directly in line with the wings of the ilium on the mid-line).

               • Clip and scrub this region. Wear gloves and use a small drape or glove wrapper.

               • Insert an epidural needle into the caudal portion of the LS site with the needle angled at approximately 45° from vertical.

               • Slowly advance the needle until the epidural space is entered.

                    √ 'Hanging' drop often works (aspiration of fluid in the hub of the needle as the epidural space is entered).

                    √ Several 'pops' will be felt.

                    √ 'Walking off' the bone is the most definitive determination of proper placement of the needle.

               • STOP as soon as the space is entered and slowly inject the drug.

                    √ The drug should inject easily if the needle is in a space.

                    √ If the drug does not inject easily, back up a VERY tiny amount and try again.

               • Once drug has been injected, remove needle and proceed with surgery.

                    √ If local anesthetic drugs have been used, may want to lay patient with surgical side down for about 5 minutes.

               • Opioid epidurals do NOT affect motor function of the rear limb or diaphragm. Local anesthetic drugs can affect motor function but rarely do (volumes that are described here do not migrate far enough cranially to affect the diaphragm so ventilation is not impaired).

          o Complications include ineffective block (by far most common complication), epidural hematoma or abscess, hyperalgesia (VERY rare). Contraindications include bleeding disorders (to prevent hematomas) and skin disease over the LS space (to prevent abscesses). Abnormal pelvic anatomy (either from congenital lesions or trauma) may make epidurals difficult.

          o Epidural catheters are fairly easily placed in larger dogs and can be maintained for several days to allow continuous or intermittent delivery of analgesic drugs.