Sodium Channel Blockers and Local-Regional Anesthesia

Benefits and Overview of Local-Regional Anesthesia

Definition and General Utility
- Local blocks are extremely effective, inexpensive, and easy to incorporate into daily veterinary practice.
- They work by blocking pain impulses that originate in the periphery, preventing them from reaching the central nervous system (CNS).
Specific Clinical Benefits
- Decreased requirements for inhalant anesthetics.
- Stabilized vital parameters during anesthesia (decreased variation).
- Prevention of continuing tissue damage and the development of peripheral and central sensitization.
- Smoother patient recoveries.
- Reduced requirement for post-operative analgesia and sedation.
- Faster overall healing times.
- Improved satisfaction for both staff and patients.

Questions & Discussion

Question 1
- Prompt: What part of the pain pathway do local blocks aim to prevent?
- Options:
  - A) Transduction
  - B) Transmission
  - C) Modulation
  - D) Perception
- Contextual Answer: Slide 78 notes that local techniques block the initial transduction of noxious stimuli to prevent pathway propagation.
Question 2
- Prompt: If local anesthetic toxicity occurs, what can be used to help to decrease the effects?
- Options:
  - A) Saline infiltration at the site of the local block
  - B) IV Epinephrine
  - C) Lipid emulsion infusion
  - D) Naloxone
- Answer: C) Lipid emulsion infusion.
Question 3
- Prompt: Where does the spinal cord typically end in a dog?
- Options:
  - A) $T13-L1$
  - B) $L6-L7$
  - C) $L7-S1$
  - D) Mid-sacrum
- Answer: B) $L6-L7$ .

Pharmacology of Local Anesthetic Agents

Mechanism of Action
- Local anesthetics act as sodium channel blockers.
- They deactivate sodium channels on the nerve membrane, which prevents depolarization and inhibits the generation and propagation of nerve impulses.
Commonly Used Drugs
- Lidocaine:
  - Onset: $1-5\,\text{minutes}$ .
  - Duration: $30-90\,\text{minutes}$ .
  - Dose (Dog): $4-6\,mg/kg$ .
  - Dose (Cat): $2-4\,mg/kg$ .
- Bupivacaine:
  - Onset: $3-20\,\text{minutes}$ .
  - Duration: $2-6\,\text{hours}$ .
  - Dose (Dog): $1-2\,mg/kg$ .
  - Dose (Cat): $1\,mg/kg$ .
- Nocita (Liposome-encapsulated bupivacaine):
  - Onset: $20-30\,\text{minutes}$ .
  - Duration: Up to $72\,\text{hours}$ .
  - Dose (Dog): Total of $5.3\,mg/kg$ .
  - Dose (Cat): Total of $10.6\,mg/kg$ .
- Ropivacaine:
  - Onset: $3-20\,\text{minutes}$ .
  - Duration: $2-6\,\text{hours}$ .
  - Dose (Dog): $1-3\,mg/kg$ .
  - Dose (Cat): $1-2\,mg/kg$ .
- Mepivacaine:
  - Onset: $2-20\,\text{minutes}$ .
  - Duration: $1-3\,\text{hours}$ .
  - Dose (Dog): $3-5\,mg/kg$ .
  - Dose (Cat): $2-3\,mg/kg$ .
Special Considerations for Nocita
- Extended release formulation (up to $72\,\text{hours}$ ).
- FDA approved for both dogs and cats.
- Dosed at $0.4\,ml/kg$ (total dose of $5.6\,mg/kg$ ).
- Does NOT contain preservatives; it must be kept refrigerated and discarded within $3\,\text{days}$ of the first puncture.
- Ideal for skin incisions and surgical site infiltration but NOT recommended for motor nerve blockade.
Chemical Properties and Tissue Environment
- Local anesthetics are basic drugs.
- $pKa$ of Lidocaine: $7.9$ .
- $pKa$ of Bupivacaine: $8.1$ .
- Normal tissue pH is $7.4$ . In an acidic environment (inflamed or infected tissue), these drugs tend to be highly ionized.
- Non-ionized molecules are the pharmacologically active form; therefore, local anesthetics can be LESS effective in acidic pH environments.

Adjuncts to Local Anesthesia

Dexmedetomidine (Alpha-2 agonist)
- Causes peripheral vasoconstriction, which decreases drug absorption and increases block duration.
- Preparation: Goal concentration is $1\,mcg/ml$ .
- Mixing: Add $0.02\,mls$ of $50\,mcg/ml$ dexmedetomidine to $10\,mls$ of $5\,mg/ml$ bupivacaine.
Buprenorphine (Partial mu agonist opioid)
- Has intrinsic sodium channel blockade properties.
- Can extend block duration from $6\,\text{hours}$ up to $18\,\text{hours}$ .
- Preparation: Goal concentration is $5-10\,mcg/ml$ .
- Mixing: Add $0.17\,mls$ of $0.3\,mg/ml$ buprenorphine to $10\,mls$ of $5\,mg/ml$ bupivacaine.

Complications, Contraindications, and Toxicity

Complications: Infection, Hematoma, Local anesthetic toxicity, Allergic reaction, and Nerve injury.
Contraindications:
- Active skin infection at the block site.
- Coagulopathy.
- Widespread infection or sepsis.
- Pre-existing neuropathy in the target area.
Local Anesthetic Toxicity Profiles
- Lidocaine: Primarily associated with neurotoxicity. Symptoms include sedation, trembling, seizures, and coma.
- Bupivacaine: Primarily associated with cardiotoxicity. Symptoms include arrhythmias and cardiovascular collapse.
Toxicity Rescue Protocol
- Lipid emulsion can be used as a rescue agent.
- Protocol: $4\,ml/kg$ IV bolus given over $2\,\text{minutes}$ , followed by a CRI at $0.5\,ml/kg/hr$ for $2-5\,\text{hours}$ .
- The bolus can be repeated if clinical signs (CSs) do not resolve.

Dental and Head Nerve Blocks

Indicational Scope: Dental extractions, gingival biopsies, oral surgeries (tongue, palate, mandibulectomy, maxillectomy), and rhinoscopy.
Anatomy: Trigeminal Nerve (CN V)
- Ophthalmic nerve: Sensory fibers for eyes, forehead, and nasal cavity.
- Maxillary nerve: Sensory fibers for upper lip, upper teeth, upper gingiva, nares, hard/soft palate, and cheeks.
- Mandibular nerve: Sensory fibers for lower lip, teeth, gingiva, part of tongue, and chin. Provides motor fibers to the muscles of mastication.
Specific Blocks
- Infraorbital Nerve Block:
  - Blocks rostral aspect of maxilla, including the carnassial tooth (upper 4th premolar).
  - Technique: Palpate infraorbital foramen; insert needle $1-2\,mm$ into canal; lift needle to confirm; aspirate and inject.
  - Note: Cats and small dogs have very short infraorbital canals.
- Maxillary Nerve Block:
  - Blocks the entire upper jaw and face.
  - Extraoral Technique: Insert needle perpendicular behind the cranial border of the zygomatic arch and cranial to the ramus of the mandible.
  - Intraoral Technique: Palpate notch at the ventral zygomatic arch caudal to the last maxillary molar; insert curved needle $1-3\,mm$ .
- Inferior Alveolar (Mandibular) Block:
  - Blocks lower lip, teeth, gingiva, and part of the tongue.
  - Technique: Palpate nerve inside the mouth behind the last molar (feels like a string); insert needle externally remaining medial to the mandible; feel for the bleb formation.
- Mental Nerve Block:
  - Blocks rostral to the mental foramen. Needle insertion at the level of the foramen (not necessarily inside).
Volume Selection per Site
- Cats/Small Dogs: $0.1-0.3\,mls$ .
- Medium Dogs: $0.3-0.5\,mls$ .
- Large Dogs: $0.5-1.5\,mls$ .

Ocular and Auricular Blocks

Auricular Block:
- Targets: Base of ear, inner auricular cartilage, and external ear canal.
- Technique: $22-25\,g$ needle; insert $1\,\text{inch}$ rostral to the vertical canal, directed toward the V formed by the zygomatic arch and vertical canal.
- Volume: $0.5-3\,mls$ per site.
Retrobulbar Block:
- Indications: Enucleation, evisceration, intraocular surgery.
- Technique: $1.5\,\text{inch}\;22\,g$ needle curved at $20-40\,\text{degree}$ angle. Insert halfway between lateral canthus and mid-lid; walk needle around bony orbit caudal to globe.
- Volume: $0.5-3\,mls$ .

Limb Blocks

Three Point Carpal Block:
- Targets: Radial, Ulnar, and Median nerves to desensitize digits and pads.
- Radial nerve: Insert needle where carpus articulates.
- Ulnar nerve: Insert at angle behind the carpal pad.
- Median nerve: Insert behind the first digit.
- Volume: $0.1-0.3\,mls$ per site.
Ring Block: Subcutaneous line of local anesthetic all the way around the dorsum and ventrum of the paw.
RUMM Nerve Block (Radial, Ulnar, Median, and Musculocutaneous):
- Blocks front limb distal to the elbow (does NOT fully block the elbow).
- Radial Nerve Site: $1/3$ up humerus length from lateral epicondyle, between triceps and brachialis.
- Ulnar/Median/Musculocutaneous Site: $1/2$ up humerus length from medial epicondyle, medial to brachialis near the brachial artery.
- Volumes:
  - Cats/Small Dogs: $0.1-0.3\,mls/site$
  - Medium Dogs: $0.5-1.5\,mls/site$
  - Large Dogs: $1.5-3\,mls/site$

Thoracic, Abdominal, and Genital-Urinary Blocks

Line or I/V Block: Used for incisions, mass removals, or wound repairs. Use an L or V pattern.
Intercostal Nerve Block:
- Indications: Thoracotomy, lateral thoracic wall lacerations, chest tube placement, flail chest (does NOT block viscera).
- Technique: Palpate rib in proximal $1/3$ quadrant; place needle on caudal border at $90\,\text{degrees}$ . Block two nerves cranial and two nerves caudal to site.
- Volumes:
  - Cats/Small Dogs: $0.1-0.3\,mls/site$
  - Medium Dogs: $0.5-1\,mls/site$
  - Large Dogs: $1-3\,mls/site$
Testicular Block:
- Inject into the body of the testicle toward the spermatic cord.
- Dose: $1\,mg/kg$ bupivacaine OR $4\,mg/kg$ lidocaine (divided between both testicles). Usually $0.2-2\,mls$ per testicle.
Mesovarium Block:
- Infiltrate the mesovarium after elevating the ovary.
- Dose: Lidocaine $2\,mg/kg$ ( $0.5-3\,mls$ per side).
Cavitary Lavage:
- Peritoneal or thoracic lavage with bupivacaine ( $1-2\,mg/kg$ ) prior to closing.
- Can be diluted with saline to total volume: $0.4-0.6\,ml/kg$ .

Epidural Anesthesia and Analgesia

Indications: Hind limb surgery (TPLO), caudal soft tissue surgery, laparoscopy, C-sections, and pancreatitis.
Anatomy and Landmark (L7-S1):
- Spinal cord ends around $L6-L7$ in dogs but can reach $S2$ in cats.
- Landmarks: Wings of ilium and the transverse process of $L7$ .
Technique:
- Insert needle directly caudal to the $L7$ transverse process perpendicular to the vertebral column.
- Advance through the ligamentum flavum (feel the "pop").
- Check for CSF or blood. Inject only if resistance is absent (air bubble in syringe should not compress).
Epidural Agents:
- Lidocaine: $1-3\,mg/kg$ . Duration: $1-3\,\text{hours}$ .
- Bupivacaine: $0.2-0.5\,mg/kg$ . Duration: $3-6\,\text{hours}$ .
- Morphine: $0.05-0.15\,mg/kg$ . Duration: $12-20\,\text{hours}$ . Side effects include pruritus and urinary retention.
Caudal (Sacrococcygeal) Epidural:
- Anesthetizes tail, perineum, penis, vagina, urethra, colon, and anus.
- Location: $S-C1$ or $C1-C2$ . Dose: $0.1-0.2\,mls/kg$ .

Advanced Techniques: Ultrasound Guidance

Increases efficacy and decreases complications via visualization.
Common US-Guided Blocks:
- Axillary brachial plexus, Sub-scalene brachial plexus.
- Inguinal femoral, Saphenous, Sciatic.
- Transverse abdominis plane (TAP) block, Quadratus lumborum block.

Lidocaine Continuous Rate Infusions (CRI)

Benefits: Maintains steady plasma levels, eliminates peaks/dips, and aids in multi-modal analgesia.
Contraindications/Cautions: Cats are more sensitive to neurologic side effects; limit to $1-4\,\text{hour}$ infusions and reduce dose or avoid.
CRI Dosing Table

Drug	Loading Dose	CRI Rate
Lidocaine	$0.5-2\,mg/kg$	$1.5-4\,mg/kg/hr\,\text{(dog)}, 0.5-2\,mg/kg/hr\,\text{(cat)}$
Hydromorphone	$0.025-0.1\,mg/kg$	$0.02-0.05\,mg/kg/hr$
Methadone	$0.1-0.5\,mg/kg$	$0.02-0.2\,mg/kg/hr$
Fentanyl	$2-10\,mcg/kg$	$2-10\,mcg/kg/hr$
Butorphanol	$0.05-0.3\,mg/kg$	$0.05-0.3\,mg/kg/hr$
Ketamine	$0.25-1\,mg/kg$	$100-600\,mcg/kg/hr$
Dexmedetomidine	$0.25-1\,mcg/kg\,\text{(+/-)}$	$0.25-2\,mcg/kg/hr$

Clinical Summary: By blocking the initial transduction of noxious stimuli, local and regional techniques prevent the expansion of the pain pathway and reduce peripheral/central sensitization.