Local and Regional Anesthesia – BCSE Study Guide

Overview and Clinical Importance

Local and regional anesthesia represents one of the most powerful tools in veterinary pain management. By blocking sodium channels in peripheral nerves, local anesthetics provide complete interruption of nociceptive transmission, offering profound analgesia that cannot be matched by systemic analgesics alone. These techniques reduce inhalant anesthetic requirements, minimize systemic drug exposure, and provide extended postoperative pain control.

On the BCSE, expect questions covering local anesthetic pharmacology (mechanism, onset, duration, toxic doses), specific nerve block techniques (dental, limb, epidural), and recognition and treatment of local anesthetic systemic toxicity (LAST). This domain integrates with pharmacology, anatomy, and surgery content.

Section 1: Local Anesthetic Pharmacology

Mechanism of Action

Local anesthetics block voltage-gated sodium channels in neuronal membranes, preventing depolarization and action potential propagation. This blocks both sensory (pain) and motor nerve transmission when used at appropriate concentrations.

Key Concept: Local anesthetics exist in equilibrium between ionized (charged) and non-ionized (uncharged) forms. The non-ionized form crosses the nerve membrane, while the ionized form binds to the sodium channel receptor from inside the cell.

MEMORY AID - pKa and Onset: "Lower pKa = Faster Action." Drugs with pKa closer to physiological pH (7.4) have more non-ionized molecules available to cross membranes. Lidocaine (pKa 7.7) acts faster than bupivacaine (pKa 8.1).

Physicochemical Properties Affecting Drug Action

Three key properties determine local anesthetic characteristics:

• pKa (Dissociation Constant): Determines ONSET. Lower pKa = more non-ionized drug at physiological pH = faster membrane penetration = faster onset.
• Lipid Solubility: Determines POTENCY. Higher lipid solubility = better nerve membrane penetration = greater potency. However, highly lipid-soluble drugs may become trapped in myelin, slowing onset.
• Protein Binding: Determines DURATION. Higher protein binding = longer attachment to sodium channel receptor = longer duration of action.

MEMORY AID - "PPL": pKa = onset (P), Protein binding = duration (P), Lipid solubility = potency (L). Think "PPL" - the Properties that determine Performance of Local anesthetics.

Classification: Amides vs. Esters

Local anesthetics are classified by their chemical linkage as either amides or esters. This classification affects metabolism and allergenicity.

MEMORY AID - Amide Recognition: Amide local anesthetics have TWO "i's" in their name: LidocaINE, BupivacaINE, MepivacaINE, RopivacaINE. Esters have ONE "i": ProcaINE, TetracaINE.

Commonly Used Local Anesthetics

Lidocaine Hydrochloride

Lidocaine is the most widely used local anesthetic in veterinary medicine due to its rapid onset and intermediate duration. It is available in 1% and 2% concentrations, with or without epinephrine.

• Onset: Rapid, 1-2 minutes (less than 5 minutes)
• Duration: 60-120 minutes
• pKa: 7.7 (closest to physiological pH)
• Recommended dose (Dog): 4-6 mg/kg
• Recommended dose (Cat): 2-4 mg/kg (cats are more sensitive)
• IV toxic dose (Dog): Approximately 20-22 mg/kg causes CNS toxicity (seizures)

Bupivacaine Hydrochloride

Bupivacaine provides the longest duration of action among commonly used local anesthetics, making it ideal for postoperative analgesia. However, it is the most cardiotoxic and should NEVER be administered intravenously.

• Onset: Slower, 5-10 minutes (up to 20 minutes for large nerves)
• Duration: 4-6 hours (up to 8-12 hours reported)
• pKa: 8.1 (farther from physiological pH, hence slower onset)
• Recommended dose (Dog): 1-2 mg/kg
• Recommended dose (Cat): 1 mg/kg maximum
• IV toxic dose (Dog): 4.3-8 mg/kg causes CNS toxicity; cardiovascular collapse at 20-22 mg/kg

Local Anesthetic Drug Comparison Table

Clinical Pearl: Ropivacaine is structurally similar to bupivacaine but is LESS cardiotoxic and causes LESS motor blockade, making it a safer alternative for procedures where motor function preservation is important.

Section 2: Nerve Blocks

Dental Nerve Blocks

Dental nerve blocks are essential for oral surgery in dogs and cats. They reduce inhalant requirements, provide superior intraoperative analgesia, and extend pain control into the postoperative period. Always aspirate before injection to avoid intravascular administration.

Infraorbital Nerve Block

• Nerves Blocked: Infraorbital nerve, superior alveolar nerves
• Area Desensitized: Upper lip, nose, skin rostral to infraorbital foramen, ipsilateral maxillary incisors, canine, and premolars 1-3
• Landmark: Palpate infraorbital foramen dorsal to the maxillary 3rd premolar (dogs) or 3rd premolar (cats)
• Technique: Insert needle intraorally just apical to PM3, advance only a few mm into foramen entrance. Apply digital pressure over foramen for 1 minute for caudal spread
• Volume: 0.2-1.5 mL per side

Inferior Alveolar (Mandibular) Nerve Block

• Nerves Blocked: Inferior alveolar nerve
• Area Desensitized: Ipsilateral mandible, all mandibular teeth, lower lip, chin
• Landmark: Mandibular foramen on medial aspect of ramus; angular process palpable extraorally
• Technique (Intraoral): Palpate mandibular foramen on lingual surface of ramus caudal to last molar, inject at foramen
• Technique (Extraoral): Insert needle just cranial to angular process, advance along medial ramus, center over ventral mandibular notch
• Volume: 0.2-1.5 mL per side

MEMORY AID - Dental Block Rule: "INFRAORBITAL = Upper teeth, INFERIOR ALVEOLAR = Lower teeth." Think: INFRAorbital is IN the upper jaw (maxilla), INFERIOR is below (mandible).

Mental Nerve Block

• Nerves Blocked: Mental nerve (terminal branch of inferior alveolar)
• Area Desensitized: Ipsilateral mandibular incisors, rostral lower lip
• Landmark: Middle mental foramen, located ventral to PM1-PM2 interspace at 2/3 mandibular height
• Volume: 0.2-1.0 mL per side

Peripheral Limb Nerve Blocks

Brachial Plexus Block

The brachial plexus block desensitizes the entire forelimb distal to the shoulder. It is commonly used for thoracic limb surgeries including fracture repair, amputation, and mass removal.

• Nerves Blocked: Radial, ulnar, median, and musculocutaneous nerves
• Technique: Insert needle medial to shoulder joint, advance toward costochondral junction of first rib. Aspirate and inject while withdrawing needle
• Volume: 0.2-0.3 mL/kg divided along plexus

Sciatic and Femoral Nerve Blocks

Combined sciatic and femoral nerve blocks provide analgesia for pelvic limb surgeries including stifle surgery (cruciate repair, patellar luxation), tibial fractures, and amputations.

Sciatic Nerve Block:

• Landmark: Groove between greater trochanter and ischial tuberosity
• Technique: Inject at 1/3 distance from greater trochanter toward ischial tuberosity
• Area Blocked: Caudolateral thigh, stifle, and all structures distal to stifle (except medial aspect)
• Volume: 0.1 mL/kg

Femoral Nerve Block:

• Landmark: Femoral triangle on medial thigh (bordered by sartorius, pectineus, iliopsoas muscles)
• Technique: Palpate femoral pulse, inject dorsomedial to femoral artery
• Area Blocked: Medial thigh, stifle, and saphenous distribution
• Volume: 0.1-0.2 mL/kg

MEMORY AID - Limb Block Coverage: "Sciatic = Stifle and South" (posterior/lateral limb below stifle). "Femoral = Front of thigh" (anterior/medial thigh and stifle). Need BOTH for complete pelvic limb analgesia.

Section 3: Epidural Anesthesia

Anatomy and Indications

Epidural anesthesia involves injection of drugs into the epidural space - the area between the dura mater and the vertebral canal wall. This space contains fat, blood vessels, and nerve roots of the cauda equina.

Key Anatomical Points:

• Spinal cord termination (Dog): L6-L7 in medium/large dogs
• Spinal cord termination (Cat): L7-S1 (more caudal, higher risk of dural puncture)
• Injection site: Lumbosacral space (L7-S1) most common in dogs and cats
• Epidural space diameter: 2-4 mm in medium dogs, less than 3 mm in cats

Technique

Step-by-Step Procedure:

• Position patient in sternal recumbency with hindlimbs pulled forward OR lateral recumbency
• Palpate wings of ilium - an imaginary line between them crosses the L7-S1 space
• Palpate the lumbosacral space as a depression on midline between L7 and sacrum
• Aseptically prepare the site and wear sterile gloves
• Insert spinal needle (20-22g) perpendicular to skin at approximately 45 degrees from vertical
• Feel for "pop" as needle penetrates ligamentum flavum
• Confirm placement using hanging drop or loss-of-resistance technique
• Aspirate to check for blood or CSF
• Inject drugs slowly

Techniques to Confirm Epidural Space Entry:

• Hanging Drop Technique: Place a drop of saline in needle hub; negative pressure in epidural space draws fluid into needle when space is entered
• Loss-of-Resistance: Attach saline-filled syringe; resistance decreases suddenly upon entering epidural space
• Tail/Limb Movement: Twitching may occur when needle contacts cauda equina (common in cats)

MEMORY AID - Epidural Landmarks: "Wings to the Dip" - Find the wings of the ilium with thumb and middle finger, your index finger will rest in the "dip" (lumbosacral space) at the midline.

Drugs and Dosages

Volume Guidelines: Use 0.2 mL/kg total volume up to a maximum of 6 mL for local anesthetics. Dilute with sterile saline to achieve desired volume. Volume determines cranial spread - 0.2 mL/kg typically spreads to L1-L2 level.

Contraindications and Complications

Absolute Contraindications:

• Skin infection at injection site
• Septicemia
• Coagulation disorders (thrombocytopenia, DIC)
• Abnormal pelvic anatomy preventing landmark identification

Potential Complications:

• Hypotension (from sympathetic blockade)
• Urinary retention (especially with morphine)
• Motor blockade/ataxia (with local anesthetics)
• Respiratory depression (if drug spreads too cranially)
• Epidural hematoma or abscess (rare)
• Delayed hair regrowth at shave site

Section 4: Local Anesthetic Systemic Toxicity (LAST)

Mechanism and Risk Factors

LAST occurs when local anesthetics reach toxic plasma concentrations, typically from accidental intravascular injection or rapid systemic absorption from highly vascular tissues. Both CNS and cardiovascular systems are affected due to sodium channel blockade.

Risk Factors for LAST:

• Small patient size (cats, toy breeds)
• Hepatic dysfunction (impaired amide metabolism)
• Hypoproteinemia (increased free drug fraction)
• Acidosis (shifts equilibrium toward ionized, trapped form)
• Hypoxemia
• Injection into highly vascular areas

Clinical Signs

The classic presentation is biphasic, beginning with CNS signs followed by cardiovascular collapse. However, cardiovascular signs may occur simultaneously or even first (especially with bupivacaine or under general anesthesia).

MEMORY AID - LAST Progression: "CNS before CV" - Generally, CNS toxicity (seizures) occurs BEFORE cardiovascular collapse, EXCEPT with bupivacaine where cardiac arrest may be the first sign. Remember: "Bupi Breaks the Rules - it goes for the heart first."

Treatment of LAST

IMMEDIATE ACTIONS:

• STOP administration of local anesthetic immediately
• Call for help and get lipid emulsion
• Secure airway and provide 100% oxygen
• Control seizures with benzodiazepines (diazepam, midazolam)
• Initiate CPR if needed

HIGH-YIELD NOTE - INTRALIPID PROTOCOL: 20% Lipid Emulsion (Intralipid): BOLUS 4 mL/kg IV, followed by CRI 0.5 mL/kg/min for 10 minutes. Can repeat bolus 1-2 times if needed. This is the FIRST-LINE treatment for bupivacaine cardiotoxicity!

Mechanism of Lipid Emulsion Therapy:

• "Lipid Sink" - creates a lipid compartment that sequesters lipophilic local anesthetics from tissues
• Provides direct energy substrate for impaired myocardium
• Enhances cardiac inotropy

Important Considerations:

• Do NOT use lidocaine to treat arrhythmias - it will worsen toxicity
• Avoid high-dose epinephrine (may worsen arrhythmias)
• LAST-induced cardiac arrest is resistant to standard ACLS protocols
• Prolonged CPR may be needed - do not give up early

MEMORY AID - Intralipid Dosing: "4 and 5" - Bolus is 4 mL/kg, CRI is 0.5 mL/kg/min. Keep a LAST rescue kit with 20% lipid emulsion available wherever nerve blocks are performed.

Prevention of LAST

• Calculate maximum safe dose BEFORE administration
• ALWAYS aspirate before injecting
• Inject slowly (allows time for detection of early signs)
• Use the lowest effective concentration
• Consider using ultrasound guidance for peripheral nerve blocks
• When combining local anesthetics, do not exceed maximum dose of EITHER drug
• Use epinephrine (1:200,000) to slow absorption and provide marker for intravascular injection