Local Anesthesia: Mechanism and Effects

• General anesthesia stops pain perception, but local anesthesia (LA) blocks pain signal initiation and conduction in nerves.
• LA should be part of a multi-modal analgesic plan.
• LA minimizes systemic drug requirements and wind-up pain, and can be administered before, during, or after surgery.

Mechanism of Action

• Analgesia comes directly from sodium ion channel blockade and membrane stabilization.
• Blockade occurs from inside the nerve cell, preventing nerve cell depolarization, and inhibits nerve impulse conduction.
• Patients experience a dose-related decrease in sensory, motor, and autonomic function, and the effect is temporary, as the anesthetic is metabolized or hydrolyzed.

Local Anesthetic Effects

• Small nerve fibers (C and Aδ) are blocked before large myelinated fibers.
• Sensory loss occurs before motor function loss.
• The order that sensations disappear is pain, cold, warmth, touch, joint, and, finally, deep pressure.
• Lipid solubility of LA has the following effects: A high lipid solubility equals more potent effects, and greater lipid solubility increases the length of action.
• LA speed of onset is inversely related to the drug's lipid solubility and pKa.
• The more protein-bound the LA is, the longer the duration of action (e.g. bupivacaine > lidocaine).
• Increasing LA solution concentration in a specified site increases analgesia duration.
• Systemic absorption increases (e.g. vasodilation), the duration of effect decreases.
• Lidocaine causes vasodilation, reducing its duration of action.

Local Anesthetics - Esters

• Procaine is generally combined with other drugs, like procaine penicillin.
• Tetracaine is another ester.
• Benzocaine, if sprayed on the larynx in cats, may induce methemoglobinemia.

Local Anesthetics - Amides

• Amides are more commonly used.
• Lidocaine is in this family.
• Mepivacaine is an amide.
• Bupivacaine is in this family.
• Ropivacaine is an amide.
• Lidocaine + Prilocaine (EMLA cream) is included.

Lidocaine

• A commonly used LA with a quick start time.
• The pKa is 7.9.
• Low protein binding and vasodilation contribute to a short duration of action.
• It does minimal tissue damage or irritation.
• It is safe in IV form at clinical doses.
• Other effects include anesthetic sparing, increased analgesia, antiarrhythmic, GI motility, anti-inflammatory, and anti-shock effects.

pKa and Ionization

• Dissociation constant, or pKa, denotes the pH at which 50% of an acid or base exists in an ionized condition.
• Most LAs are weak bases with pKa values between 8 and 9, and at a pH of 7.4 (physiological), they are mostly ionized.
• Un-ionized is required to cross lipid cell membrane; Therefore, infected tissue blocks is less effective.
• In an acidic environment the drug shifts to its ionized form, and in an alkaline environment the drug shifts to its unionized form.

Mepivacaine

• The local anesthetic commonly used for diagnostic nerve blocks for lameness examination in horses.
• Due to its lower pKa of 7.6, the onset is faster than lidocaine.
• The duration of action last longer than lidocaine due to to high protein bounding.
• There is less irritation or tissue damage.
• Because there are no vasodilator effects, epinephrine is not required.

Bupivacaine

• Longer time to onset when compared to lidocaine due to a higher pKa of 8.1 and an increase in lipid solubility.
• Longer duration of action.
• Has high protein-binding properties.
• There is a greater concern for systemic cardiotoxicity
• Should never be given IV!
• The Margin of safety is the lowest compared to lidocaine, mepivacaine, and ropivacaine.

Bupivacaine Liposome Injectable Suspension

• Nocita® provides bupivacaine's extended release for up to 72 hours.
• Approved by FDA in 2016 to control pain in dogs undergoing cranial cruciate ligament surgery.
• Administer as multimodal analgesia.
• Now can be used on single-use cats for onychectomy.
• Dosage is 0.4 mL/kg (maximum dose of 5.3 mg/kg).
• Use a moving needle infiltration technique while making sure to inject in all layers of the tissue.

Ropivacaine

• Similar chemical properties to bupivacaine but is a pure S-(-) enantiomer.
• The is less CV (cardiovascular) and CNS (central nervous system) toxicity than bupivacaine.
• The onset time is concentration-dependent.
• Low concentrations equal similar time to bupivacaine.
• Higher concentrations equal similar onset time to mepivacaine.
• Its actions are of similar duration to bupivacaine; typically, is more expensive.

Drug Comparison Chart

• Lidocaine (2% solution) has an onset of 10-15 minutes with a 1-2 hour duration, and a toxic dose of 6-8 mg/kg (dog) an 2-3 (cat)
• Mepivacaine (2% solution) has an onset of 5-10 minutes with a 1.5-3 hour duration, and a toxic dose of 6 mg/kg (dog) an 3 (cat)
• Bupivacaine (0.5% solution) has an onset of 20-30minutes with a 4-6 hour duration, and a toxic dose of 3-4 mg/kg (dog) an 2 (cat) Ropivacaine (Varies) has a concentration dependent onset with a 4-6 hour duration, and a toxic dose of 5 mg/kg (dog), unknown (cat), so use less than 2 mg/kg

Mixing Local Anesthetics

• Combining local anesthetics can results that are unpredictable, and shorten the duration of action.
• Attention must paid to toxic dosages, diluting 1:1 with sterile saline.

Additives to Local Anesthetics

• Opioids such as preservative-free morphine, when administered with LA for epidural or spinal anesthesia, provide analgesia lasting up to 24 hours, but may cause systemic side effects (respiratory depression, urinary retention, vomiting in awake patients, and pruritus)
• Buprenorphine is administered with LA for certain nerve blocks, e.g., dental and ring blocks
• Alpha2 agonists, like xylazine, medetomidine, and dexmedetomidine when used in small doses with LA in locoregional anesthesia, improve analgesia and duration of action, but can cause systemic side effects (hypertension & bradycardia)
• Ketamine can be added as it enhances analgesic effects blocking NMDA (N-methyl-D-aspartate), sodium, and potassium channels. Side effects may include an increase in HR and BP.

Other Drugs Having Effects on LA

• Epinephrine (vasoconstrictor) may be added to increase action duration by delaying absorption, potentially decreasing LA toxicity.
• Dose: 0.1mg : 20 mL of LA or 5 µg/mL.
• Never use on distal extremities because it may cause tissue necrosis.
• Hyaluronidase can be used to hasten onset time, but it could also lead to more systemic absorption and toxicity.
• Duration of block may also be decreased; it may be utilized in exotics.
• Sodium bicarbonate can be added to lidocaine to reduce pain on injection and possibly enhance the effect by increasing diffusion across the nerve cell membrane.
• Dose: 1:9 ratio (or 1 mEq per 10 mL of LA)

General Considerations for LA Use

• Aspirate for spinal fluid prior to injecting it to prevent serious complications.
• Stop injecting it if feel resistance to prevent intraneural injection.
• Avoid injecting in open areas (inflamed or infected and avoid area to prevent neoplasia).
• Make sure to select the correct gauge (smaller is generally better), and the select the adequate length of needle for particular patient.
• Use only sterile needles and solutions.
• Before beginning a painful procedure, wait and make sure to allow adequate time for the block to occur and set in.
• Be aware of risks of performing a local anesthesia technique as well as species-specific toxic doses before administration.
• Make sure to label drug type, if you out of your syringe dose avoid “interchange” to prevent errors administration.

Toxic Effects of LAs

• Neurologic effects

• may be harder to notice when the animal is under anesthesia

• Sudden alterations in mental state or sedation. – Agitation – Muscle twitching – Nystagmus – Seizures – Coma

• Cardiorespiratory effects: – Respiratory arrest – Cardiac dysrhythmias – Prolonged P-R interval, prolonged QRS interval, AV block, VPCs, ventricular tachycardia, ventricular fibrillation – Hypotension due to vasodilation – Myocardial depression/cardiac toxicity – Death

• With most LAs (except bupivacaine), reach CV signs quicker than producing CNS toxicity so, larger doses are required.

• Recognize cardiotoxic effects between medications so it is easily differenced.

• Lidocaine results in hypotension and bradycardia when given in overdose.

• ropivacaine and bupivacaine can can lead to sudden collapsing or ventricular dysrhythmias.

Other Potential Effects

• Cats are likely to develop Methemoglobinemia .
• Do not recommend using it in patients that are hypotensive or in a state shock to the effects of LAs causing increased sympathetic blockade > CV collapse.
• It is contraindicated to use a local anesthetic technique with coagulopathy due to possible hemorrhage. Inhalant anesthesia decreases metabolism of lidocaine; If greater than to 2 mg/kg in cats, it is advised to use lower dosage.
• Allergic reactions are rare, although it is more possible with use of ester-type LA.

Treatment

• Always see CNS signs( depression transition to excitation ) prior to CV signs unless if bupivacaine was given as accidental IV.
• Administer supportive treatment based on type signs(CNS VS CV) vs if cardiac arrest start CPR.
• If signs of nervous reactions: Provide anticonvulsants :(midazolam or diazepam @ 0.5mg/kg IV
• Always have supplemental oxygen if needed. Provide continuous ventilation and make sure patient is intubation.
• Continuous monitoring of ECG and blood pressure.
• If signs of BP issues: BP support support: consider IVF/ Vasopressors /inotropes/ anti-arrhythmia as needed.
• Early Intravenous lipid emulsion therapy (reported in rats, dogs and cats) should be administered if indicated.

Species-Specific Considerations

• Cats have limited ability to kidneys.
• Cats have limited ability to metabolize LA (due to the conjugation step needed prior to elimination via the kidneys) so they are more susceptible to toxicity from Lidocaine or/and Burpivacaine.
• Cats are more likely to have Methemoglobinemia and Heinz body anemia with certain local anesthetics.
• Benzocaine must be avoided with small exotics and ferrets.
• Measure kid and lamb for dehorning and castration procedure. An overdose must be easily administered to them due to having a small body mass.

Basic Equipment Needed

• Needles – Hypodermic – Epidural/spinal needles (sharp bevel and have a stylet, > prevents tissue from being introduced into injection – 27 to 18 g. and (2-9 cm)
• Syringes – 1-20ml – glass / low for used with epidurals

Advanced Equipment Needed

• Peripheral nerve stimulators can improve LA block decreased volume of LA decreasing unwanted side effects.
• Insulin needles that contain injection ports.
• Ultrasound-can aide in visualization of nerve.
• Epidural catheter kit
• Tuohy needle(curved tip for facilitation)
• Syringe with a Loss of resistants
• Radiopaque catheter with guide wire
• Connector and antibacterial filter

Infiltration of LA

• Line block vs. block of surgical incision
• ‘field’ block — used in biopsies, low-pain procedures.
• Splasch block- used in enucleations.
• Can preform ring blocks in cats- can be found in online link-
• Inverted ‘L’ block
• Diffusion catheter (“soaker
• Intermittent boluses of bupivacaine (q 6
• 1hr) or CRI of lidocaine

Intercostal Blocks

• Indicates analgesia with fracture of traumatized Thoracotomy, rib and chest tube placement.

• Landmarks*

• Caudal (at tail ) to edge of rib for avoidance of vein and artery.

• Always put close or as spine as distal to anesthesize region for injection.

• Always select location that is distal- for better outcomes due to overlapping for adequate nerve function.

• If going for long, Never go depth, as it would penatrate pleura

• Interpleural block

• Provides analgesia to provides abdominal- including viscera.

• Apply LA inside the abdomen prior

Intratesticular Block

• Split 2mg/kg lidocaine between the 2 testicles
• For cats decrease the dose by half Pre scrotal line block used for dogs can reverse a amount.
• When surgicaly prepping area, make sure to clip and cleaned first-prior to administering.
• It is important to use a 22-25 g. needle
• To inject syringe, Aspirate
• Lidocaine travels and maximizes in up the spermatic cord to reach desired effects within minutes.

Ophthalmic & Ear Blocks

• Retrobular allows to provides anesthesia to the eye and avoid “cancer”
• AComplications of intravasuclar injections is that it can lead to intravascular injection, harm and damage and sub. Make sure to use with caution.
• auricotempranol, which can leads paralysis of nerves. prevent blinking.

Dental nerve blocks

• Mental nerve block- used for rostral tissues
• For procedures involving rostral mandible and associated teeth, rostral lower lip
• Infraorbital nerve block- used for rostral part.
• Effectiveness of block depends on whether injection is made inside the foramen or only at the entrance.
• Maxillary nerve block-* for upper. If both nerve blocked caution is needed or.
• *Could perform bilaterally for a rhinoscopy
• if dental extractions are happening in the mandibule area.

Thoracic Limb Regional Anesthetic Blocks

• Brachial plexus block- Can provide analgesia to the elbow and structures that is can be used for the accuracy of pain in stimulates by stimulator(accuracy improved with with guided stimulator.)
• It can lead to hemorrhage due IV injection pneumothorax, and if present would suggest to be: Hornes syndrome Provides anesthesia in distoral side with RUMM. Is simple once its is made to be preformed once anatomy.

Abdominis plane block (TAP-block)

• ultrasound allows for analgesia in ventral and ventral abdominal includes(skin and Mammary tissue). Mammary chain sectioned. It all includes abdul subcutaneous and abdu muscular region
• If major abdominal procedure is happing this would helpful for patient.

Lumbosacral nerve

• Helpful in orthopedic surgeries or hindllimp. Helpful if exploratory. Will block sensories. If decreases inhalant agent must also decrease muscle relation.
• Post Op will increase a period and relief of pain when give through blocks.
• Be sure to use with preservative FREE (Duramorph:0.1 mg/kg- in order not for spinal needle used to be infected. Is advised to use spinal needle with 20-22 gauge for this.