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What are the uses of local anesthetics in small animal med?
Used in conjunction with sedation or general anesthesia
Reduce amount of general anesthetic
Minimizing cardiopulmonary depression
Provide a useful analgesia
Easy to learn and no expensive equipment
What are the uses of local anesthetics in equine medicine?
Diagnostic lameness tool,
increased use in conjunction with sedation or general anesthesia,
used in standing surgery
What are the uses of local anesthetics in food animal medicine?
Used MORE than general anesthesia
Avoid to lie down
Eliminate the dangers associated with forcible casting and restraint and prolonged recovery
What are the 2 groups of local anesthetics? What drugs are in each groups?
Ester: procaine, cocaine, tetracaine
Amide: lidocaine, bupivacaine
What determines the potency of a local anesthetic? What primarily determines the duration of a local anesthetic? KNOW THIS
Lipid solubility- potency
Protein binding- duration
How does lidocaine compare to bupivacaine? KNOW THIS
Lidocaine: quick onset, short duration, intermediate potency
Bupivacaine: long onset, long duration, high potency
What is the primary method of action for local anesthetics?
Prevent the rapid influx of sodium into nerve axons that produces an action potential
Probably work by: Direct channel blocking or Indirect interference with sodium channel function
What determines how much local anesthetic is absorbed systemically?
Not intended systemically (shortens duration and increases risk)
Depends on vascularity of site
What can we do to decrease vascularity and thereby decrease systemic absorption?
Addition of epinephrine (1:200,000) to ↓ blood flow-prolongs duration
Do NOT do this for ears, toes, tails, etc
In spinal anesthesia, what nerves are blocked first? KNOW THIS!
Sympathetic > sensory > motor
What nerves are blocked first with local anesthetics? KNOW THIS!
Small, unmyelinated FIRST
Large, myelinated LAST
Outer fibers= proximal structures
Inner fibers= distal fibers
What body systems are affected by local anesthetic toxicity? What reactions are seen?
Systemic: Cardiac/vascular, Central nervous system, Methemoglobinemia
Localized or systemic: Allergic reactions
What body system will first have toxicity to local anesthetics?
Central Nervous System
-usually before cardiovascular toxicity
What signs do animals have with CNS toxicity? Do epileptics have increase sensitivity to local anesthetics then?
Seizures (people- drowsiness, numbness of lips)
No evidence that epileptics are most sensitive
How do local anesthetics affect the cardiovascular system?
Direct vascular action- vasodilation
Indirectly: conduction blockade of autonomic fibers
Primary site of action is myocardium
-decreased excitability, conduction rate, and force of contraction
What is the most cardiotoxic local anesthetic?
What is the most widely used local anesthetic? What is its onset of action and duration of action?
Rapid onset of action
Duration variable- 1 hr without epinephrine
What reaction can lidocaine cause in the horse?
Local irritation and swelling
What is the most cardiotoxic drug? What is its onset of action and duration of action?
Relatively slow onset (~15~30 minutes)
Long duration: up to 8 hrs with epinephrine
Use for when long action is needed: post-op, long surgery
What is the most widely used drug in the horse? And why?
Causes very little swelling and edema in the area of injection, possibly as it lacks vasodilatory action.
Onset of action is faster and reliability of block greater than with prilocaine.
What kind of local anesthetic blocks do we use in the dog and cat?
Topical (surface) anesthesia
Intrasynovial anesthesia (joint block)
Spinal & epidural anesthesia
Carpal block - ring block
Brachial plexus block
When is a field block used in small animal medicine?
Minor surgery: mass removal, laceration repair etc.
What are the challenges of a field block?
NEVER inject local analgesic through infected tissues
Delayed wound healing
Where can topical anesthetics be used?
Mucous membranes, eyes, laryngeal area
Where is intrasynovial anesthesia inserted? What is it used for?
In joints, bursa, and tendon sheaths.
Useful for both diagnosis of lameness, and for general pain relief.
The local anesthetic chosen must cause minimal irritation, and great care in sterility is necessary as infection in these sites occurs easily.
Why is choosing the right local anesthetic and sterility important for intrasynovial anesthesia?
Want anesthetic that causes minimal irritation
Sterility is necessary as infection in these sites occurs easily
What is the lidocaine patch used for? How long does it last?
5 % dermal patch that is Cuttable!
Only for superficial pain
Duration can be 3 days!
What is the difference between spinal anesthesia and epidural anesthesia?
Spinal anesthesia : injection of local anesthetic around the spinal cord (Subarachnoid space) - intrathecal injection
Epidural anesthesia : injection of local anesthetic in the epidural space
-Needle enters the spinal canal, but does not penetrate the meninges. -> limited to the canal outside the dura mater
What determines the area that will be blocked with an epidural?
The site of injection: sacrocoxygeal or intercoxygeal, lumbosacral
Quantity, volume of local anesthetic injected
-Body weight (kg) / 4.5 ml/kg = total volume
Size of spinal canal
-Species, breeds, size of patient (fat/thin), pregnant
Position of animal
When is the needle inserted for an epidural for a dog or cat? TEST QUESTION!
Lumbosacral (between L7-S1)
Where does the spinal cord end in a dog? Cat? TEST QUESTION
Dog: around 4-6th lumbar vertebra
Cat: at 3rd sacral vertebra
How is an epidural done in a small animal patient?
Place the patient laterally or in sternal recumbency (personal preference)
The hair over the lumbosacral junction should be clipped and aseptically prepared
Wear sterile surgical gloves! Aseptic technique!
A 20-22 gauge, 1.5-2.5 inch spinal needle may be used; depending on the size of the animal, a longer 3.5 inch spinal needle is used for obese and large animals
What are the landmarks for an epidural on a small animal patient?
Iliac prominences on either side (wing of ilium)
Take an imaginary line between them crossing the dorsal spinous process of the last lumbar segment
What are the 2 techniques for small animal epidural? KNOW THESE!
Hanging drop technique
Lack of resistance technique
Explain how a handing drop epidural is done. KNOW THIS!!!
This involves removing the stylet of the spinal needle, filling the hub of the needle with saline or anesthetic solution, and allowing one drop to hang from the hub.
As the needle is advanced through the ligamentous structures, the drop does not move.
However, upon penetration of the ligamentum flavum, the negative pressure in the epidural space will draw the drop of solution into the needle, indicating proper placement in the epidural space.
A "pop" felt through the needle is usually encountered when the spinal needle is passed through the ligamentum flavum.
The chance for a successful "hanging drop" technique is greater in large dogs than in smaller dogs and cats.
If the "hanging drop" technique fails, the "lack of resistance" technique can be used.
Explain how the lack of resistance epidural is done. KNOW THIS!
This indicates proper placement of the injection needle in the epidural space based on the amount of resistance to the injection of air or saline.
Once in the epidural space, the injection of air, saline, or anesthetic solution will encounter minimal resistance.
A separate syringe of normal saline (3 ml or air preferred by others) should be prepared for the "lack of resistance" technique.
When minimal resistance to the saline injection is encountered, the saline syringe is replaced with a syringe-containing anesthetic, and the injection is completed.
To rule out the possibility of administering drugs into the venous sinus (presence of the blood) or subarachnoid space (presence of CSF), it is important to aspirate or allow a few seconds to check bleeding before epidural injection
What are contraindications for epidural anesthesia?
Skin infection over the injection site
Direct trauma to the injection site
What are drugs commonly used for the epidural?
Morphine (0.1 mg/kg)- preservative free morphine if you have it
Bupivacaine (0.5 mg/kg)
Dexmedetomidine (0.005 mg/kg)
Detamine (0.1 mg/kg)
Combo of drugs= Better analgesia and duration can be longer!
What nerves are blocked in the Feline Carpal block? TEST QUESTION!
Block median nerve, ulnar nerve (palmar branch and dorsal branch), radial nerve (superficial branches)
What drugs are used for cat declaws?
0.5 ml of 2% Lidocaine + 0.5 ml of 0.5% bupivacaine and give 0.2 ml per site for infiltration
Why would you use the brachial plexus block?
Provide analgesia for distal to the shoulder joint
What is blocked with the brachial plexus block? How long does it last?
Block C6, C7, C8, and T1 forming nerve plexus
Point of the shoulder is landmark
Motor blockade: 12-16 hours
Sensory blockade: 9-12 hours
What is a nerve locator?
Used for locate each specific nerve to block
Small amount of local anesthetic required
Complete block can be achieved
What does the intra-orbital foramen block?
PM3 to I1
What does the Mental foramen block?
PM2 to I1
What does the mandibular foramen block?
M3 to I1
What are some of the blocks we do in bovine/small ruminant?
Lumbosacral epidural- ataxia
Caudal epidural: Cd1-Cd2
Inverted "L" or inverted "7"
What are the advantages of the Inverted L block in ruminants?
Deposition of the analgesic away from the incision site, thus minimizing edema, hematoma, and possible interference with healing.
What are disadvantages of the Inverted L block in ruminants?
Incomplete analgesia and muscle relaxation of the deeper layers of the abdominal wall
Toxicity after injecting significant amounts of analgesic solution
Increased cost due to large doses and longer time required for injection
Where is the placement of the needle for the Proximal paravertebral block? What nerves are block?
The dorsal aspect of the transverse processes of the last thoracic (T-13) and first and second lumbar (L-1 and L-2) vertebrae is the site for needle placement.
-use last rib as landmark
The dorsal and ventral nerve roots of the last thoracic (T-13) and 1st and 2nd lumbar spinal nerves emerge from the intervertebral foramina are desensitized.
How do you know you have an effective Proximal paravertebral block?
Analgesia of skin
Scoliosis toward densensitized side- due to paralysis of paravertebral muscles, increased skin temp due to vasodilation (paralysis of cutaneous vasomotor nerves)
What is the technique of the distal paravertebral block?
Needle is inserted ventral to tips of transverse process.
Needle is withdrawn completely and reinserted dorsal to transverse process.
Desensitizes distal ends of L-1, L-2 and L-4
Where is the needle inserted for a cornual nerve block in cows? What does it sensitize?
Injected on the upper third of the temporal ridge, about 2.5 cm below the base of the horn.(nerve is superficial)
Opthalmic division of 5th cranial nerve
Onset is 10-15 mins and duration is one hour
Why is the cornual nerve block more challenging in the goat?
Two branches of the nerve (lacrimal and infratrochlear branches)
Why is the cornual nerve block not used in kids?
Total overdose of local in these very small animals
What is the Peterson block used for?
Enucleation- must also block eyelids
What are the landmarks for the Peterson block?
Coronoid process of mandible- walk off cranial
Inject at foramen orbitorotundum 20mls
What kind of epidural is done in the horse? What drugs are used? TEST QUESTION!
Caudal Epidural (NOT LUMBROSACRAL!!!)
-want horse to stay standing
What do the following blocks block in the horse: Supraorbital, Infraorbital, Auriculopalpebral?
Supraorbital: upper eyelid and forehead
Infraorbital: Lip and nose
Auriculopalpebral: Prevent eyelid movement
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