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What are indications for local anesthesia? (6)
- combine with sedatives for minor procedures (small mass removal)
- when general anesthesia is an unnecessary risk for the patient (C section)
- during general anesthesia to minimize inhalant requirement (MAC reduction)
- for pain management when surgery isn't an option (patient not stable enough yet- ex HBC)
- to provide pre-emptive and postop analgesia (epidural morphine)
- to provide a balanced approach to anesthesia
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How does procaine penecillin work?
- the procaine forms a salt with the penicillin to delay absorption and prolong the duration of effect
- the procaine in the formulation is in excess of the mount required to form the salt so there is a local anesthetic effect
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What are the commonly used local anesthetics, their time to onset, duration, and potency? (4)
- Lidocaine: Intermediate potency and Intermediate duration (1-2hrs), onset in 5-15min
- Mepivacaine: Intermediate potency and duration (2-2.5hrs), onset in 5-10min
- Ropivacaine: High potency, longer duration (4-6hrs), onset in 5-15min
- Bupivacaine: High potency and longer duration (6-8hrs), onset in 20-30min
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What are the chemical properties of the structure of local anesthetics? (similar for all of them)
- lipophilic aromatic ring
- intermediate chain
- hydrophilic secondary chain
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What are the 2 classifications of local anesthetics? What drugs are in each classification?
- Ester-linked: rapidly metabolizable by enzymes in blood, shorter duration, more rare to see allergic reactions; cocaine, procaine, chloroprocaine, proparacaine, Benzocaine
- Amide-linked: metabolized by the liver, intermediate to long duration; lidocaine, Mepivacaine, Bupivacaine, Ropivacaine
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Describe the depolarizing mechanism of action of local anesthetics.
- local anesthetics selectively bind to inactivated, closed Na+ channels--> Na+ channels remain impermeable and prohibit conduction of nerve impulses
- they bind the H-gate on the inside of the Na-channel as well as obstructing the external openings of the Na-channels
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Describe the acid-base properties of local anesthetics and how this affects their action.
- local anesthetics are weak bases, meaning they dissociate into ionized and un-ionized forms in water
- ionized form is more water soluble, so they are put in acid solutions so most of it is ionized in the bottle
- once injected, it is in a more neutral environment and the amount of un-ionized drug increases
- the un-ionized form is lipid soluble and diffuses rapidly through cell membranes to reach Na-channels
- Once the drug crosses the lipid barrier, the un-ionized drug become protonated (ionized)
- once the local anesthetic reaches the site of action (Na-channel), the ionized form is responsible for its activity
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_____________ drug crosses cell membranes; _____________ drug blocks sodium channels.
- Un-ionized (hydrophobic); ionized (charged)
- [once the lipophilic un-ionized drug crosses the cell membrane, it becomes protonated/ charged, and this ionized drug blocks the Na-channels]
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Toxicity of local anesthetic relates to...
the concentration of local anesthetic in the systemic circulation (plasma conc) based on dose, absorption rate, and metabolism
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What are the clinical signs of local anesthetic toxicity in order of appearance? (4)
- seizures (first)
- apnea
- hypotension
- death (last)
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What are methods of delivering local anesthetics? (6)
- topical- MM's
- topical on intact skin- very limited absorption, not very effective
- SQ injection
- injected around a nerve
- epidural injection
- intra-articular injection
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What local anesthetic can be used as a topical eye drop? Why would we use this?
- Proparacaine
- to desensitize the cornea for minor procedures and tonometry
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Which local anesthetics comes as a spray and are used for intubation? Which one shouldn't be use in cats and why?
- Lidocaine and Benzocaine
- Benzocaine (cetacaine)- causes methemoglobinemia in cats
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What is ELMA cream and what is it used for?
- Eutectic Mixture of Local Anesthetic- Lidocaine and prilocaine
- Absorbed through intact skin
- Takes a while to start working but lasts ~ an hour
- May cause local irritation or rarely systemic effects
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Define eutectic.
a mixture in which the individual components are soluble in each other and the resulting compound has a lower melting point than the individual components
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How are lidocaine patches used?
- transdermal lidocaine patches alongside incisions
- effective for 24 hours
- no known side effects
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How does pKa affect the action of a local anesthetic?
- the ratio of ionized to un-ionized drug depends on the pKa of the drug and the pH of the environment
- as the pKa of a local anesthetic is increased, less free diffusible (un-ionized) drug is available at tissue pH (neutral), delaying the onset of action
- EX- the pKa of bupivacaine is higher than that of lidocaine, making bupivacaine slower onset of action
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How does the local environment to which a local anesthetic is injected affect the action of the drug?
- the local environment depends somewhat on the blood supply
- local anesthetics are less effective in an acidic environment (tissue with infection and swelling) because there is less un-ionized drug present to cross the cell membrane
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As protein-binding of the local anesthetic increases,...
- the duration of anesthetic effect also increases
- EX- lidocaine is more protein-bound than procaine, therefore it lasts longer
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As the lipid-solubility of a local anesthetic drug increases, ...
- potency and toxicity are also increased.
- EX- bupivacaine is more lipid-soluble than lidocaine, therefore it is more potent and potentially more toxic
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How does the additive of hyaluronidase affect the action of a local anesthetic drug?
- Hyaluronidase is an enzyme that breaks down hyaluronic acid and facilitates the spreading of the local anesthetic, speeding up the onset of action
- it also enhances the systemic absorption so potentially makes the drug more toxic
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How does the addition of epinephrine to the local anesthetic drug affect its activity?
epinephrine is a vasoconstrictor, so it reduces vascular absorption, prolonging the duration of local anesthetic effect
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How does adding sodium bicarbonate affect the activity of local anesthetic drugs?
- Alkalinization of local anesthetic solutions shortens the onset of neural blocker (faster-acting), enhances the depth of sensory and motor blockade (more potent), and increases the spread of epidural blockade
- it does this by increasing the amount of local anesthetic in the lipid-soluble form (un-ionized)
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How does the addition of liposomes affect the activity f local anesthetic drugs?
- liposomes are vesicles consisting of bilayers of phospholipid surrounding an aqueous phase
- it acts as a barrier to drug diffusion from the liposome, resulting in a slow-release preparation with a prolonged duration of action
- also decreaed toxicity
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How might pregnancy affect the action of local anesthetic drugs?
duration of ester-linked local anesthetics may be prolonged d/t reduced plasma cholinesterase activity
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Local anesthetic blockade can affect... (4)
- sensory function
- autonomic function
- motor function
- all 3 at once
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The more ________, the harder it is to block nerve conduction; this has clinical implications for...
- myelin
- larger nerve fibers have a myelin sheath and are not easily blocked with a local anesthetic
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Describe the classification of the 6 different types of peripheral nerve fibers, their function, and their size (which has implications for how easily they are blocked by a local anesthetic).
- [Largest- most resistant to block] A-alpha- motor
- A-beta- pressure sensation
- A-gamma- proprioception
- A-delta- pain, temperature sensation
- B- preganglionic, sympathetic
- [Smallest- easily blocked] C- pain
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How does the topography of nerve fibers affect local anesthesia?
- nerve fibers on the outside of the nerve (mantle fibers) supply the proximal parts of the body (ex. shoulder, pelvis)- these are blocked first
- nerve fibers located in the center of the nerve (core fibers) supply the more distal parts of the body (ex. fingers, toes)- delayed block
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Describe the clinical implications of nerve fiber topography and progression of anesthetic blockade.
- diffusion of anesthetic is from mantle of the nerve to core of the nerve
- shoulders and pelvis become anesthetized first, fingers and toes become anesthetized last
- spread of anesthesia may take up to 20 minutes
- make sure to provide adequate time if you are working on a more distal body part
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Describe how nerve fiber topography affects the RECOVERY from anesthetic blockade.
- recovery occurs from mantle to core because of the larger blood supply around the nerve
- sensation first returns to shoulder and pelvis, last returns to fingers and toes
- ie. if the sensation in the fingers and toes has returned, the patient is fully recovered from local anesthetia
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Describe a line block.
slow injection of local anesthetic while advancing the needle along the line of the proposed incision
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Describe a pain buster.
equipment to allow for long term administration of local anesthetics into a wound to provide extended analgesia (sewn into wound)
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What are techniques of local infiltration of local anesthetic? (4)
- tissue infiltration/line blocks
- "pain busters"
- lidocaine patches
- EMLA, lidocaine cream
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When using local tissue infiltration, keep total dose of lidocaine below _________.
10mg/kg
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Why would a local anesthetic come mixed with epinephrine?
- used to reduce absorption and increase local anesthetic effect and duration
- local vasoconstriction, resulting in decreased uptake into vasculature and away from the site
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When shouldn't you use local anesthetic mixed with epinephrine and why?
epi should not be injected into tissues supplied by end arteries (ears, tail)because of the risk of severe vasoconstriction, local ischemia, and necrosis
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____________ must be avoided during tissue infiltration because...
Intravascular injections; it will be carried away from the target site and excessive doses of lidocaine IV may cause seizures, apnea, CVS collapse, and death.
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Describe the technique and dose for local tissue infiltration.
multiple intradermal or subcutaneous injections of 0.5mL of lidocaine
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What are the blocks of the limbs? (3)
- three point or ring block in cats (forelimb)
- brachial plexus blocks and RUMM blocks (forelimb)
- intravenous regional anesthesia (Bier block) (forelimb or hindlimb)
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What types of spinal blocks do we use in vetmed?
lumbosacral and coccygeal epidurals
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Describe the three point block sued in cats?
ropivacaine 0.3mL SQ at the radial nerve, median nerve, and dorsal and palmar branches of the ulnar nerve [to block the forelimb]
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Describe a brachial plexus block.
- block the radial, median, ulnar, musculocutaneous, and axillary nerves
- blocks from the elbow, distally
- 3 techniques:"blind technique", ultrasound guided, or peripheral nerve stimulator
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Describe the blind technique for the brachial plexus block.
- sedate patient
- palpate for the point of the shoulder
- insert needle medial to the shoulder joint and direct the needle parallel to the vertebral column toward the costochondral junction
- with opposite hand, lift leg
- palpate for the axillary artery, which is located near the nerve of the brachial plexus
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Describe the nerve stimulator technique for a brachial plexus block.
- anesthetize patient
- palpate for the point of the shoulderinsert needle medial to the shoulder joint and direct the needle parallel to the vertebral column toward the costochondral junction
- with opposite hand, lift leg
- use teflon-sheathed needle, so only the sharp tip of the needle with elicit the carpal extension
- this is how you know when you're t the right location
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Describe the ultrasound guided technique for a brachial block.
- dog in dorsal recumbency with limbs naturally flexed
- axillary area scanned with transducer oriented in parasagittal plane and rotated to obtain an optimal short axis view of the axillary vessels
- advance needle until triceps brachii muscle twitches
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Describe the effects of a RUMM block.
midhumeral block of the radial, ulnar, musculocutaneous, and median nerves for sensory blockade of the elbow, antebrachium, and paw
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Describe the effects of IV regional anesthesia.
for anesthesia of an extremity distal to a tourniquet
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Describe the technique of IVRA.
- place IV catheter
- esmarch bandage placed (very tight vet wrap, starting at the toes to push all the blood out of the distal limb)
- rubber tourniquet around the limb proximal to the elbow (forelimb) or proximal to the hock (hindlimb)
- esmarch bandage removed
- Lidocaine (only lidocaine for this block!) injected IV distal to the tourniquet
- MAXIMUM OF 2 HOURS then remove tourniquet
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What are potential complications with IVRA? (2)
- reversible shock is tourniquet is left on >4 hours
- death d/t sepsis and endotoxemia if tourniquet left on >8 hours
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What is the difference b/w a spinal and an epidural?
- Epidural goes in epidural space: effects last longer
- Spinal goes into subarachnoid space: can confirm your location if you get spinal fluid in your needle, can get more cranial effects (can be bad...)
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Where does the spinal cord end in the 4 main species?
- Dogs: L6-L7
- Cats: L7-S3 (very variable!)
- Horses: S2
- Cows: S1
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How do you perform an epidural in a dog?
- a spinal needle with stylet is placed at the midline of the lumbosacral space
- needle should be right on midline with your fingers on the wings of the ilium
- a distinct pop is felt when the needle is advances through the interarcuate ligament
- should not have blood or spinal fluid
- should have minimal resistance to injection
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Describe the effects of a lumbosacral epidural in a dog?
- 1.5 hours after lidocaine
- 3-5hrs after bupivicaine or ropivacaine
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What are the advantages of epidural anesthesia? (4)
- good muscle relaxation
- post-op analgesia
- minimal effects on the body
- low cost
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What are potential complications of epidurals? (5)
- inadequate anesthesia d/t faulty technique
- animal awake during surgery (might move)
- surgery time is limited
- hypotension, resp depression, or apnea after excessive blockade
- accidental administration into the subarachnoid space (must decrease dose!)
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What are contraindications to performing epidurals? (3)
- hypovolemia and hypotension
- bleeding disorder
- skin infection
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What are the advantages of using epidural opioids? (5)
- profound somatic and visceral pain relief
- no motor blockade
- no sensory blockade
- no depression of sympathetic nervous system
- reversal of systemic side effects by naloxone
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Describe epidural/ spinal in cats.
- due to anatomical variation, more likely to perform a spinal than an epidural
- usually perform coccygeal epidural for local anesthetics
- can do lumbosacral with opioids
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Xylazine is sometimes used for epidural anesthesia in...
cattle and horses (may get systemic effects- sedation)
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