Local Anesthetics Flashcards
What was the first local anesthetic and when was it introduced?
Cocaine- 1884
What is unique to cocaine as a LA?
It causes profound vasoconstriction
others will cause vasodilation
Is cocaine still in use today? If yes, what is it typically used for?
Yes! It is a schedule II drug typically used for nasal surgery in humans (or other highly vascular surgeries)
What are the two classifications of LAs?
Ester-CO-
Amide -NHC-
What is the general structure of LAs?
Aromatic ring and hydrophilic portion joined by a hydrocarbon chain
They are classified by the hydrocarbon chain
What are the esters?
Cocaine, procaine, chloroprocaine, tetracaine
What are the amides?
Lidocaine, mepivicaine, bupivicaine, etodicaine, prilocaine, ropivacaine
What is the general MOA of LAs?
Bind to sodium channels in nerve membranes slowing rate of depolarization
Threshold potential cannot be reached and action potentials are not propagated
T/F: LAs are weak acids.
False- all LAs are weak bases
pK ~7.7-9
What kind of tissues will increase ionization of LAs?
Acidic tissues (ie infection esp) which makes them inactive
What three things affect potency, onset, and duration?
- pK
- protein binding
- lipid solubility
Will a higher or lower pK increase onset time?
Lower- closer to tissue pH
Will a greater or lesser affinity to protein binding increase duration of action?
High protein binding affinity
Will a higher or lower lipid solubility increase potency?
Higher the lipid solubility, higher the potency
What is the only LA that can be given IV or for IVRA?
Lidocaine
What is lidocaine onset/duration?
Fast onset, short duration
Is lidocaine absorbed transcutaneously?
Yes
What are the systemic/IV effects of lidocaine?
- Antiarrhythmic
- Decrease MAC
- Analgesic
- Free radical scavenger
- Improves GI motility
What is the onset/duration of bupivicaine?
Immediate onset, moderate duration (3-8hr)
T/F: Bupivicaine has the lowest cardiovascular toxicity.
FALSE- highest toxicity, do not give IV
What is mepivacaine used for?
Nerve blocks, intra-articular analgesia (large animals)
What is the onset/duration of mepivacaine?
Fast onset, short duration (1.5-3)
What is the onset/duration of ropicacaine?
Intermediate onset, moderate duration (3-8hr)
T/F: Ropivacaine is less cardiotoxic than bupivacaine.
True
What is procaine commonly used for?
Procaine penicillin G suspensions (PPG)
What are tetracaine and proparacaine commonly used for?
Ophthalmic preparations
What is benzocaine commonly used for?
Historically used as a laryngeal spray for intubation
What does systemic absorption depend on?
- Site of injection
- Dosage
- Drug Characteristics
- If epinephrine is used
What site of injection has the fastest absorption?
Intercostal blocks
How are esters metabolized?
Hydrolysis of plasma cholinesterases
How are amides metabolized?
Microsomal liver enzymes
Are amides or esters more likely to accumulate?
Amides
Do amides or esters have a metabolite that may cause allergic reactions?
Esters
PABA may cause subsequent allergic reactions
Why is epinephrine used with LAs?
To prolong duration of block via vasoconstriction
Why is bicarbonate used with LAs?
Shorten onset and prolong duration, decrease sting on injection
Why would LAs be combined?
Shorter onset and longer duration combining lidocaine and bupivicaine?
Not really useful overall and may increase toxicity
What drugs cause methemoglobinemia?
Benzocaine and prilocaine
Benzocaine + tetracaine (cetacaine) laryngeal spray especially in cats (discontinued used in vet med)
T/F: Neurotoxicity is concentration dependent and can cause permanent damage.
True, although permanent damage is rare with normal clinical use
What drugs are the most neurotoxic?
Spinal lidocaine most, bupivacaine
What kinds of LAs should be used for spinal anesthesia/epidurals to avoid neurotoxiticy?
Preservative free
T/F: Chondrotoxicity is concentration and time dependent.
True
What is the most chondrotoxic LA?
Bupivicaine, no longer administered IA
What is the least chondrotoxic LA?
Mepivacaine (carbocaine)- most often used for equine lameness dx
What is systemic toxicity caused by?
Excessive plasma concentrations due to inadvertent IV infection or excessive doses/intolerant patient
What are the signs of systemic toxicity with lidocaine?
Depression/sedation, twitching, CV signs
What are the signs of systemic toxicity with bupicavaine?
Cardiovascular collapse and death
If epinephrine is used, what is the first sign of accidental IV injection?
Increased HR
What are the goals of local anesthesia?
- Provide analgesia before, during, and after a procedure
- May allow decreased dosage of systemic drugs
- Prevents central sensitization
- Important for multimodal anesthesia
What is the order of the nerve blockade?
Smaller, less myelinated fibers first
- B fibers- pre-gang. sympathetic
- A-delta and C fibers- pain
- A-gamma fibers- prorioception
- A-beta fibers- touch/pressure
- A-alpha fibers- motor
What is the exception to the order of nerve blockade?
Brachial plexus- motor neurons blocked fist
motor nearer to periphery
What are the three types of local anesthesia?
- Topical
- Subcutaneous
- Line block
What are the three types of regional anesthesia?
- IVRA
- Peripheral- individual nerves or areas
- Central (neuraxial)- epidural or spinal
What type of anesthesia is a bier block?
Intravenous regional anesthesia
What is the procedure for a bier block?
Tourniquet is placed and anesthetic is injected to a peripheral vein and allowed to diffuse into target tissues
What LA is used for bier blocks and why?
Lidocaine only- it will be released into systemic circulation after procedure is finished
What is the difference between epidural and spinal anesthesia?
Epidural- injection into the space between the dura and vertebrae
Spinal- injection into the space between the dura and spinal cord, into CSF
Where are epidurals used?
Tail/perineum, hindlimb, and abdominal/thoracic pain/procedures
Where does the spinal cord end?
Dogs- L6-L7
Cats- L7-S3
Horses, ruminants, pigs- mid sacrum
How should the doses be adjusted if CSF is encountered?
Decreased by 50%
What are contraindications to epidurals?
CHINAS
- Coagulopathy
- Hypovolemia
- Infection at site
- Neoplasia at site
- Anatomy- Inability to palpate landmarks
- Sepsis
What are adverse effects of epidurals?
- Hypotension
- Motor block/paralysis (concerning for LA)
- Hematoma
- Infection
- Neurotoxicity
What is a nerve locator?
Electrostim unit with an insulated needle that produces a motor response when needle is close to a nerve
Can ultrasound be used to locate nerves?
Yes
