Exam 3: Lecture 18/19 - Locoregional Anesthesia Flashcards
Why should I use a local block?
-General anesthesia prevents the perception of pain
-Blocks the initiation & conduction of action potentials in nerve fibers
What does a local block not stop?
-Transduction
-Transmission
-Modulation of pain
What should always be considered as part of a multi-modal analgesic plan?
-Local anesthetic drug
When would we administer a local anesthetic drug?
-Pre-emptively (before sx)
-Intraoperatively
-Postoperatively
What does administering a local block prior to sx do?
-Decreases the stress response
____ can allow sx to be performed in a patient without the use of general anesthesia (more commonly utilized in large animals)
-Local anesthetic drug
What does local anesthetic drug reduce the amount of?
-Reduces amount of induction and maintenance drugs required if given before surgical stimulation
-When given prior to surgical stimulation, it can reduce the potential for development of “wind up” pain
What is the mechanism of action of local anesthetic drugs?
- BH+ <-> B + H+ (packaged salt)
- To cross the phospholipid bilayer, has to disassociate b/c ionized form is not going to pass through lipid membrane
- Base comes off so it can cross through the membrane
- Has to re-form and re-ionize so it can come in and block Na channel inside the nerve cell
Analgesia of local anesthetic drugs is a direct result of
Sodium ion channel blockade & membrane stabilization
-blockade occurs from inside the nerve cell
-Nerve cell depolarization is prevented, which blocks the conduction of nerve impulses
With a local anesthetic drug, the patient has a dose-dependent loss of
-Sensory
-Motor
-Autonomic function
What are the important nerve fibers for local anesthetic effects?
Small diameter nerve fibers:
-C fibers
-Alpha delta dibers
What order does sensation disappear in with local anesthetic effects?
- Pain
- Cold
- Warmth
- Touch
- Joint
- Deep pressure
What is the lipid solubility of LA drugs?
-Highly lipid soluble = more potent effects
-Increased lipid solubility = increased duration of effect
The speed of onset of LA is inversely proportional to the drug’s
lipid solubility & pKa
What is important about the protein binding ability of LAs?
-More highly protein-bound LA have a longer duration of action
(ex: bupivacaine > lidocaine)
If we increase the concentration of LA in a given area, we will _____ the duration of analgesia
Increase
As systemic absorption increases for LAs (ex: due to vasodilation), what happens to the duration of effect?
-Decreases
(ex: lidocaine causes vasodilation so it has a shorter duration of action)
What are the two main types of local anesthetics?
-Esters
-Amides
What are examples of ester LAs?
-Procaine (typically combined w/ other drugs)
-Tetracaine
-Benzocaine (use in cats may cause methemoglobinemia)
What are examples of amide LAs?
-Lidocaine
-Mepivacaine
-Ropivacaine
How do we know if an LA is a ester or an amide?
-Amides will have an “i” before the caine
What drug?
-Commonly used LA w/ fast onset time (b/c low pKa of 7.9)
-Short duration of action b/c low protein binding & ability to cause vasodilation
-Minimal damage or irritation to tissue
-Safe to give IV
-Lidocaine
What are some effects of Lidocaine when given IV?
-Anesthetic sparing (MAC sparing)
-Augments analgesia
-Antiarrhythmic
-GI promotility
-Anti-inflammatory
-Anti-shock
What is the pKa (dissociation constant)?
-The pH at which 50% of an acid or base is in the ionized form
Most LAs are weak bases with a pKa between _____. So they are mostly _____ at a pH of 7.4 (physiologic)
-8-9
-Mostly ionized
What does it mean b/c most LAs have a pKa between 8-9?
-The closer we get to our body’s pH of 7.4, those drugs will have a bit more probability to cross over the membrane
In an acid environment, what happens to the LA?
-Shifts to the ionized form
In an alkaline environment, what happens to the LA?
-Shifts to an unionized form
What happens if we have more of a substance in an ionized form?
-It will take longer and be harder for that drug to have an effect on the body
What drug?
-Commonly used in LA for equine diagnostic nerve blocks in lameness exams
-Faster onset time b/c lower pKa of 7.6
-Highly protein bound, so longer duration of action
-Less irritation or tissue damage
-Absence of vasodilator effects (no need to use w/ epinephrine)
-Mepivacaine
What drug?
-Longer time to onset due to higher pKa of 8.1 & increased lipid solubility
-Longer duration of action due to high protein binding
-Margin of safety is lowest
-Greater concern for systemic cardiotoxicity (should never be given IV)
-Bupivacaine
What is “Nocita”?
-Bupivacaine liposome injectable suspension
-Provides an extended release of bupivacaine (up to 72 hours)
What drug?
-Similar chemical properties as bupivacaine, but is the pure S-(-) enantiomer (causes less CV & CNS toxicity than bupivacaine)
-Concentration dependent onset time
-Similar duration of action to bupivacaine
-Ropivacaine
What happens with low concentrations of Ropivacaine?
-Similar time to bupivacaine
What happens with high concentrations of Ropivacaine?
-Similar onset time to Mepivacaine
Why would we use Bupivacaine instead of Ropivacaine?
-Usually more $$ than Bupivacaine
What drug?
-Concentration = 2% (20 mg/mL)
-Onset = 10-15 mins
-Duration = 1-2 hours
-Toxic dose = 6-8 mg/kg dog, 2-3 mg/kg cat
-Lidocaine
What drug?
-Concentration = 2% (20 mg/mL)
-Onset = 5-10 minutes
-Duration = 1.5-3 hours
-Toxic dose = 6 mg/kg dog, 3 mg/kg cat
-Mepivacaine
What drug?
-Concentration = 0.5% (5 mg/mL)
-Onset = 20-30 minutes
-Duration = 4-6 hours
-Toxic dose = 3-4 mg/kg dog, 2 mg/kg cat
-Bupivacaine
What drug?
-Concentration = varies
-Onset (min) = concentration dependent
-Duration = 4-6 hours
-Toxic dose = 5 mg/kg dog, unknown for cats
-Ropivacaine
Can we just combine local anesthetics?
NOPE!
-Results can be unpredictable and duration of action shorter
-Have no clue what you’ll actually end up with
What 3 things can be additives to local anesthetics?
-Opioids
-Alpha 2 agonists
-Ketamine
What can adding an opioid to local anesthetics do?
-Can prolong the duration of action of LA (often doubles it)
-Example of synergy (analgesia can last up to 24 hours)
What is an example of an opioid additive to local anesthetics?
-Preservative-free morphine (commonly admin. w/ LA for epidural or spinal anesthesia)
-Buprenorphine admin. with LA for certain nerve blocks
What is the problem with using an opioid additive to local anesthetics?
-Systemic side effects (respiratory depression, urinary retention, vomiting if awake, and pruritis) can occur
What alpha 2 agonists can be used as additives to local anesthetics and what does adding them accomplish?
-Small doses of xyazine, medetomidine & dexmedetomidine
-Used in certain locoregional anesthetic technique to improve analgesia & duration of action
What are the systemic side effects associated with using an alpha 2 agonist as an additive to LA?
-Hypertension & bradycardia
What drugs have effects on LA (other than the additives)?
-Epinephrine
-Hyaluronidase
-Sodium bicarbonate
What effect does Epinephrine have on LA?
-Epi = vasoconstrictor
-May be added to increase the duration of action as it delays absorption
-Can potentially reduce the toxicity of LA
What effect does Hyaluronidase have on LA?
-Can be used to hasten onset time
-Could also lead to more systemic absorption & toxicity
-Duration of block may be decreased (may be utilized in exotics)
What effect does Sodium Bicarbonate have on LA?
-Can be added to lidocaine to reduce pain on injection and possibly enhance the effect of the LA by increasing diffusion across nerve cell membrane
What should you always do before injecting LA?
-ALWAYS aspirate for blood
What should you watch out for when injecting LA?
-Never inject if you feel resistance b/c will decrease the chance of intraneural injection
-Avoid injecting in to an inflamed/infected area or if neoplasia is present
When selecting the correct gauge needle for LA, what is the general rule of thumb?
-Smaller is generally better
How should you plan your LA block?
-Allow adequate time after administration for onset of the block before proceeding w/ a painful procedure
-Understand the potential risks of doing the LA technique as well as species specific toxic doses prior to administration
When going to give LA, you should always label your syringe & do not _____
“interchange” drugs
-Ex: giving same dose of another LA drug if you are out ofthe one you planned to use
What are two categories of toxic effects LAs can cause that you have to be aware of?
-Neurologic
-Cardiorespiratory
What are the neurologic toxic effects we can see with administration of LAs?
-Sudden alteration in mental state or sedation
-Agitation
-Muscle twitching
-Nystagmus
-Seizures
-Coma
What are the cardiorespiratory toxic effects we can see with administration of LAs?
-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), larger doses are required to produce signs of _____ toxicity than to produce ____ toxicity
CV system toxicity than to produce CNS toxicity
-(Bupivacaine skips straight to cardiorespiratory effects)
The ______ effects of LAs differ between drugs
Cardiotoxic effects
What will an overdose of lidocaine result in?
-Hypotension & bradycardia
What do toxic doses of Ropivacaine & Bupivacaine cause?
-Produce sudden CV collapse or ventricular dysrhythmias that are resistant to treatment
____ LA potential effect is more likely to occur in a cat
Methemoglobinemia
LAs are not recommended in patients that are ______, because _____
-Hypotensive or in shock
-LAs may cause increased vasodilation due to sympathetic blockade -> CV collapse
Using an LA would be contraindicated in what type of patient?
-A patient with coagulopathy due to possible hemorrhage
Inhalant anesthesia _____ the metabolism of lidocaine in cats
decreases (more than 2 mg/kg)
_____ reactions are rare, but could occur with the use of ester-type LAs
Allergic reactions
What LA systemic toxicity signs are seen first, and what are the exceptions to that?
-Usually see CNS signs (CNS depression -> CNS excitation) prior to CV signs
-Unless bupivacaine was given IV accidentally, OR patient has decreases hepatic function
What should you do if you suspect an LA toxicity?
-STOP administration of the LA! Start CPR if cardiac arrest
How would you treat an LA systemic toxicity?
Depending on clinical signs (CNS vs CV), treat supportively:
-Anticonvulsants (midaz. diaz.)
-Supplemental oxygen
-Continuous monitoring of ECG & blood pressure
-Intravenous lipid emulsion therapy
What are feline species specific LA consideration?
-Cats have limited ability to metabolism LA so are more susceptible to toxicity from lidocaine & bupivacaine
-Cats at an increased risk for developing methemoglobinemia & Heinz body anemia w/ certain local anesthetics
_______ (LA) use should be avoided in ferrets and other small exotics
-Benzocaine
What species are easy to overdose due to small body mass & high vascularization of horn bed & need special consideration if using an LA?
-Kids & lambs
-Carefully calculate the dosages if dehorning or castration procedures are planned
What is the basic equipment needed for LA?
Needles
-Hypodermic
-Epidural/spinal needles (sharp bevel w/ stylet)
-27-18g, 2-9 cm
Syringes
-1-20 mL
-For epidurals -> glass syringe or low-resistance plastic syringe useful
What are some advanced equipment needed for LA?
-Peripheral nerve stimulators (improves accuracy & decrease volume of LA to limit side effects)
-Insulated needles w/ inj. port
-Ultrasound (aids in visualization of n.)
-Epidural catheter kit (if more ongoing epidural needed)
What is included in the epidural catheter kit?
-Tuohy needle (curved up to facilitate advancement of an epidural catheter)
-Loss of resistance syringe
-Radiopaque catheter w/ guidewire
-Connector & antibacterial filter
What are some specific local anesthesia techniques in SA?
-Infiltration anesthesia & “Splash” blocks
-Intratesticular (IT) blocks
-Intra-articular blocks
-Brachial plexus nerve blocks
-RUMM nerve blocks
-Digital nerve blocks
-Opthalmic blocks
-Intravenous regional (BIER) blocks
-Intercostal nerve blocks
-Intra(inter)pleural analgesia
-Maxillary nerve block
-Mandibular nerve block
-Epidural anesthesia & spinal analgesia
-Ultrasound guided nerve blocks
-Nerve-stimulator guided nerve blocks
-Soaker catheters
-Lidocaine patches
What are the different infiltrations of LA?
-Line block vs. block of surgical incision
-“Field” block
-“Splash” block
-Ring block in cats
-Inverted “L” block
-Diffusion catheter (“Soaker catheter”)
What is a “line block” infiltration of LA?
-Block to the side of the surgical incision
What is a “field” block good for?
-Quick biopsies or things that are not super painful
What is a “splash” block good for?
-Ex: after enucleation you can “splash” LA on there to provide analgesia
What is the ring block in cats used for?
-Removing declaws in cats
When is the diffusion catheter (“soaker catheter”) LA used?
-Placed in incision during closure
-Intermittent boluses of bupivacaine or CRI of lidocaine can be utilized to achieve analgesia
What is shown by this image?
-Diffusion catheter (“soaker catheter”) LA
What are the indications for an intercostal LA block?
-Analgesia for thoracotomy, rib fractures, & prior to chest tube placement
What are the landmarks for an intercostal LA lock?
-Caudal edge of rib, avoid v. & a.
-As proximal to spine as feasible to anesthetize region distal to injection
-Block 2-3 intercostal spaces cranial & caudal to areas affected for better results (b/c overlapping n. supply)
-Avoid depth of needle going past rib as it may penetrate pleura
What block is shown here?
-Intercostal block
What is an interpleural infiltration block (LA)?
-Aka intrapleural block
-For post-op management of thoracotomy/sternotomy to provide analgesia to thoracic cavity & cranial abdominal cavity (e.g. pancreatitis)
-Can do one time block or place an indwelling thoracic drain/catheter for repeated blocks
What is an intraperitoneal infiltration block (LA)?
-Provides analgesia to abdominal cavity (specifically viscera)
-LA applied in abdomen prior to closing linea alba
What block is shown here?
Intratesticular block
What are the considerations for doing an intratesticular block?
-In canine: split 2 mg/kg lidocaine between 2 testicles (decrease dose by half for cats)
-Can preserve small amount for pre-scrotal line block in dogs
-Clip & prep surgical area
-Use 22-25 g. needle
-Aspirate prior to injection
-Lidocaine travels up spermatic cord to achieve max. effect in a few min
What is the opthalmic LA block?
Retrobulbar
-Provides anesthesia to the eye (including conjunctiva, cornea, & uvea)
What are the complications associated with a Retrobulbar LA block?
-Hematoma
-Intravascular injection
-Nerve damage
-Subarachnoid injection
What is the ear LA block?
-Auriculotemporal nerve & great auricular nerve block
What does the auriculotemporal nerve & great auricular nerve block do?
-Provides analgesia for the external ear canal & auricular pinna
What can happen with the auriculotemporal nerve & great auricular nerve block?
-Could produce temporary paralysis of nerves -> prevent the patient from blinking, so recommended to lubricate the eye q 2hrs
What are the 4 main dental nerve blocks?
-Mental nerve block
-Infraorbital nerve block
-Maxillary nerve block
-Mandibular nerve block (i.e. inferior alveolar)
What is the mental nerve block used for?
-Procedures involving rostral mandible & associated teeth, rostral lower lip
What is the infraorbital nerve block used for?
-For procedures involving rostral part of maxilla (skin, lip, maxilla, maxillary teeth, palate & nasal cavity)
What does the effectiveness of the infraorbital nerve block depend on?
-Whether injection is made inside the foramen or only at the enterance
What is the maxillary nerve block used for?
-Procedures involving maxilla, upper teeth, nares/nasal cavity, palate & upper lip
-Could perform bilaterally for a rhinoscopy
What is the mandibular nerve block (i.e. inferior alveolar)
-For procedures involving the mandible, lower lip, & dental extractions
What are the thoracic limb regional anesthetic blocks?
-Brachial plexus block
-Radial/ulnar/median/musculocutaneous (RUMM block)
-Cervical paravertebral
What is a Brachial plexus block?
-Can provide analgesia to the elbow & structures distal to it
How can we improve the accuracy of the brachial plexus block?
-Peripheral nerve stimulator
What risk(s) is/are associated with the brachial plexus block?
-Risk of hemorrhage or IV injection, pneumothorax, & Horner’s syndrome
What is the RUMM block (radial/ulnar/median/musculocutaneous) used for?
-Provides anesthesia to distal thoracic limb (carpus & paw)
-Simple to perform once you locate the anatomy
What is the cervical paravertebral block used for?
-Provides analgesia & muscle relaxation to upper thoracic limb (including proximal shoulder & humerus)
-Difficult to perform (mostly referring vet will be utilizing this)
What block provides intravenous regional anesthesia?
-Bier block
How do we perform a bier block?
-Access vein in leg (cephalic or saphenous) distal to tourniquet site
-Desanguinate limb by wrapping w/ Esmarch bandage (start distal & wrap proximally)
-Place tourniquet proximally & unwrap Esmarch bandage
-Inject lidocaine IV; maximum effect in 5-10 minutes
-Never leave tourniquet on more than 90 minutes
What are the pelvic limb regional anesthetic blocks?
-Femoral/saphenous nerve block
-Sciatic nerve block
What do we use the femoral/saphenous nerve block for?
-Commonly combined w/ sciatic nerve block to achieve anesthesia of pelvic limb (distal to mid femur)
How can we decrease the time for completion of the block, the rate of complications & also less LA used with the femoral/saphenous nerve block?
-Using an ultrasound
What is the sciatic nerve block used for?
-This block alone is sufficient to perform surgery of the foot & hock
When should the femoral/saphenous nerve block & sciatic nerve block be combined?
-Always combine the two blocks for surgeries involving the stifle!
What drugs are most commonly used with the pelvic limb regional anesthetic blocks?
-Bupivacaine 0.5% combined with dexmedetomidine
What is a “TAP-block”?
-Ultrasound guided transverse abdominis plane block
What is the TAP-block used for?
Provides analgesia to the ventral & lateral abdominal wall including:
-Skin
-Mammary glands
-Abdominal subcutaneous tissue
-Abdominal muscles & parietal peritoneum
When are lumbosacral epidural & spinal blocks useful?
In orthopedic surgeries of the hindlimb & also for abdominal exploratory surgery
-Blocks sensory, motor, & sympathetic fibers
-Decreases concentration of inhalant agent (MAC reduction) & causes muscle relaxation
Lumbosacral epidural & spinal blocks provide post-op analgesia for
-12-24 hours
What are some considerations for administering lumbosacral epidural & spinal blocks?
-Clip & sterile prep. Sterile gloves
-Use preservative-free morphine (Duramorph) and/or LA
-20-22 gauge spinal needle
What are the epidural landmarks?
-Cranial border of wings of ilium
-Line connecting wings crosses dorsal spinous process of L7
-Directly caudal feel depression of lumbosacral junction (L7-S1)
What recumbency should the animal be placed in for an epidural?
-Sternal or lateral recumbency
What is the “procedure” for an epidural?
-Sternal or lateral recumbency
-Place needle in center of depression, on dorsal midline & direct bevel of needle rostrally
-Advance needle until it penetrated ligamentum flavum (popping sensation)
-If bleeding occurs, likely advanced too far (dogs)
-Test dose of 0.5-1 mL of air or saline to test for resistance. Aspirate gently to insure needle not in vein
-Inject slowly over 1-2 minutes
What should you do if CSF is present when injecting an epidural?
-1/2 total volume of drug is administered as a “spinal or intrathecal”
What are some contraindications for epidurals?
-Patients w/ increased intracranial pressure
-Clotting disorders
-Hypovolemic patients
-Degenerative central or peripheral axonal diseases
-Anatomical abnormalities
-Skin infected at epidural site
What is a problem associated with epidurals?
-Urinary retention
What is a caudal epidural?
-Newer technique described to facilitate catheterization in male cats w/ urethral obstruction (or other perineal procedures)
-Avoids the need to induce general anesthesia in compromised patients & provides pain relief
When do we use intra-articular bocks and what do we use for them?
-Usually inserted sterilely after joint exploration & closure
-Can use lidocaine or bupivacaine
-Morphine = useful for chronically inflamed joints
How can we topically apply LA?
-Lidocaine patches
-Topical cream
-Proparacaine opthalmic solution
-Lidocaine sprayed on larynx
-LA applied to urethra
What are tips for success with local anesthesia techniques?
Have a game plan!
-Appropriate equipment readily available
-Review anatomy
-Understand LA drug pharmacology
-Know potential complications & contraindications
-Have treatment plan prepared in case of complication
-Prepare another plan in case block does not work
-Practice, practice, practice
-Keep up w/ current literature
