Lecture 18 & 19: Locoregional Anesthesia (Exam 3) Flashcards
Why should I use a local block
- General ax prevent the perception of pain but it doesn’t stop the transduction, transmission, & modulation of pain
- LA block the initiation & conduction of action potentials in nerve fibers
T/F: LA should always be considered as part of a multi-modal analgesic plan
True
Describe the admin of LA
- Can be pr, intra, or post op
- Administering prior to sx will decrease the stress response
- Often allows sx to be performed in a px w/o the use of general x (more used in Large animals)
- Reduces the amount of induction & maintenance drugs req if given before surgical stimulation
- When given prior to surgical stimulation it can reduce the potential for dev of “wind up” pain
What is the MOA of LA
- Analgesia is a direct result of the NA ion channel being blocked & the membrane being stabilized
- Blockade occurs from inside the nerve cell
- Nerve cell depolarization is prevented which block the conduction of nerve impulses
What happens to the px when given LA
- Has a dose dep loss of sensory, motor, & autonomic fxn
- This effect is transient as the local ax is metabolized or hydrolyzed
What do LA do to small diameter (C & Ag) nerve fibers
- Blocks preferentially prior to large myelinated fibers
- Loss of sensation occurs w/ varying degrees of loss of motor fxn
- C > Ag > Ay / Ab / Aa
What is the order that sensation disappears
- Pain
- Cold
- Warmth
- Touch
- Joint
- Deep pressure
How does the lipid solubility of an LA changes its effects
- High lipid soluble = more potent effects
- Increased solubility = increased duration of effect
- Speed of onset of the LA is inversely proportional to the drugs lipid solubility & pKa
How does the protein binding ability of an LA changes its effects
More highly protein bound LAs have a longer duration of action
T/F: Decreasing the concentration of LA in a given area will increase the duration of analgesia
False; increasing the concentration
How does the systemic absorption of an LA changes its effects
- As systemic absorption increases (like due to vasodilation) the duration of the effect decreases
- Lidocaine causes vasodilation so it has a shorter duration of action
List some ester local ax
- Procaine
- Tetracaine
- Benzocaine
List some amide local ax
- Lidocaine
- Mepivacaine
- Bupivacaine
- Ropivacaine
- Lidocaine + prilocaine
Describe lidocaine
- Commonly used LA w/ a fast onset time (low pKa of 7.9)
- Short duration of action b/c of low protein binding & ability to cause vasodilation
- Min damage or irritation to tissue
- Safe to give IV @ clinical doses
- Effects when given IV: ax sparing, augments analgesia, antiarrhythmic, GI promotility, anti inflammatory, & antishock
What is pKa (dissociation constant)
The pH @ which 50% of an acid or base is in the ionized state
What is the pKa of most LAs
They are mostly bases w/ a pKa btw/ 8 & 9 so they are mostly ionized @ a pH of 7.4
What happens to LAs in an acidic environment? What about alkaline?
- Acidic: The drug shifts to the ionized
- Alkaline: Drug shifts to an unionized form
Describe Mepivacaine
- Commonly used LA in equine for diagnostic nerve blocks in lameness exams
- Faster onset time compared to lidocaine (pKa of 7.6)
- Highly protein bound so longer duration of action than lidocaine
- Less irritation or tissue damage
- Absence of vasodilator effects so no need to use w/ epinephrine
Describe Bupivacaine
- Longer time to onset compared to lidocaine b/c of high pKa (8.1) & increased lipid solubility
- Longer duration of action b/c of high protein binding
- Margin of safety is the lowest compared to lidocaine, mepivacaine, & ropivacaine
- Greater concern for system cardiotoxicity so it should never be given IM
Describe bupivacaine liposome injectable suspension
- Nocita provides extended release of bupivacaine for up to 72 hours
- FDA approved in 2016 to control pain in dogs having cranial cruciate ligament sx
- Use a moving needle infiltration tech being sure to inject in all layers of tissue
Describe ropivacaine
- Similar chemical properties as bupivacaine (is the pure s-(-) enantoiomer)
- Less CV & CNS toxicity than bupivacaine
- Concentration dep onset time (low -> similar to bupivacaine & high -> similar onset to mepivacaine)
- Similar duration of action to bupivacaine
- More expensive the bupivacaine
Fill out the chart:
What happens if you combine local anesthetics
- Results can be unpredictable & duration of action shorter
- Need to pay attention to toxic Dosages
- Don’t do it
What can be additives to LAs
- Opioids
- Alpha 2 agonists
- Ketamine
Describe opioids
- Preservative free morphine commonly admin w/ LA for epidural or spinal ax
- Analgesia can last up to 24 H
- Has systemic side effects that can occur
Describe alpha 2 agonists
- Small doses of xylazine, medetomidine, & dexmedetomidine used in certain LA techs to improve analgesia & duration of action
- Can have systemic side effects
Describe ketamine
- Enhances analgesic effects by blocking NMDA, sodium, & potassium channels
- Side effects may include an increase in HR & BP
How does epinephrine effect LAs
- Vasoconstrictor
- May be added to increase duration of action b/c it delays absorption
- Can potentially reduce toxicity of LA
- Never use on distal extremities b/c may cause tissue necrosis
How does Hyaluronidase effect LAs
- Used to hasten onset time
- Could lead to more systemic absorption & toxicity
- Duration of block may be decreased
- May be utilized in exotics (reptiles)
How does sodium bicarbonate effect LAs
- Added to lidocaine to reduce pain on injection
- Possible enhance the effect of the LA by increasing diffusion across nerve cell membrane
What are general consideration for the use of LA
- Always aspirate for blood prior to injecting to prevent serious complications
- Never inject if you feel resistance (this will decrease the chance of intraneural injection)
- Carefully select the correc gauge (smaller is generally better) needle & length for a particular px
- Be sure to use sterile needles & solutions for each px
- Allow adequate time after admin for the onset of the block before proceeding w/ a painful procedure
- Understand the potential risks of doing the LA tech as well as species specific toxic doses prior to admin
- Label the syringe & do no interchange drugs
What are neurologic toxic effects of LA
- Sudden alteration in mental state or sedation
- Agitation
- Muscle twitching
- Nystagmus
- Seizures
- Coma
What are cardiorespiratory effects of LAs
- Respiratory arrest
- Cardiac dysrhythmias
- Hypotension due to vasodilation
- Myocardial depression/cardiac toxicity
- Death
What is required to produce signs of CV system toxicity than to produce CNS toxicity
Larger doses
What are some other potential effects?
- Methemoglobinemia more likely to occur in a cat
- Don’t recommend using in a px that are hypotensive or in show (cause increased vasodilation due to sympathetic blockade -> CV collapse)
- Contraindicated to use a local ax tech in a px w/ coagulopathy due to possible hemorrhage
- Inhalant ax decreases metabolism of lidocaine in cats
- Allergic rxn are rare but could occur w/ the use of ester type LA
Haw can you tx LA systemic toxicity
- Usually see CNS signs before CV unliss bupivacaine was given IV or the px has decreased hepatic fxn
- Stop admin of the LA
- Start CPR if cardiac arrest
- Anticonvulsants
- Supplemental oxygen, intubation, & ventilation
- Cont moinitoring of ECG & blood pressure
- Consider IVF, vasopressors, inotropes, & antiarrhythmics as needed
- IV lipid emulsion therapy
What are some species specific considerations of LAs
- Cats have limited ability to metabolize LA so they are more susceptible to toxicity from lidocaine & bupivacaine
- Cats are @ an increased risk for dev methemoglobinemia & heinz body anemia w/ certain local ax
- Avoid use of benzocaine in ferrets & other small exotics
- Carefully calculate dosages for kids & lambs if dehorning or castration procedure planned (easy to overdose them b/c small body mass & high vascularization of horn bud)
What basic equipment is needed
- needle (27 to 18 g; 2 to 9 cm)
- Syringes (1 to 20 mL)
What advanced equipment is needed for LAs
- Peripheral nerve stimulators used to improve accuracy of LA blocks
- Insulated needles w/ injection port
- Ultrasound
- Epidural catheter kit
Describe an infiltration of LA
- Line block: going right to the side of the incision
- Field block for biopsy
- Splash block for enucleation
- Ring block in cats for declaw
- Inverted L block used more in food animal
- Diffusion catheter is placed in the incision during closure
Describe intercostal blocks
- Indications: analgesia for thoracotomy, rib fractures, & prior to chest tube placement
- Landmarks: Caudal edge of rib (avoid vein & artery), as proximal to the spine as feasible to anesthetize the region distal to injection, block 2 to 3 IC spaces cranial & caudal to the areas affected for better results due t overlapping nerve supply, & avoid depth of needle going past rib as it may penetrate the pleura
Describe an interpleural block (intrapleural block)
- For post op management of thoracotomy/sternotomy to provide analgesia to the thoracic cavity & cranial abdominal cavity
- Can do a one time block or place an indwelling thoracic drain/catheter for repeated blocks
Describe an intraperitoneal block
- Provides analgesia to the abdominal cavity (specifically viscera) LA applied in the abdomen prior to closing the linea alba
Describe a intratesticular block
- Split 2mg/kg lidocaine btw/ the 2 testicles
- Can be performed in cats, just decrease dose by half
- Can reserve a small amount for pre-scrotal line block in dogs
- Clip & prep the surgical area
- Use a 22-25 g needle
- Aspirate prior to injection
- Lidocaine travels up the spermatic cord to achieve max effect in a few mins
Describe a retrobulbar block
- Provides ax to the eye including the conjunctiva, cornea, & uvea
- Complications include hematoma, IV injection, nerve damage, & subarachnoid injection
- Use with caution
Describe an auriculotemporal nerve & great auricular nerve block
- Provides analgesia for the external ear canal & auricular pinna
- Could produce temporary paralysis of nerves which prevents the patient from blinking so recommended to lubricate the eye every 2 hours
Describe a mental nerve block
For procedures involving rostral mandible & associated teeth, & rostral lower lip
Describe an infraorbital nerve block
- For procedures involving the rostral part of the maxilla (skin, lip, maxilla, maxillary teeth, palate, & nasal cavity)
- Effectiveness of block depends o whether the injection is made inside the foramen or only @ the entrance
Describe a maxillary nerve block
- For procedures involving the maxilla, upper teeth, nares/nasal cavity, palate, & upper lip
- Could perform bilaterally for a rhinoscopy
Describe a mandibular nerve block (inferior alveolar)
For procedures involving the mandible, lower lip, & dental extractions
Describe a brachial plexus block
- Can provide analgesia to the elbow & structures distal to it (accuracy improved w/ peripheral nerve stimulator)
- Risk of hemorrhage or IN injection, pneumothorax, & Horners syndrome
Desribe a radial/ulnar/median/musculocutaneous block (RUMM block)
- Provides ax to distal thoracic limb (carpus & paw)
- Simple to perform once you locate the anatomy
Describe a cervical paravertebral block
- Provides analgesia & muscle relaxation to upper thoracic limb (including the proximal shoulder & humerus)
- Difficult to perform
Describe an IV regional anesthesia block (bier block)
- Access vein in leg (cephalic or saphenous) distal to tourniquet site
- Desanguinate limb by wrapping w/ an esmarch bandage (start distal & wrap prox)
- Inject lidocaine IV (max effect in 5 to 10 min)
- Never leave tourniquet on more than 90 mins
Describe a femoral/saphenous nerve block
- Commonly comboed w/ the sciate nerve block to achieve ax of the pelvic limb (distal to mid femur)
- The use of the ultrasound decreases the time for completion of the block
- The rate of complications & also less LA is used
Describe a sciatic nerve block
- This block alone is sufficient to perform sx of the foot & hock
- Always combine the two blocks for surgeries involving the stifle
What is used for a pelvic limb regional ax blocks
Bupivacaine combined w/ dexmedetomidines most commonly used
What does an ultrasound guided transverse abdominis plane block (TAP) do
Provides analgesia to the ventral & lateral abdominal wall including skin, mammary glands, abdominal sub Q tissue, & abdominal muscles/ parietal peritoneum
Describe a lumbosacral epidural & spinal blocks
- Useful in ortho sx of the hindlimb & also for abdominal exploratory sx
- Blocks sensory, motor, & sympathetic fibers
- Decreases concentration of inhalant agent (MAC reduction) & causes muscle relaxation
- Post op analgesia for 12 to 24 H
- Clip & steril pre w/ sterile gloves
- Use preservative free morphine
- 20 to 22 gauge spinal needle
Describe the epidural & its landmarks
- Cranial border of the wings of the ilium
- Line connecting the wings crosses dorsal spinous process of L7
- Directly caudal feel depression of lumbosacral jxn (L7-S1)
- Sternal or lateral recumbency
- Place needle in center of depression on dorsal midline. Direct bevel of needle rostrally
- Advance the needle until it penetrates ligamentum flavum (popping sensation)
- If bleeding occurs the needle probably has been advanced too far
- Test dose .5 to 1 ml of air or saline to test for resistance. Aspirate gently to insure needle is not in vein
- Inject slowly over 1 to 2 mins
