Local anesthesia Flashcards
Local anaesthetics block
voltage-gated sodium channels and prevent membrane depolarisation.
pH of Local anaesthetics
LA are weak bases
Voltage-gated sodium channels may be:
Resting (R)
Open or active (O)
Closed or desensitized (I)
LA only affect
open or closed channels.
Therefore local anaesthetics are able to inhibit the development and transmission of electrical currents.
To get inside the cell, LA has to be in what form?
unionized form
It has to be lipid soluble as well.
To bind with the receptor, the molecule has to be in what form?
ionized form
In the cell LA has to be ionized.
This is possible, because LA are weak bases.
In veterinary medicine amide-linked local anaesthetics are used. Give 5 examples of these.
Lidocaine
Bupivacaine
Mepivacaine
Ropivacaine
Prilocaine
Molecular characteristics of LA that influence pharmacodynamics.
pH
dissociation constant (pKa)
lipid solubility
protein binding
The dissociation constant (pKa) partly determines the onset of action of an LA.
pKa – pH at which half the drug is present in the unionised form and half in the ionised form.
Those LA agents with pKa values near body pH offer
a faster onset of action.
Lipid solubility partly determines
potency.
Protein binding partly determines
duration of action.
Tissue penetrance depends on a number of factors (3).
If the tissue is inflamed, it tends to have lower pH and therefore?
pKa, local pH, molecular size etc.
If the tissue is inflamed, it tends to have lower pH and therefore the onset of block is delayed.
Reasons we should we use LA? (5)
Important part of balanced anaesthesia
Reduces the effect of pain to cardorespiratory system.
Reduces the effect of pain to endocrinological system.
Better post-op analgesia
Reduces perioperative complications
Balanced anaesthesia = (3)
Reduced dosages
Reduced side effects
Faster recovery
Side-effects of LA (5)
Allergic reactions
Local tissue injury/neurotoxicity
Systemic toxicity
Vasodilation and hypotension
May also block somatic nerves causing paralysis
Signs of systemic toxicity from LAs
CNS signs: depression, sedation, convulsions, apnea
Cardiovascular: hypoxemia, respiratory arrest, cardiovascular collapse
Treatment of LA toxicity
Lipid emulsion (20% intralipid i.v. boluses, then CRI, more boluses if needed. Maximum total dose 12ml/kg)
In turn, Lipid toxicity signs:
Dyspnoe
Pyrexia
Seizures
Coma
Coagulation abnormalities
Hepatic dysfunction
Pruritus
Pancreatitis
Corneal lipidosis
It is easier to avoid toxicity than to treat it!
Local anaesthesia techniques
Infiltration aensthesia
Head blocks
Neuraxial blocks
Epidural
Infliltration anaesthesia (3 strategies)
Line and ring blocks (pictured)
- For superficial growth removals
- Numbs the skin and subcutaneous tissue
Incision inflitration
- For surgery sites
- Numbs the skin
Wound-catheters
- For extensive wounds
- Needs special catheter
Head blocks include (6)
Maxillary nerve block
Infraorbital nerve block
Inferior-alveolar/mandibular nerve block
Mental nerve block
Retrobulbar block
Auricular block
Maxillary nerve block.
Numbs what?
Where is it placed and how?
Numbs:
- Dorsal part of the nose (cavity)
- Upper lip
- Teeth rostral to M2
- Skin and mucosa
Extraoral
- Parallel to hard palate, a little bit more rostral
- If you feel bone, pull back 2-3mm
Intraoral
- Needle bent to 90 degree angle, behind the last molar
Infraobrital nerve block
Numbs what?
Where is it placed and how?
Risks?
Numbs:
- Teeth rostral to infraorbital foramen
- Skin and mucosa rostrally
- Upper lip
- Partly nose, and dorsal aspect of the nose.
Risks:
- Puncturing eyeball (in cats)
- Injecting LA into the eye (cats, brachys)
- Higher risk of hitting the vein and artery.
Inferior alveolar nerve block.
Numbs what?
Where is it placed and how?
Numbs:
- All the teeth in mandibula (unilateral)
- Lower lip, corner of the mouth
- Skin and mucosa rostrally
Extraoral
- Palpate the concavity where the nerve should be.
- Insert the needle through skin, feel it with your fingertip.
Intraoral
- From M1 in a 30 degree angle towards the nerve.
(Middle) Mental block.
Numbs what?
Where is it placed and how?
Numbs:
- Teeth rostral to I3
- Lower lip and frenulum
- Skin and mucosa
Insert the needle rostral and ventral to labial frenulum.
Retrobulbar block.
Numbs what?
Where is it placed and how?
Only for what use?
Complications?
Numbs:
- Conjunctiva
- Upper and lower lid
- Eyeball
- Forehead
ONLY FOR ENUCLEATION!
Complications:
- Injecting the eye
- Damaging the eye/vessels
- Injecting into CSF
- Oculocardiac reflex (bradycardia)
TECA-BO
Total Ear Canal Ablation and Bulla Osteotomy (TECA-BO)
Auricular blocks.
Numbs what?
Where is it placed and how?
For what use?
Complications?
Numbs
- Outer ear
- Ear canal
Great for TECABO + othematoma (auricular hematoma)
Complications:
- Facial nerve paralysis
- „drooping“ upper lid
Intercostal block.
What is it, what are you blocking?
Indications?
Complications?
You have to block 2-3 intercostal spaces cranial and caudal to the site.
Indications:
- Thoracotomy
- Flail chest, rib fractures
- Chest drains
Complications:
- Hemothorax
- Pneumothorax
- Lung damage
What are Neuraxial blocks?
Spinal and epidural blocks.
Spinal = LA is injected into CSF that surrounds spinal cord.
- Better analgesia
- Higher risk
- Complicated to perform
Epidural (extradural) = LA is injected to the outer side of dura mater. Can be cranial/high or caudal/low but this does NOT indicate placement on the animal. This refers to total amount injected!
Indications for an Epidural
Surgery on hindlibs, tail, perianal region
Abdominal surgery
Pelvic surgery (depends on the surgery)
Thoracic surgery
Acute pancreatitis
FATE (feline aortic thromboembolism)
Contraindications for an Epidural
Obese animal (hard to find landmarks)
Fractures of pelvis or spine
Neuropathies
Hypovolemia
Coagulopathy
Infection
High ICR
Explain Cranial/caudal and high/low epidurals.
Low epidural or caudal epidural
- Animal keeps the motoric function of hindlimbs.
High epidural or cranial epidural
- Motoric function of hindlimbs is not preserved.
Terminology depends on the amount injected not on the injection site!
Caudal epidural = smaller amount
Cranial epidural = larger amount
Epidural local anesthetic drugs rule of thumb, volume per 5-7kg?
Max volume?
You can mix them with what?
Not more than 1ml per 5-7kg
Not more than 6ml
Can mix with opioid or alpha2-agonist, or add epinephrine.
Method for epidural administration.
Animal in lateral or sternal recumbency.
Use your thumb and middle finger to find iliac wings. With your index finger find the lumbosacral space; L7-S1.
Insert the spinal needle perpendicular and on the midline. Always use spinal needle!
Needle goes through skin, subcut tissue,
Dorsal supraspinous ligament, Interspinous ligament, Ligamentum flavum (through this you’ll feel a „click“).
Be sure there isn’t any blood or CSF coming out of the needle.
How to determine the correct placement when administering an epidural?
You feel a specific click when you penetrtae lig. flavum.
Loss of resistance when injecting.
- Fill the syringe with 3ml NaCl (0,9%) and 1ml air.
- Air bubble is not compressing and fluid is sucked in.
Drop-technique (Gutierrez technique)
- After going through skin and subcutaneous tissue, the stylet is removed.
- A drop of NaCl (0,9%) is added to the needle cap.
- When you are in the correct place, the drop is sucked in.
Complications of epidurals.
Hypotension
Possible supression of reflex-tachycardia
Vasodilation (relative hypovolemia)
Hypoventilation, apnea
Effect is too cranial (C5/C6)
Hypothermia
Vasodilation, suppressed shivering
Increased bleeding in surgery site
Systemic toxicity
Poor hair growth
Urine retention
Constipation
Itchiness
