Spinal anesthesia involves administering local anesthetic into the subarachnoid space.
The spinal canal extends from the foramen magnum to the sacral hiatus. The boundaries of the bony canal are the vertebral body anteriorly, the pedicles laterally, and the spinous processes and laminae posteriorly
Three interlaminar ligaments
The spinal cord is invested in three meninges
The pia mater
The dura mater
Extends from the attachment of the dura at S-2 to the cerebral ventricles above. The space contains the spinal cord, nerves, cerebrospinal fluid (CSF), and blood vessels that supply the cord.
Central nervous system effects
Determinants of spread Major factors Baricity of solution Position of patients (except isobaric solution) Dose and volume of drug injected (except isobaric) Minor factors Level of injection Speed of injection/barbotage Size of needle Physical status of patients Intra-abdominal pressure
Determinants of duration Drug used Dose injected Presence of vasoconstrictors Total spread of blockade
Transient paresthesias ， Spinal hematoma ， back pain ， Bloody tap ， Transient neurologic syndrome ， Postdural puncture headache
Hypotension , Bradycardia
Apnea , Dyspnea
Nausea and vomiting , Urinary retention
Correct placement of needle
Hanging drop technique
Loss of resistance
Epidural anesthesia is achieved by introducing local anesthetics into the epidural space
Determinants of the level of epidural blockade
Volume of local anesthetic
Speed of injection.
Spread of epidural blockade.
Determinants of onset and duration of epidural blockade
Selection of drug .
Addition of epinephrine
Addition of opioid.
pH adjustment of solution.
Catheter complications ： The catheter can be inserted into an epiduralvein ， Inability to thread the epidural catheter ， Catheters can break off or become knotted ， Cannulation of the subdural space
Unintentional subarachnoid injection
Local anesthetic overdose.
Postdural puncture headache.
Direct spinal cord injury
Caudal anesthesia is obtained by placing local anesthetic into the epidural space in the sacral region
Complications The complications of caudal anesthesia are similar to those of epidural anesthesia
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Fossum, Fundamentals of orthopedic surgery and fracture management, p. 825 Techniques in Small animal Surgery , 2 nd Edition, 2002. Drug Dose Onset (min) Duration (hours) Lidocaine 2% 1 ml/ 3.4 -4.5 kg 10 1-1.5 Bupivacaine (0.25 or 0.5%) preservative free 1ml/4.5kg 20-30 4.5-6 Fentanyl 0.001 mg/kg 4-10 6 Oxymorphone 0.1 mg/kg 15 10 Morphine (preservative free) 0.1 mg/kg 0.03 mg/kg (cats) 23 20 Buprenorphine 0.003-0.005 mg/kg diluted w/saline 30 12-18
Techniques of Administration
Ideal if lateralized orthopedic problem
Surgical site down for maximal infiltration of agent (at least 5min)
Pelvic limbs drawn cranial
Stretches out ligamentum flavum
Expands intervertebral space
Needle Selection con’t… Patient size Gauge and length Alternative gauge and length Cats and small dogs 25 G; 1” 22 G; 1.5” Medium to large dogs 22 G; 2.5” 21 G; 2.5” Very fat, large or giant breed dogs 21 G; 3”
If intrathecal reduce dose by 40-75% of epidural
Reduce dose by up to 75% in pregnant patients
Engorgement of epidural vessels
Decreases volume of space
Keep head elevated for 10 min.
Place affected limb down for 5 min.
α -2 agonists
Prudent to choose preservative free when possible
No reports of neurotoxicity in animals after one dose
Avoid especially with:
Avoid antioxidant Na metabisulfite (in local anesthetics that contain epinephrine) intrathecally
Not a true preservative
Local Anesthetics: Disadvantages
Relatively short duration of action
Possibility of unwanted motor blockade
Potential blockade of spinal sympathetic nerves
Cause or aggravate hypotension
Quick onset of action
Analgesia with minimal motor blockade
High affinity for Na channels (potential for cardiotoxicity)
Lidocaine for injection or preservative free 1.0-2.0%