Unit 8: Regional Anesthesia Flashcards
What are the 5 divisions of the spinal column? How many vertebrae are present in each?
Cervical = 7
Thoracic = 12
Lumbar = 5
Sacrum = 5 fused
Coccyx = 4 fused
*33 vertebrae total
What are the anatomic boarders of the facet joint on the vertebrae?
Superior articular process of one vertebra
Inferior articular process of the vertebra directly above
*injury to facet joint can compress spinal nerve that exits the respective foramina
What ligament covers the sacral hiatus? What is the significant of this?
Sacrococcygeal Ligament
-it is punctured during caudal approach to the epidural space
Order the 5 ligaments of the spinal column from posterior to anterior
-Supraspinous Ligament
-Interspinous Ligament
-Ligamentum Flavum
-Posterior Longitudinal Ligament
-Anterior Longitudinal Ligament
What ligaments are penetrated during the midline and paramedian approach to the epidural space?
Midline Approach:
- supraspinous
- interspinous
- ligamentum flavum
Paramedian Approach
- ligamentum flavum
List the structures/spaces between the skin and spinal cord as they would be encountered during a subarachnoid block
-Skin
-SubQ tissue
-Muscle
-Supraspinous ligament
-Interspinous ligament
-Ligamentum flavum
-Epidural space
-Dura mater
-Subdural space
-Arachnoid mater
-Subarachnoid space
-Pia mater
-Spinal cord
What are the boundaries of the epidural space?
Cranial Border = Foramen Magnum
Caudal Border = Sacrococcygeal Ligament
Anterior Border = Posterior Longitudinal Ligament
Lateral Border = Vertebral Pedicles
Posterior Borders = Ligamentum Flavum and Vertebral Lamina
What happens when you accidently inject local anesthetic into the subdural space during a SAB? How about during an epidural?
Subdural space = potential space between the dura and arachnoid mater
Inadvertent injection of LA into the space yields the following:
-epidural dose –> high spinal w/ delayed onset (15-20 min)
-spinal dose –> failed spinal
What is Batson’s plexus? What is its significance?
Batson’s Plexus = epidural veins that drain venous blood from the spinal cord
-valveless veins pass through the anterior and lateral regions of the epidural space
-obesity and pregnancy increase intra-abdominal pressure causing engorgement – associated w/ increased risk of needle injury or cannulation during neuraxial techniques
What is the plica mediana dorsalis? What is its significance?
Band of connective tissue between the ligamentum flavum and dura mater
-existence remains controversial
-if it does exist it could create a barrier that impacts the spread of medications within the epidural space
What are the significant dermatomes levels and related cutaneous innervation?
-C6 = 1st digit (thumb)
-C7 = 2nd and 3rd digits
-C8 = 4th and 5th digits
-T4 = Nipple line
-T6 = Xiphoid process
-T10 = Umbilicus
-T12 = Pubic symphysis
-L4 = Anterior knee
What is the site of action for spinal anesthesia and epidural anesthesia?
Spinal Anesthesia Site of Action:
- myelinated preganglionic fibers of the spinal NERVE ROOTS
- LA also inhibits neural transmission in the superficial layers of the spinal cord
Epidural Anesthesia Site of Action:
- diffuse through DURAL CUFF before they can block the nerve roots
- LA also leaks through the intervertebral foramen to enter the paravertebral area
What factors significantly affect the spread of LA in the subarachnoid space?
Controllable Factors:
- baricity
- pt position during and after block placement
- dose
- site of injection
Non-Controllable Factors:
- volume of CSF
- density of CSF
What factors do not significantly affect the spread of LA in the subarachnoid space?
-Barbotage
-Increased intra-abdominal pressure (coughing/labor)
-Speed of injection
-Orientation of bevel
-Addition of vasoconstrictor
-Weight
-Gender
What is the primary determinant of spread for epidrual anesthesia?
Volume
What is the order of blockade of spinal anesthesia? What are the blockade level differences?
-Autonomic fibers are blocked first
-Sensory fibers are blocked second
-Motor neurons are blocked last
*autonomic blockade = 2-6 dermatomes higher than sensory block
*sensory block = 2 dermatomes higher than motor block
How is differential blockade different with epidural anesthesia than with spinal?
There is no autonomic differential blockade with epidural anesthesia
Sensory block = 2-4 dermatomes higher than motor block
What are the characteristics of type A-alpha nerve fibers?
Heavy myelination
Function = skeletal muscle (motor) and proprioception
12-20 um diameter
Velocity = +++++
Block onset = 4th
What are the characteristics of type A-beta nerve fibers?
Heavy myelination
Function = touch and pressure
5-12 um diameter
Velocity = ++++
Block onset = 4th
What are the characteristics of type A-gamma nerve fibers?
Medium myelination
Function = skeletal muscle (Tone)
3-6 um diameter
Velocity = +++
Block onset = 3rd
What are the characteristics of type A-delta nerve fibers? What order block onset?
Medium myelination
Function = fast pain, temp, touch
2-5 um diameter
Velocity = +++
Block onset = 3rd
What are the characteristics of type B nerve fibers?
Light myelination
Function = preganglionic ANS fibers
3 um diameter
Velocity = ++
Block onset = 1st
What are the characteristics of type C sympathetic nerve fibers? What is the block onset order?
No myelination
Function = postganglionic ANS fibers
0.3-1.3 um diameter
Velocity = +
Block onset = 2nd
What are the characteristics of type C dorsal root nerve fibers? What is the block onset order?
No myelination
Function = slow pain, temp, touch
0.4-1.2 um diameter
Velocity = +
Block onset = 2nd
What is the order of block onset for the different nerve types?
First = B fibers
Second = C fibers (sympathetic and dorsal root)
Third = A-gamma and A-delta fibers
Fourth = A-alpha and A-beta fibers
What are the cardiovascular effects of neuraxial anesthesia?
Sympathectomy vasodilates the arterial and venous circulations –> Reduction in venous return, CO, and BP
-volume loading with ~15mL/kg and vasopressors will minimize hypotension
Bradycardia is caused by:
-blockade of preganglionic cardioaccelerator fibers at T1-T4 (promotes relative increase in parasympathetic tone)
-unloading of cardiac mechanoreceptors (Bezold-Jarish reflex)
-unloading of the stretch receptors in the SA node
What are the respiratory effects of neuraxial anesthesia?
Negligible effects on minute ventilation, tidal volume, RR, dead space, and arterial blood gas tensions in the HEALTHY patient
Accessory muscle function is reduced – impairment of the intercostal muscles (inspiration/expiration) as well as abdominal muscles will decrease pulmonary reserve
-important for pt w/ severe COPD
Apnea is usually result of brainstem hypoperfusion (Decreased blood flow to ventilatory centers in the medulla) – not a result of phrenic nerve paralysis or high concentrations of LA in the CSF
How does neuraxial anesthesia affect the neuroendocrine response to stress?
Diminishes the surgical stress response – inhibits afferent traffic originating from the surgical site
-reduces circulating levels of catecholamines, renin, angiotensin, glucose, TSH, and growth hormone
How does neuraxial anesthesia affect GI function?
Inhibit the sympathetic tone –> increase parasympathetic tone to the gut –> relaxes the sphincters in the gut and increases peristalsis
How does neuraxial anesthesia affect renal and hepatic blood flow?
As long as systemic blood pressure is maintained – hepatic and renal blood flow and function are unchanged
What is the risk of neuraxial anesthesia in the pt with coagulopathy? What lab values are considered contraindications to neuraxial technique?
Risk of spinal or epidural hematoma
Contraindicated in significant pathologic or therapeutic coagulopathic states:
-platelet count <100,000
-PT, aPTT, and/or bleeding time twice the normal values
What cardiac pathologies present a risk of hemodynamic collapse with neuraxial anesthesia?
Valve lesions with fixed stroke volume
-severe aortic stenosis
-severe mitral stenosis
-hypertrophic cardiomyopathy
What is the risk of a neuraxial technique in the patient with intracranial hypertension?
Increased chance of brain herniation with sudden change in CSF pressure
What is the relationship between neuraxial anesthesia and multiple sclerosis?
Intrathecal technique may exacerbate symptoms
-no good data to support this
-use lower dose and concentration of LA if doing a spinal for pt with MS
What is the specific gravity of CSF?
1.002 - 1.009
What is baricity? How does it influence your selection of local anesthetic?
Baricity = density of a local anesthetic solution relative to the CSF
-Isobaric solution = similar baricity to CSF (LA remains in place)
-Hyperbaric = higher density (LA sinks)
-Hypobaric = lower density (LA rises)
*general rule - solutions in dextrose are hyperbaric – saline are isobaric – water are hypobaric
*Procaine 10% in water = exception (it is hyperbaric)
How does a hyperbaric solution distribute in the sitting patient? How about the supine patient?
Hyperbaric solution will settle to the lowest point of the spinal canal
-Sitting = sink and anesthetize the sacral nerve roots (Saddle block)
-Supine = slide down the lumbar lordosis and pool in the sacrum and thoracic kyphosis
How does a hypobaric solution distribute in the sitting patient? How about the supine patient?
Hypobaric solution will settle to the highest point of spinal canal
-Sitting = rise towards brain (not a good idea)
-Supine = float toward the lower lumbar region
What are the two classifications of spinal needles? What are examples of each?
Cutting Tip – Quincke and Pitkin
Non-Cutting Tip:
-Pencil point – Pencan, Sprotte and Whitacre
-Rounded bevel – Green
What are the pros and cons of each type of spinal needle?
Cutting Tip:
-Pros = requires less force
-Cons = higher risk of PDPH, less tactile feel, needle more easily deflected, more likely to injure cauda equina
Non-Cutting Tip:
-Pros = lower risk of PDPH, more tactile feel, needle less likely to deflect, less likely to injure the cauda equina
-Cons = requires more force
What are the three types of epidural needles? How are they different from each other?
Differ in the angle of the needle tip
-Crawford = 0 degrees
-Husted = 15 degrees
-Tuohy = 30 degrees (curvature plus blunt tip helps prevent dural puncture)
*needle angle increases in alphabetical order
How do you dose a caudal anesthetic in a child and adult for the following block heights:
-sacral
-sacral to low thoracic (~T10)
-sacral to mid thoracic
Sacral Block Height:
- pediatric dose = 0.5 mL/kg
- adult dose = 12-15 mL
Sacral to Low Thoracic (~T10):** common dose
- pediatric dose = 1 mL/kg
- adult dose = 20-30 mL
Sacral to Mid Thoracic:
- pediatric dose = 1.25 mL/kg
- adult dose = N/A
What are the absolute and relative contraindications to caudal anesthesia?
Absolute Contraindications:
-spina bifida
-meningomyelocele of sacrum
-meningitis
Relative Contraindications:
-pilonidal cyst
-abnormal superficial landmarks
-hydrocephalus
-intracranial tumor
-progressive degenerative neuropathy
What is the mechanism of action of neuraxial opioids?
Inhibit afferent pain transmission in the substantia gelatinosa (lamina 2) of the dorsal horn
Neurotransmission is reduced by:
-decreased cAMP
-decreased calcium conductance (presynaptic neuron)
-increased potassium conductance (postsynaptic neuron)
Epidural opioids also diffuse into the systemic circulation – delivered to opioid receptors throughout the body
Do neuraxial opioids cause sympathectomy, skeletal muscle weakness, and/or changes in proprioception?
Neuraxial opioids DO NOT cause these
What are the doses for commonly used intrathecal opioids?
Sufentanil = 5-10 mcg
Fentanyl = 10-20 mcg
Meperidine = 10 mg
Morphine = 0.25 - 0.3 mg
What are the doses for commonly used epidural opioids?
Sufentanil = 25-50 mcg (10-20 mcg/hr)
Fentanyl = 50-100 mcg (25-100 mcg/hr)
Hydromorphone = 0.5-1 mg (0.1-0.2 mg/hr)
Meperidine = 25-50 mg (10-60 mg/hr)
Morphine = 2-5 mg (0.1-1 mg/hr)
Rank the opioids from most lipophilic to most hydrophilic
Lipophilic:
- Sufentanil
- Fentanyl
- Meperidine
- Hydromorphone
- Morphine
Hydrophilic
How does lipophilicity affect rostral spread of opioids in the subarachnoid space?
Hydrophilic drugs (morphine, meperidine) tend to remain in the subarachnoid space and travel towards the brain – rostral spread
Lipophilic drugs (fentanyl, sufentanil) tend to diffuse out of the subarachnoid space and enter the systemic circulation
Compare and contrast PK/PD profiles of hydrophilic and lipophilic opioids used for spinal anesthesia
-Hydrophilic opioids stay in CSF longer than lipophilic
-Hydrophilic have extensive more rostral CSF spread with wide band of analgesia
-Lipophilic have minimal and less rostral CSF spread with narrow band of analgesia
-Hydrophilic site of action = rexed laminae 2&3
-Lipophilic site of action = rexed laminae 2&3 + systemic
-Hydrophilic have delayed (30-60 min) onset compared to lipophilic fast (5-10 min) onset
-Hydrophilic duration is longer (6-24 hrs) than lipophilic (2-4 hrs)
-Hydrophilic has less systemic absorption than lipophilic
-Hydrophilic has early and late respiratory depression – lipophilic only has early
-Higher incidence of N/V and pruritus w/ hydrophilic
What are the four most important side effects of neuraxial opioids? Which is most common?
- Pruritus (most common)
- Respiratory depression
- Urinary Retention
- N/V
Which local anesthetic can reduce the efficacy of epidural opioids?
2-Chloroprocaine
Which neuraxial opioid can reactivate herpes simplex labialis?
Epidural Morphine
-best explained by the cephalad spread of morphine to the trigeminal nucleus
-presents 2-5 days after administration
What is the pathophysiology of postdural puncture headache? How does it present?
Puncture of the dura causing CSF to leak from subarachnoid space –> CSF pressure is lost, cerebral vessels dilate
Brainstem sags into the foramen magnum –> stretches the meninges and pulls on the tentorium
Classic presentation = fronto-occipital headache
-may be accompanied by nausea, emesis, photophobia, diplopia, and tinnitus
-upright position (gravity) makes headache worse
What are the risk factors for PDPH?
Patient Factors:
-higher risk = younger age, female, pregnancy
-lower risk = older age, male, non-pregnant
-no effect on risk = early ambulation
Practitioner Factors:
-higher risk = cutting tip needle, larger diameter needle, using air for LOR, needle perpendicular to long-axis of neuraxis
-lower risk = non-cutting tip, smaller diameter, using fluid for LOR, needle parallel to long-axis, continuous spinal catheter (if placed after wet tap)
-no effect on risk = continuous spinal catheter (if placed after spinal block)
How do you treat PDPH?
-Bed rest
-Hydration
-NSAIDs
-Caffeine (cerebral vasoconstriction)
-Epidural blood patch (definitive treatment)
*opioids are not used