1. Neuraxial Anesthesia & Local Anesthetic Dosing Flashcards
adult spinal cord end
L1-L2
conus medullaris
end of spinal cord
cauda equina
nerve root bundle that begins at the conus medullaris
cauda equina innervates
legs and bladder
adult dural sac end
S2
kid spinal cord end
L2-L3
kid dural sac end
S3
layers from skin to CSF 7
1 skin 2 supraspinous ligament 3 interspinous ligament 4 ligamentum flavum 5 epidural space 6 dura mater 7 subarachnoid space
lordosis (convex) is where in spine
cervical 7
lumbar 5
kyphosis (concave) is where in spine
thoracic 12
kyphosis
posterior curvature of spine
lordosis
anterior curvature of the spine
scoliosis
lateral curvature of the spine
how many cervical vertebrae
7
how many thoracic vertebrae
12
how many lumbar vertebrae
5
how many sacral vertebrae
5
how many coccygeal vertebrae
4
how many total vertebrae
33
how many cervical nerve roots
8 pairs
how many thoracic nerve roots
12 pairs
how many lumbar nerve roots
5 pairs
how many sacral nerve roots
5 pairs
how many coccygeal nerve roots
1 pair
how many pairs of spinal nerve roots total
31
what are nerve roots covered by
dural sheath
why is it important that nerve roots are covered by dural sheath? 2
- roots close to spinal cord float in dural sac and pushed away by advancing needle
- nerve blocks close to intervertebral foramen carry risk of subdural injection
what is the most common starting insertion site for a spinal or lumbar epidural?
L3-4 interspace
L4-5 is acceptable; L2-3 may be considered if lower attempts fail
tuffiers line
line between superior aspects of iliac crests and estimates L4 body
what is the T10 (umbilicus) dermatome needed for? 3
1 spontaneous vaginal delivery (SVD)
2 inguinal surgery
3 testicular surgery
what is the T4 (nipple) dermatome needed for?
c-section
why is it convenient that T4 is the most dependent area of the spine in the supine position?
lay pts down after spinal then it will go to the correct height usually
it helps prevent the spread of local anesthetic above T4 and prevents high spinal
T5-L1
vasomotor tone
what happens when T5-L1 are blocked with spinal or epidural
vasodilation and hypotension
“sympathectomy”
nearly all pts with spinals in supine position will have a degree of sympathectomy
are sympathectomies more common with spinals or epidurals?
spinals
what is the earliest sign of sympathectomy?
nausea and vomiting
T1-T4
cardiac accelerator fibers
what can happen if the block rises above T4?
significant bradycardia bc you are blocking the sympathetics to the heart
C3-C5
phrenic nerve
if the block goes above this then the pt will go apneic
C6-C8
hands/fingers
the pt will experience tingling/numbness or weakness in their fingers
what do you do if they start to get numbness in their fingers?
place the pt in reverse trendelenburg
T4-T5
carina
T6
xyphoid process
T7
inferior border of scapulae
T8-L1
kidney
S2-S4
bladder
sympathetic blockade
blocking the nerves up to that level will have the ability to produce hypotension and bradycardia
sensory blockade
blocking nerves up to that level will produce an absence of pain but NOT of movement/touch
motor blockade
blocking nerves up to that level will block the pts ability to move those limbs
levels of sympathetic vs sensory vs motor blockade
sympathetic is two levels higher than sensory
sensory is two levels higher than motor
nerves are more easily blocked if they are: 2
1 smaller
2 myelinated
differential blockade order from easiest to hardest block
autonomic>sensory>motor
what is the goal of an epidural?
to stop the needle in the epidural space and not puncture the dura
what type of needle is used for an epidural?
17ga tuohy needle
4 epidural advantages
1 we can give analgesia as long as necessary
2 more control over analgesic level
3 less profound sympathectomy
4 better preservation of motor function is possible (less dense block)
disadvantages to epidural
1 not as dense as spinal (not as comfortable if they have to do a c-section)
2 high propability of PDPH (headache) if needle puncture
3 onset of action is longer for epidurals (several minutes)
4 more potential for local anesthetic toxicity with epidural
walking epidural
epidural where it is either:
only narcs
lower dose local
-it preserves motor function, good for post op pain control
spinal anesthesia description (3)
1 dura punctured
2 single shot of drug given (preservative free)
3 smaller needles are used
two spinal pencil point needles
whitacre
sprotte
sprotte
whitacre needle
smallest opening
CSF aspiration slow and hard
sprotte needle
longer opening
CSF aspiration easier
higher chance of injecting epidurally
spinal cutting type needle
quincke
quincke
cuts through ligaments better but makes larger hole in dura
what is the 18ga introducer needle used for
the spinal needle can be placed through the introducer to guide it to the correct spot
advantage of introducer needle
much less bending of spinal needle
commonly used if spinal needle is smaller than 22ga
22ga spinal needle advantage and disadvantage
advantage-
18ga introducer not needed
disadvantage-
higher risk of spinal headache
common uses for the 22ga spinal needle
elderly pts (lower risk of headache) obese pts
advantage to 25ga spinal needle
way less likely for spinal headache
what is the most common size spinal needle for adults?
25ga
disadvantage to 25ga spinal needle
more likely to bend when passing ligaments ** used with introducer
advantage to 27ga spinal needle
smallest hole in dura and least likely headache
disadvantage to 27ga spinal needle
highest chance of bending through ligaments
when is the 27ga spinal needle used?
CSE
combined spinal epidural where it is placed through the espocan needle
continuous spinal anesthesia
catheter placed into intrathecal space
problem with continuous spinal anesthesia
microcatheter use for continuous spinal anesthesia is contraindicated due to pooling of local anesthetic
what is the risk associated with microcatheters
neurotoxicity and cauda equina syndrome
pooling of local anesthetic
practical use of continuous spinal anesthesia
problem: accidentally wet tap someone during epidural placement
fix: just thread epidural catheter into the intrathecal space through touhy needle and give lower doses
4 management steps of continuous spinal anesthesia
1 sterile technique critical
2 catheter threaded 2-3cm intrathecal space
3 analgesia is usually maintained with local anesthetic boluses NOT infusion
4 appropriate dosing intervals are anywhere from 45-90 min
what should you do before and after each continous spinal injection
flush with previously aspirated CSF after each injection
baricity
how dense (heavy) the drug is compared to CSF and the density determines whether the drug will sink or rise
3 types of baricity
hyperbaric
hypobaric
isobaric
hyperbaric
spinal drug is denser than CSF and drug will sink
how to make drug hyperbaric
adding an equal volume of 10% dextrose/glucose to the local anesthetic
where does a hyperbaric spinal move when placed in supine pt
the hyperbaric drug tends to move to T4 because it is the most dependent area of the spine
how do we know that glucose must interfere with the hyperbaric drugs absorbtion?
shorter time to peak concentration
shorter duration of action than plain local anesthetics
what happens if you allow pt to remain sitting after placing a hyperbaric spinal?
saddle block anesthetizes:
sacral nerves
buttocks
perineal
thighs
what does a saddle block anesthetize?
sacral nerves
buttocks
perineal area
inner thighs
what types of procedures are saddle blocks used for
genitourinary
2nd stage labor pain
hypobaric drug
spinal drug is lighter than CSF and drug will rise
how do you make a drug hypobaric
add sterile water
how much sterile water do you add to a drug to make it hypobaric
depends on source!!
larger volume
3mL per 1mL
1mL per 1mg
what is the most common use for a hypobaric spinal?
hip surgery
pt in lateral position with operative hip up
isobaric spinal drug definition
spinal drug has the same specific gravity as CSF and will remain at the level of injection
how do you make a drug isobaric?
add equal volume of CSF or normal saline to the local
does baricity apply to spinals and epidurals?
no just spinals
how long until the baricity of the spinal settles?
10-15min
then shouldn’t rise or sink based on position
CSE
combined spinal epidural technique
CSE technique (6 steps)
1 touhy needle placed in epidural space
2 27ga spinal needle through touhy into the intrathecal space
3 perform spinal block into intrathetcal space
4 remove spinal needle
5 thread epidural catheter through touhy needle
6 remove touhy needle
what is a common needle set for the CSE?
espocan kit
17 ga tuohy needle with hole
27ga spinal needle
advantages CSE 3
1 more dense block than epidural
2 decr risk of PDPH
3 postop analgesia with epidural w/o using duramorph
disadvantage to the CSE
can’t perform a test dose through the epidural catheter bc the pt is already numb from the spinal
dural puncture epidural technique 5
1 17ga epidural needle placed
2 spinal needle though tuohy and punctures dura
3 spinal needle removed without dosing
4 epidural catheter placed into epidural space
5 some of the local anesthetic leaks into the intrathecal space through the small hole
advantages to dural puncture epidural technique 3
1 faster sacral onset
2 greater sacral spread of local
3 lower incidence of unilateral block
disadvantage dural puncture epidural technique
small chance of post dural puncture headache
epidural summary points
longer analgesia possible
more control of analgesic level
more gradual, less profound sympathectomy
less dense block
spinal summary points
limited analgesic duration
single shot (cant alter dose)
rapid potent sympathectomy
denser block
sacral hiatus
site of needle insertion for caudal block
what is the sacral hiatus covered by?
sacrococcygeal ligament
sacral cornu
bony pominence to either side of sacral hiatus
palpate these for landmarks
is a caudal block a spinal or epidural?
epidural block but differs because
performed at sacral level
dosed with single shot of drug
caudal block technique 5
1 palpate sacral hiatus
2 insert needle through sacral hiatus at 45 deg angle
3 advance cephalad until pop
4 advance cannula over the needle and remove needle
5 aspirate before inject
when is it possible for the epidural to cause spinal cord or nerve root damage?
in the lumbar or thoracic region
what will happen when epidural needles get too close to nerve roots?
patients experience parasthesias and the anesthetist can redirect
can lumbar and thoracic epidurals be done when the patient is asleep?
no they must be awake to reduce the risk of nerve injurt
can caudal blocks be done when the patient is asleep?
yes because they are so far away from the spinal cord or roots
what is the most popular block for children?
caudal block
3 advantages to caudal blocks
performed on asleep pts
more reliable perineal anesthesia (than lumbar)
less likely dural puncture and nerve damage
5 disadvantages to caudal blocks
dural can still be punctured rectum can be punctured technically more difficult in adults twice as much local anesthetic required than lumbar epidural higher risk of urinary retention
8 factors that affect neuraxial spread of local anesthetics
1 total mg dose 2 total volume injected 3 addition of epi 4 addition of narcotic 5 height of patient 6 positioning 7 weight of patient 8 age
how does total mg dose affect spread?
higher doses spread more
how does volume injected affect spread
the higher volume will spread more
does dose or volume have greater affect on spread
dose
how does the addition of epi spread the block?
it prolongs the block but doesnt raise the level
how does addition of narcotic affect the block
increases the density “strength” of the block
how does the height of the patient affect the block
the shorter you are the more likely it will travel too high
how does trendelenburg effect block
more cephalad spread of local
how does reverse trendelenburg effect block
less cephalad spread of local
how does lateral effect block
block will be more one sided
how does the weight of the patient effect spread of block
the more obese (heavy) the patient: higher it will spread and a lower local dose is required
why does local anesthetic spread higher in heavier patients?
increased intraabdominal pressure
compression of inferior vena cava
engorgement of epidural veins
decreases CSF volume
how does age effect spread of block
geriatric pts have: lower dosing requirements and shorter onset (reduced CSF volume; decrease in nerve fibers and decreased conduction velocity)
8 absolute contraindications to neuraxial anesthesia
1 refusal 2 infection at injection site 3 serve hypovolemia 4 coagulopathy (epidural hematoma) 5 severe aortic stenosis 6 severe mitral stenosis 7 sepsis 8 elevated ICP
what could infection at injection site or sepsis lead to
meningitis or epidural abscess
what must the platelet count be for OB before neuraxial blockade?
> 80,000- 100,000
why should aortic/mitral stenosis be avoided with neuraxial blocks?
sympathectomy drops preload and afterload and those should be maintained with aortic stenosis
why avoid neuraxial block in patients with elevated ICP?
cant tolerate sympathectomy
high MAP is needed to perfuse the head with elevated ICP
Cerebral perfusion pressure= MAP-ICP
relative contraindications for neuraxial anesthesia 4
1 neurologic deficiencies (MS) (worsening symptoms)
2 sepsis
3 previous back surgery (may effect spread)
4 severe COPD (may rely on accessory muscles to breath)
11 potential complications of neuraxial blocks
pruritus nausea and vomiting (from hTN) urinary retention parasthesia (short term) nerve/ spinal cord injury backache PDPH transient neurologic symptoms (TNS) cauda equina syndrome (CES) epidural abscess epidural hematoma
what is the incidence of back pain following spinal anesthesia?
25%
possible etiologies of back pain: 5
1 regular common backache from needle or lying flat 2 transient neurologic symptoms (TNS) 3 cauda equina syndrome (CES) 4 epidural abscess 5 epidural/spinal hematoma
what is a epidural abscess caused by
infection potentially after back surgery or neuraxial block
what are the symptoms of a epidural abscess
back pain intensified by spine percussion
signs of infection (fever, increases WBC)
sensory AND motor deficits
how is an epidural abscess diagnosed?
ct scan
treatment of epidural abscess
surgical decompression via laminectomy
how are the symptoms of epidural abscess and epidural hematoma different?
hematoma has faster onset and the WBC count should be normal
how are epidural hematomas treated
immediate surgical evacuation
what is transient neurologic symptoms?
someone who experiences back pain without motor deficits
resolves on own
what are some hypothesis about what causes TNS
lithotomy position
intrathecal vasoconstrictors
highly concentrated local
lidocaine
which is more serious CES or TNS?
CES because it includes back pain and motor deficits and/or bladder and bowel dysfunction
etiology of CES
nerve root/spinal cord trauma
highly concentrated local anesthetics
continuous spinal anesthesia through microcatheter
which needs a neurology consult? CES or TNS
CES
post dural puncture headache
more likely to occur with wet tap from tuohy needle during epidural
less likely with spinal because headache is proportional to the size of the hole
if someone gets a wet tap what is the likelihood of PDPH
80% chance
what are the two options for if you wet tap a patient
thread catheter ~2cm intrathecally for continuous spinal anesthesia
remove needle and start another epidural higher level
if you are older are you more or less likely to get a PDPH?
less likely as you get older
symptoms of PDPH
headache bilateral frontal occipital and extends to neck
aggravated by standing or sitting
why does standing make the headache worse
venous return decrease
epidural veins engorge
push out more CSF and worsen the headache
3 treatments for PDPH
autologous blood patch
analgesics, caffeine, generous fluid admin
neostigmine and atropine combination
what is the gold standard for PDPH treatment
blood patch, 90-99 success rate
regional anesthesia advantages (compared to GA) 4
1 decreases anesthetic requirements (decreases postop N/V)
2 decrease respiratory complications
3 decreased surgical blood loss
4 decreases incidence of thrombosis
how many “i” in esters
one
how many “i” in amides?
more than one “i”
how are esters metabolised
plasma esterases
what do esters produce as a byproduct and why does that matter
p-aminobenzoic acid PABA
it is associated with allergic reactions
how are amides metabolized?
by the liver
which is more likely to cause an allergic reaction? tetracaine or marcaine?
tetracaine because it is an ester
what is the pH of local without epi
6-7
acidic so it prolongs shelf life
what is the pH of local with epi
4-5
more acidic because epi is unstable in basic environments
what local is the longest acting spinal local?
tetracaine (pontocaine)
which drug is the most common for spinal anesthesia
marcaine 0.75%
which local’s duration is least affected by addition of epi?
marcaine
marcaine duration
2 hours
how long does adding phenylephrine to tetracaine increase the duration of block
70-100%
how long does adding epi to tetracaine increase the duration of block
40-60%
how long does adding clonidine to tetracaine increase the duration of block
50-70%
which spinal local promotes early ambulation?
spinal mepivacaine
which spinal local is best for ultra short procedures?
spinal chloroprocaine
which spinal local is associated with TNS and CES
lidocaine
what are the 4 most common local anesthetics for labor epidural dosing
marcaine
ropivacaine
lidocaine
chloroprocaine
marcaine advantages 2
motor sparing
longest duration of epidural locals
longest lasting
marcaine disadvantages 3
1 less effective at blocking the larger sacral nerves
2 slowest onset
3 very cardiotoxic
what is the implication of marcaine being less effective at blocking larger sacral nerves
higher chance of losing their effectiveness during stage two labor
toxic dose marcaine with and without epi
2.5mg/kg w/o epi
3mg/kg w/ epi
what is the treatment for marcaine toxicity
CPR
intralipid 20%
Ropivacaine (naropin) 0.2%
similar pharmacology to marcaine
less cardiotoxic and more expensive
what is the max dose of ropivacaine
3mg/kg
advantages of lidocaine 2% or 1.5% with epi
more effective at blocking larger sacral nerves
fast onset
disadvantages of lidocaine 2% or 1.5% with epi
more significant motor blockade (could inhibit pushing)
neurologic symptoms if the toxic dose is exceeded
highest risk of TNS and CES (intrathecal hyperbaric lido)ch
what is the toxic dose of lidocaine with epi
7mg/kg
what is the toxic dose of lidocaine without epi
4mg/kg
chloroprocaine (3%) advantages
fastest epidural onset
minimal drug transfer across placenta
when are you most often going to use chloroprocaine
emergency c section
why does chloroprocaine have such a rapid onset
pseudocholinesterase metabolism
chloroprocaine 3% disadvantages
highest degree of motor block shortest duration (redosed frequently)
which local should you use for vaginal delivery?
marcaine
which local should you use if epidural stops working
lidocaine
5 most common situations an epidural is bolused include
initial test dose loading dose dose to increase blocks density raising the block to t4 gradually raising the block to t4 immediately
what should you do before bolusing an epidural?
verify they have stable vital signs prior to bolusing and monitor for 10 mins after
aspirate everytime to rule out intravascular or intrathecal injection
signs of intravascular injection
increase HR
tinnitus
oral/tongue numbness
sings of intrathecal injection
immediate numbing of the legs
what is the initial test dose
5mL of 1.5% lido with 1:200,000 epi
accidental IV injection ruled out by absence of:
tachycardia
mouth/tongue numbness
ringing in ears
accidental intrathecal injection ruled out by
not having immediate numbness
epidural loading dose
additional 5mL loading dose after test dose to speed up the onset of block
higher risk of sympathectomy
why dose to increase the density of epidural block ?
patchy block when it starts to wear off, disconnect pump and bolus 5mL
when would you raise the epidural block gradually to t4?
non emergent c section
how to raise the epidural block level gradually
gives initial 10mL of local, wait 3 min check level
give 5mL of local, wait 3 min, check level
give another 5mL of local if still not high enough
if local is bolused too fast what will happen
risk of high block and hypotension is increased
if the anesthetist waits too long in between boluses what happens?
the block density will increase but the block wont rise
what is the local of choice for raising an epidural block from t10 to t4?
lidocaine has longer duration than chloroprocaine
Chloroprocaine is faster onset
what are common preservatives in local anesthetics?
sulfites (bisulfite, metabisulfite)
parabens (methylparaben)
EDTA
do multi dose or single dose vials have preservatives?
multidose vials
methylparaben
bacteriostatic preservative added to multidose vials
potential anaphylactoid symptoms
what is methylparaben contraindicated for?
epidural and intrathecal
what are methylparaben free solutions referred to as?
MPF
sulfites (and citric acid) are added to what?
local that are premixed with epi to prevent degradation of epi
causes more pain on injection
what has intrathecal injection of sulfites resulted in?
arachadonitis
anaphylactoid rxns
what are sulfites contraindicated for?
spinals
are sulfites okay to use for epidurals?
yes
what is EDTA used for?
prolong shelf life and allow autoclaving to sterilize glass vial
what is EDTA been linked to in patients?
epidurally injected associated with severe pain at injection site
can you use chloroprocaine for spinals?
yes
recently approved by FDA
what two things should you check before injecting epudural or intrathecal
preservative free
“for spinal or epidural use”
can you use a local with preservatives for a bier block?
no
why is bicarb added?
added to lidocaine or chloroprocaine to speed up onset by bringing closer to pKa
how long until the local will precipitate when bicarb is added
6hr
effects of adding alpha agonist?
prolongs block duration
limits toxic side effects
enhances analgesic quality
what local is effected the most with added epi
tetracaine
what local is effected the least with added epi
bupivacaine
if the local is _____ lipid soluble then added epi is less significant.
more
can alpha agonists cause analgesia?
yes by directly inhibiting sensory and motor neurons
what does clonidine do to BP
greater decrease in BP
duramorph
morphine without preservatives
which causes more respiratory depression in fetus and urinary retention? morphine or fentanyl?
morphine
advantages of neuraxial opioids
analgesia
no motor blockade
no sympathectomy
disadvantages of neuraxial opioids
pruritus (itch)
delayed respiratory depression
nausea/vomiting
the more lipid soluble drugs they have a _____ onset and _____ duration
faster onset
shorter duration
____ lipid soluble drugs exit the central nervous system quickly
more
duration of epidural narcotics
twice as long as spinal because epidural dose is higher
spinal dosing of opioids
should not be dosed if it is outpatient procedure
what are the three forms a drug can be in?
non polar
polar/neg charged (loss of H+)
polar/pos charged (gain of H+)
what is the form a drug takes determined by? 2
drugs pH
pH of environment the drug is placed in
if you have higher degree of nonpolar drug then the onset is
faster
do the ionized or nonionized portion of the drug cross the lipid membrane?
nonpolar (nonionized)
if an acidic drug is placed in a basic environment then it will become:
negatively charged and slow the onset of the drug
if a basic drug is placed in an acidic environment then it will become
positively charged and it will slow the onset of the drug
what is the highest possible portion of nonpolar drug
50%
when you place a drug in an ideal pH then it will
have the highest portion of nonpolar drug and fastest onset possible
pKa
pH of the drugs environment that will result in the drug having 50% ionization and 50% nonionization
pH of lidocaine
6.5
pKa of lidocaine
7.9
what would you do to bring the pH of lidocaine closer to its pKa?
add sodium bicarb
do basic drugs have a high or low pKa
high
do acidic drugs have a high or low pKa
low
the drug will have a ____ onset the closer the drugs pKa is to physiologic pH 7.4
faster
are local anesthetics by themselves more acidic or basic?
basic
are local anesthetics in the vial more acidic or basic?
acidic
acid added so it wont precipitate
adding bicarb to local anesthetic will 4
1 bind up excess acid
2 make drug less pos charged
3 increase the pH
4 speed up onset of action
how much lower is the pH of local when the solution contains epi?
1-1.5 units lower
factors that determine the onset of local anesthetic 4
how ionized the local is
how close locals pH is to the pKa
how close the locals pKa is to physiologic pH
how lipid soluble the local is
effect of higher lipid solubility on local anesthetics onset and duration
slow onset
long duration
effect of higher lipid solubility on other drugs such as fentanyl onset and duration
fast onset
short duration
factors that determine local anesthetic potency 3
how concentrated (more=potent) how lipid soluble (higher=potent) total dose (more=potent)
