Neuraxial Anesthesia Flashcards
Neuraxial Anesthesia
- can be used in combo with GA or alone or after for post-op analgesia
- single dose or catheter for intermittent boluses or continuous infusion
Benefits of Neuraxial Anesthesia
- In high risk pts may decrease risk of venous thrombosis, PE, cardiac comps, bleeding, resp depression, transfusion requirements, PNE, vascular graft occlusion
- Post-op epidural analgesia may reduce mech vent need & time to extubation in abd or thoracic surgery pts
- Epidural/spinals (regional anesthesia) for C-section has decreased mortality & morbidity compared to GA
Spinal Column General Anatomy
from foramen magnum to L1 in adults, L3 in children
Dura Mater + Arachnoid Mater usually stuck together
Subarachnoid space = CSF
Pia mater adherent to spinal cord
Filum Terminale
an extension of the Pia mater which attaches the end of the spinal cord (conus medullaris) to the coccyx
Spinal Vs. Epidural
spinal requires less medication = a dense sensory and motor blockade
epidural requires more = a differential blockade
Differential Blockade
sympathetic is most cephalad: 1-2 segments above sensory
motor is most caudal
Primary Site of Action for Neuraxial Anesthesia & Physiologic Response
the nerve root
results from inhibited sympathetic and unopposed parasympathetic
Posterior V Anterior Nerve Roots
Posterior: somatic & visceral sensation
Anterior: efferent motor & autonomic outflow
Nervous System Breakdown
CNS: brain & spinal cord
PNS –> autonomic –> SNS & PNS
PNS –> somatic –> sensory & motor
Sympathetic Nervous System
thoracolumbar - T1-L2
most preganglionic synapse with postganglionic in the paravertebral ganglia
stellate ganglion: inferior cervical + first thoracic
T1-T4 are cardiac accelerator fibers
massive sympathetic response
tachycardia, bronchodilation, dry mouth, diaphoresis
anatomically & functionally more systemic
parasympathetic
craniosacral outflow
CN 3: midbrain
CN 7: pons
CN 9 & 10: medulla
Sacral segments S2, S3, S4
more selective & localized
massive parasympathetic response
bronchoconstriction (wheezing), bradycardia, miosis, vomitting, defecating, seizing
BP Effects of Neuraxial Anesthesia
varying level of decreased HR/BP
Hypotension: vasomotor tone is mostly determined by T5-L1 sympathetic fibers which innervating arterial & venous smooth muscle
- vasodilation of veins = reduced preload & CO
- vasodilation of arteries = impaired compensatory vasoconstriction
HR effects of neuraxial anesthesia
bradycardia.
a high blockade may inhibit the cardiac accelerator fibers at T1-T4
there is unopposed vagal tone which can = sudden cardiac arrest
Treatment of hypotension/bradycardia r/t neuraxial anesthesia
first line: ephedrine
glycopyrolate or atropine can be used for sympathetic bradycardia
IVF bolus 5-10ml/kg IF appropriate renal & cardiac function
head down = autotransfusion
Cephalad block effect on hemodynamic instability
more cephalad block = more hemodynamic instability
Pulmonary Effects of Neuraxial Anesthesia
usually minimal (phrenic nerve - innervating diaphragm - at C3-5)
severe CLD may require accessory muscles to breathe - these can be inhibited
post-op thoracic epidural analgesia in high risk patients is BENEFICIAL - decreased risk of PNE & respiratory failure - improves Oxygenation - decreases duration of mech vent
GI effects
unopposed vagal stim = small contracted gut & increased peristalsis- helpful for intestinal surgery
post-op epidural analgesia = decreased opioid need = quicker return of gut function
hepatic blood flow reduced with any type of anesthesia
GU effects
renal blood flow maintained
urinary retention - impairment of PNS & SNS - intraop catheter OR careful IVF admin & monitor for bladder distention
Major Indication for Neuraxial Blocks
Breast/thoracic/major abd - epidural + GA
Hip/knee replacement - can do spinal/epidrual alone
Absolute Contraindications to Neuraxial anesthesia
coagulation abnormalities severe hypovolemia increased ICP infection at injection site thrombolytic/fibrinolytic therapy
Relative Contraindications to Neuraxial anesthesia
aortic/mitral stenosis severe LF outflow obstruction sepsis severe spinal deformity pre-existing neurological defects uncooperative pt
Anticoagulants + Neuraxial Blockade
bleeding in the closed space of spinal canal = hematoma = pressure on spinal cord or caudal equina = infarction or ischemia = paraplegia or severe neurologic injury
Warfarin
interval from last dose to placement/removal: 4-5 days
Heparin
interval from last dose to placement/removal:
IV OR SubQ: 4-6 hrs & verify aPTT
Clopidogrel
interval from last dose to placement/removal: 5-7 days
interval from placement/removal to next dose:
- with loading dose: 6 hrs
- without: immediate
Argatroban
thrombin inhibitor
AVOID neuraxial anesthesia
Aspirin & NSAIDS
okay to continue
Prominent Landmarks
T7 - level with inferior tip of scapula
L4 - tip of iliac crests - Tuffiers Line
L4-L5: largest intervertebral space
S2: bw posterior superior iliac spine
thoracic vs cervical/lumbar vertebrae
thoracic spinous processes point much more caudal & therefore require a greater cephalad angle
Passing of Needle layers
skin - subQ tissue - muscle - SSL - ISL - ligament flavum - epidural space - dura mater - subdural space - arachnoid mater - subarachnoid space
Midline Approach Steps
- palpate & identify space
- clean - set up sterile
- skin wheel with LA with 25g needle
- introducer slightly above caudal spinous process with 10 degree cephalad angle
- remove stylet: CSF should drip
- attach syringe - gently aspirate - slowly inject 0.5ml/s
- complete, aspirate again to check placement
- remove all together
- assess level of blockade by testing temp or pin prick sensation
Paramedian approach
can bypass calcified ligaments
1 cm lateral and directed toward middle of the interspace
ligamentum flavum will be first resistance met
Dermatomes
C5: clavicles C6: thumb C8: ring & little fingers T4: nipples T10: umbilicus S2, 3, 4: perineum L1,2,3,4: anterior & inner surfaces of lower limbs
Spinal Needles & Catheters
sharp cutting tip: Quincke
blunt tip: Whitacre
25g
smaller gauge + blunt tip = reduced risk of PDPH
Factors influencing the level of a spinal block
baricity
positioning
drug dosage - larger dose = more cephalad
site of injection - more cephalad = more cephalad block
Baricity
migration depends on its density relative to CSF (1.003)
hyperbaric = denser/heavier
hypobaric = lighter/less dense
Positioning + baricity
Head up: hyperbaric settles caudal & hypobaric migrates cephalad
lateral: hyperbaric will have greater effect on the dependent side & hyperbaric will have a greater effect on the non-dependent side
isobaric tends to remain AT the level of injection
Baricity Supine position
anatomy will limit migration
T4-T7 is the most dependent area
T4 is the apex of the thoracolumbar curve - limiting spread
other factors affecting height of block
pt height, age, intraabd pressure, direction of bevel on injection, anatomy
spinal additives
all are preservative free
vasoconstrictors (A agonists & epi) & opioids prolong duration and/or improve quality
Epidural Anesthesia General
wider range of applications
can be cervial, thoracic, lumbar
surgical anesthesia, OB, post-op pain, chronic pain
single shot, boluses, or continuous
motor block can range from none to complete
slower onset (10-20 min)
less dense - segmental block
can provide analgesia without motor blockade
segmental block
a band of blockade at certain nerve roots, leaves those above/below unblocked
Epidural Needles
16-18g blunt bevel 15-30 degree gentle curve at tip - helps to push away dura after passing through the ligament flavum Tuohy Needle avg adult passes 4-6 cm obese pt may pass 8 cm
checking epidural placement
aspirate - confirm absence of blood
test dose - 3ml of LA with 5mcg/ml epidural
a 30 BPM increase in HR in 30s lasting 30s may mean you are intravascular
immediate profound motor block may mean you are intrathecal
Factors Affecting Level of Epidural Block
Drug Vol: 1-2ml per segment to be blocked
- to get T4 from L4-L5 would need 12-24ml
Pt height: shorter may require less per segment, closer to.1
interspace decreases with age - require less
additives usually help quality more than prolonging duration
Spinal Analgesia patho
diffuses into substantia gelatinous (Rexed lamina II)
unites with opioid receptors of primary pain afferent
Mu-1, Mu-2, kappa, delta
spinal opioid analgesia primarily Mu-2
increased frequency of pruritus & urinary retention
Intrathecal Opioid
diffuses out of intrathecal space slowly
onset of analgesia is slow, duration is prolonged
early ventilatory depression does not occur - systemic uptake is minimal
rostral speed = late resp depression (6-12hrs after)
Epidural Opioid
onset slow, duration prolonged
systemic uptake is greater here = early ventilatory depression
late depression also - rostral spread
epidural placement
sudden loss of resistance as you pass the ligamentum flavum
Hitting bone
superficially - needle hitting a lower spinous process
deeper - midline - may be hitting an upper spinous proccess
deeper - medial - may be hitting a lamina
pain or persistent paresthesias on injection of drugs
withdraw needle and re-direct
Caudal epidural anesthesia
usually for pends in combo with GA for procedures beneath umbillicus
penetration of sacrococcygeal ligament covering the sacral hiatus (the unfused S4/S5) - groove above coccyx bw sacral Cornu (bony prominences)
three areas of neuraxial complications
adverse/exaggerated physiological responses
catheter placement/injury
drug toxicity - SLAT or caudal equina syndrome
Adverse/Exaggerated physiologic responses
urinary retention high block total spinal cardiac arrest horner syndrome
catheter placement complications/injury
PDPH backache neural injury bleeding- epidural hematoma incorrect placement - inadequate anesthesia, infection, inadvertent intravascular or intrathecal block
PDPH
occurs w/I 72 hrs
intracranial hypotension from leak of CSF = sagging of brain & supporting structures
S/S: HA, N/V, stiff neck, visual changes
Risk Factors: pregnancy, female, young age, low BMI, cutting needle, large gauge
Tx: rest, blood patch, caffeine, lie flat
High/Total Spinal
rapid ascending sympathetic/sensory/motor block bradycardia hypotension dyspnea difficulty swallowing
can progress to unconsciousness & respiratory depression
Spinal-Epidural Hematoma
progressive motor & sensory block with bladder/bowel dysfunction
Lidocaine
amide - 4.5mg/kg -- with epi: max 7mg/kg OR 500mg fast onset DOA: 1-2 hr intermediate potency
Bupivicaine
amide
2mg/kg – with epi: max 3 mg/kg OR 200mg
SLOW onset
DOW: 3-10 hr - LONG acting
has a high affinity for cardiac proteins - can produce cardiac toxicity
Mepivacaine
amide
4.5mg/kg – with epi: 7mg/kg OR 500 mg
intermediate onset
DOA: 2-4hrs
Procaine
Ester
7mg/kg
spinal primarily
SLOW onset
DOA: 1-2hrs
Chloroprocaine
ester
11mg/kg – with epi: 14mg/kg OR 1g
FAST onset
DOA: 30min-1hr
MOA of LAs
produce a reversible blockade of conduction of electrical impulses at the alpha subunit of sodium channels
ONSET: pain - temp - touch - pressure - motor
Metabolism of LAs
esters - hydrolysis via tissue & plasma cholinesterase
amide - liver via CYP system
LA Properties
all are weak bases
pKa: the pH at which there is an equal fraction of ionized & non-ionized
lower pKa = greater non-ionized fraction = faster onset
once in the nerve the charged IONIZED fraction binds the Na++ channels
DOA of LA
more lipid soluble = longer
higher potency = longer
more protein bound = longer
