Spinal Blocks Flashcards
What is the term used to describe any type of anesthesia in the spinal canal or area?
Neuraxial
How can you differentiate amide from ester LAs?
- amides have an âiâ in their name
- Esters do not
All LAs are weak ________.
Bases
What does pKa tell you?
PKa= the pH where 50% of drug is ionized and 50% is non-ionized
- Lower pKaâ> faster onset, greater fraction of molecules will exist in unionized form and will cross cell membrane easily (becomes ionized once inside the cell)
What is true if a LA is more ionized?
Will stay where you inject it- not enter cell- takes longer to work
A drug with a pKa of 9.0 will have a slower onset than one with a pKa of 8. Why is this?
The closer the pKa is to physiologic pH (7.4), the faster the onset
- pKa tells us how much is available and how fast it will cross the membrane
Describe the MOA for spinal anesthesia.
- blocks nerve conduction
- impaired propagation of action potential neurons
- decreases rate of rise of action potential threshold so that threshold potential is not reached
- interact directly with Na channel receptors - inhibits Na+ influx on channel (blocks Na channel open from the inside)
- action potentials may start, but never reach threshold to continue sending msg down the axon
The intracellular environment is more acidic than the extracellular. Why is this important?
- the acidic environment want to give up H+
- it gives the H+ to the LA (weak base), making it ionized
- LA is now active inside the cell and can go block the Na+ channelâ> waits until channel opens and then block it from inside the cell
What are physiochemical factors of LAs that affect neural blockade?
LIPID SOLUBILITY:
- increases potency
- LAs more readily cross nerve membranes
PROTEIN BINDING:
- high protein binding = prolonged duration of effect
PKA:
- determines speed of onset of block
Wha this important regarding the size and function of nerve fibers?
- thin fibers are more easily blocked than thick
- myelinated fibers are more readily block than unmyelinated â> LAs produce block at nodes of Ranvier
In which order are nerve fibers blocked?
Bâ> 3”m, light myelination
Câ> 0.3-1.3 ”m, no myelination
A gamma and deltaâ> 2-6”m, moderate myelination
A alpha and Ăâ> 5-20 ”m, heavy myelination
What is the sequence of anesthesia?
ATPTP MVP
Autonomic (sympathectomy, peripheral vasodilation) Temperature (loss of ...) Pain * Touch Pressure * Motor Vibration Proprioception
How are esters metabolized?
Ester linkage readily cleaved by plasma cholinesterase
â> t 1/2â 1 min (in circulation)
- a product of its metabolism is p-aminobenzoic acid, which is what people may have an allergic reaction to
How are amides metabolized?
Via liver mechanisms
- in liver disease may be prone to adverse reactions
- elimination t 1/2= 2-3 hours (does not mean effect lasts this long)
What is baricity?
Density of medication relative to density/specific gravity of CSF
What does adding epi to a LA do?
- prolongs duration
- varies by type of LAâ> if short acting, LA will benefit from adding epi. If LA is longer acting than epi then no use adding epi
- decreases systemic toxicity
- decreases rate of absorption
- increases intensity of block
- decreases surgical bleeding, if injected near incision
- assists in evaluation of test dose
- lets you know if you are in a blood vessel (donât want LA to go systemic)
- inject epi 1st (instead of LA)- watch VS to see if it goes systemicâ> increase in HR
When would you not add epi to the LA?
- block is in area of poor circulation
- fingers, toes, penises
- IV regional- Bier block
- IV, stop blood flow (tourniquet), inject LA- LA seeps out into nerve
- if epi present, will go systemic with tourniquet let down
- IV, stop blood flow (tourniquet), inject LA- LA seeps out into nerve
- hx of severe uncontrolled HTN, CAD, arrhythmia, hyperthyroidism, uterine-placental insufficiency
- phenylephrine
What benefits does adding sodium bicarbonate to LA have?
- increases pHâ> increases concentration of non-ionized base
- increases rate of diffusion across membrane- speeds onset of block
- buffers pain- acids sting
What is the dose of sodium bicarbonate to add to LA?
- 1 mEq/10mL lido or mepivicaine
- 0.1mEq/10mL bupivicaine (avoids ppt of drug)
What effect does adding an opioid to LA have?
- adding 50-100”g fentanyl:
- shortens onset
- increases the level of block
- prolongs duration
- modulates pain transmission
- action is synergistic with action of LA
Opioid use with LA has what side effect on respiration?
- fentanyl: risk of early respiratory depression
- morphine: risk of early (local absorption) and late respiratory depression (systemic)
T/F True allergic reactions to LAs are common.
FALSE
- ) syncope, vaso-vagal, and tachycardia are NOT allergic reactions
- ) no reaction with amidesâ> pt may react to preservative if sensitive to PABA
- ) esters metabolite is similar to PABA â> may have allergic reaction to esters, but rare
How can systemic toxicity/OD of LA be minimized?
- aspirate before injecting
- test dose with epi containing solution
- use small increment volumes (5mL at a time)
- *** ALWAYS aspirate between injections **
- proper technique during bier block â> wait at least 20-30 min before tourniquet release
What are symptoms of LA CNS toxicity?
- light headed ness
- tinnitus
- metallic taste
- visual disturbance
- numbness of tongue and lip
May progress to: higher doses ℠14 ”g (on 0-28”g scale)
- muscle twitching
- loss of consciousness
- grand mal seizures
- coma
All toxicity tests were performed using which LA?
Lidocaine
Which respiratory gas has a major effect on toxicity?
CO2
- if you start to see signs of toxicityâ> HYPERVENTILATE pt, donât let CO2 climb!
What is the treatment of CNS toxicity?
- administer O2
- for seizure activity:
- midazolam 1-2 mg, propofol, thiopentanol
What are signs of CV toxicity?
- decreased contractility
- decreased conduction
- loss of peripheral vasomotor tone
- CV collapse
- IV injection of BUPIVICAINE or ETIDOCAINE may result in CV collapse, refractory to treatment because of high degree of tissue binding
How do you treat CV toxicity?
- administer O2 (CO2 exacerbates it)
- support circulatory volume, vasopressors and inotropes
- ACLS if indicated (DONT GIVE ANY MORE LIDOCAINE- use amio instead)
- TxV-tach with cardiversion
- prolonged CPR needed until cardio toxic effects subside once drug is redistributed â> 40 min or longer
- lipid emulsion
What is the dose for lipid emulsion?
- bolus: 1.5mg/kg over 1 min
- 0.25 mL/kg infusion
How is post op morbidity and mortality effected when neuraxial block is used ?
Both may be decreased when neuraxial block is used, either alone or in combination with GA
When spinal analgesia is used, the risk if which complications is reduced?
- venous thrombosis
- PE
- cardiac complications
- vascular graft occlusion
- respiratory depression and PNA
- blood loss/transfusion (donât really know why)
- allows earlier return of GI function
What is a spinal?
Subarachnoid block
- intrathecal- same thing, used when using narcotics
What are typical doses for spinal, epidural an peripheral nerve block?
- spinal: 1-2mL of LA
- epidural: 10-30mL
- peripheral nerve block: 20-30mL
Whatâs beneficial about using a spinal?
- easy to perform
- uses less LA
- less discomfort (smaller volume)
- more intense sensory and motor block
Indications for a spinal:
Surgery of lower abdomen, lower extremities or perineum
- typically place in lumbar area and will spread up to ~ T6,7â> xyphoid
What are key points for a pre-op exam?
- Always document specific baseline neuro deficits
- if s/s infection avoid spinal
- current medsâ> anticoags cause hematoma/paralysis
- cardiac disease
- AORTIC STENOSIS: fixed CO, dilation decreases Coronary Artery perfusion
What are ABSOLUTE contraindications to a spinal?
1 is patient refusal!
- doing so is battery, threatening to do it is assault - pt lack of cooperation - high ICP - coagulopathy - spina bifida - severe Aortic Stenosis
What are relative contraindications for a spinal?
- Coag that can be corrected
- hypovolemia (correctable)
- spinal cord disease
- surgical time- spinal will only last so long
- difficult airway-may be back up plan
- musculoskeletal deformities-kyphosis, scoliosis, previous spine fusion
- document all pre-existing neuropathies
What are signs of a spinal/epidural hematoma?
- new onset weakness or sensory deficit to Lower extremities
- new onset back pain
- new onset bowel/bladder dysfunction
*** must diagnose (MRI and NS c/s) and surgically decompress within 8 hours for best outcome
What is the difference clinically, between a spinal hematoma and infection of the spine?
- Infection will have similar s/s, but occurs 3-4 days later.
- Hematoma s/s will show up within 1st 24 hours
What are some landmarks you should know really well?
- T4: Nipple line
- T6, 7: Xyphoid Process
- T 10: Umbilicus
- C8: Pinky (watch for bradycardia- close to cardiac accelerators)
- C6: Thumb
- T1-T4: Cardiac Accelerators
What are suggested minimum levels for spinal anesthesia?
- lower extremity â-> T 12
- hip, vagina, uterus, bladder, prostateâ> T10
- lowers ext. TQ, testes, ovariesâ> T8
- lower intra-abdominalâ> T6
- other intraabdominalâ> T4
How many vertebrae are in each section of the back?
- 7 cervicalâ> 8 nerve roots (all other areas have 1 root/vertebrae)
- 12 thoracic
- 5 lumbar
- 5 sacral
- 4 coccygeal
What is the part of the vertebra called where you feel for needle placement?
Spinous process- needle goes in indention between
What are the three intralaminal ligaments?
- supraspinous ligament: connects apices of spinous process.
- most posterior
- interspinous ligament: connects spinous processes, 1 for each level
- ligamental flavum: connect caudal edge of vertebra above to cephalad edge of lamina below
- most anterior
- thickest/densest later- may feel a âpopâ as needle passes through
Where does the spinal cord end?
Adults: L1
Peds/infants: L3
Dural sac ends ~S2
What makes up the caudal equina?
- conus medullaris, lumbar sacral, and coccygeal nerve roots
What 3 meninges cover the spinal cord?
- dura mater:
- tough fibrous sheath
- runs the length of the rod
- arachnoid: between dura and pia mater
- pia mater: inner layer around cord
What is the epidural space?
Potential space between ligament up flavum and dura mater
- once the needle is here it creates a space to inject fluid- when fluid is gone, space goes away
- 10-30mL injected - more layers to absorb through
What is a spinal?
LA is injected into the subarachnoid space
- 1-2 mL injectedâ>only has to go through pia mater to touch spinal cord
- fast and all or nothing- blocks everything from that point down
What are some facts about CSF?
- clear, colorless fluid filling subarachnoid space
- total CSF volume = 140mL
- 30-80 in spinal canal, the rest in the brain - CSF SG= 1.004-1.009 @ 37ËC
- CSF reabsorbed to maintain pressure 10-20 mH2o
What are factors that affect the level of spinal blockade?
- drug dose: directly proportional to level
- drug volume: > volume = further spread
- turbulence of CSF: turbulence increases spread
- rapid injection, barbatoge (aspirate, inject, aspirate, inject), coughing, excessive pt movement
How many mcg are the following concentrations:
0.5%
7.5%
2%
0.5%â> 5 mcg
7.5%â> 75mcg
2%â> 20mcg
What else affects the level of spinal blockade?
- increased intra-abdominal reassure
- pregnancy- pressure dilates epidural vessels and vena cava- more pressure on spinal canal makes it smallerâ> more spread
- obesity, ascites, abdominal tumors
- spinal curvature- lumbar lordosis, thoracic kyphosis
- baricity of LA
How would you make an LA hyper-hypo- or isobaric?
- hyperbaric â> add dextrose to drug ( 5-8% dextrose)
- SG > 1.0015
- sinks in CSF - hypobaricâ> mix with sterile water
- SG <0.999
- floats in CSF - isobaricâ> mix with preservative free saline
- theoretical advantage is predictable spread, independent of position
* increasing dose will affect duration and more spread to higher dermatome - it is not possible to prepare a solution that is precisely isobaric
Using which type of needle tip decreases incidence of post-dural puncture headache?
Non- cutting tip needle
What is the land mark for L4, and where is the conus medullaris?
L4= iliac crest
Conus medullaris= T12 - L2 in 90% of people
Describe the midline approach?
- flat to 10Ë
- needle advanced through skin in same plane as spinal process with slight cephalad angle toward interlaminal space
- if you hit bone, likely need to angle needle up a little
Describe the paramedian approach?
- helpful when pt cannot maximally flex back or when spinous ligament ossified
- needle placed 1-1.5 cm lateral to midline of selected interspace
- needle aimed medically and slightly cephalad- passes lateral to supraspinous ligament
- if lamina contacted- redirect needle and walk of in medial and cephalad direction
How do you find the destination for a spinal?
- use the line across the iliac crestâ> intersects spinous process of L4 or L3-L4 interspace
- spinal anesthesia is usually done at L2-L3, L3-L4, and L4-L5 spaces
What are key points for prep for a spinal?
- prepare a larger area than you think you need
- if kit contaminatedâ> get a new oneâ> contamination is NEUROTOXIC
- check stylette and needle integrity
- inject skin with 1% lidocaine
- betadine only works if dry
- chloroprep must dry for 3 minutes
What are key points for needle placement in a spinal?
- insert with bevel parallel to longitudinal fibers (decreases incidence of PDPH)
- advance until resistance is felt going through ligamentum flavum
- pop felt as it passes by ligamentum flavum - want slow drip of CSF free flowing - may rotate in 90Ë increments until good flow established
- ** if parathesias with needle placement â> immediate withdrawal of needle and reposition
How do you administer anesthetic during a spinal?
- aspirateâ> slowly inject â> re-aspirate to confirm needle tip still in subarachnoid space
- gently remove needle as pt placed in desired position
- in hyperbaric solution- when you aspirate you will se a little swirl of fluid (not seen in hypobaric solution)
What do you do during the onset of the block?
- ascending level assessed with pinprick or alcohol swab
- closely monitor VS at least once/min until deemed stable â> HR, BP, RR
- if hyovolemic BP will really drop
- fixation of LA takes ~ 20 minutes
Where are the autonomic, sensory and motor levels of the block?
- level of autonomic block should be highest level of spinal (smaller C fibers affected fasest)
- sensory level is 2 spinal levels below autonomic
- motor level is 2 spinal levels below sensory level
A > S > M
What is the CV response to a spinal?
- hypotension: proportionate to the degree of sympathetic block achievedâ> higher block = more hypotensive
- vascular bed dilation- venous and arterial
- decreased SVR with decreased venous return as a result
- works in hypovolemic pt â> give IVF 500-1000mL bolus before you start
- HR usually will not change-unless cardiac accelerators involved (T1-T4)
What is the treatment of CV response?
- if hx allows: pre-load with 15mL/kg IVF (500-100mL)
- O2 mask
- vasopressors
- atropine: 0.4-0.8
- epiâ> drug of choice
- CPR
Why is epi the drug of choice for CV response?
Mixed alpha and beta agonist- treats bradycardia and hypotension
And increased peripheral SVR
T/F You should just expect CV effects and be prepared for Bradycardia and hypotension.
True
Especially in elderly: HTN and dehydrated
Pregnant females: healthy vessels really dilate
What are pulmonary effects of an autonomic block?
- usually minimal- even with high level block
â> chronic lung disease rely on accessory muscles more- accessory muscles impairedâ> ineffective coughing/clearing secretions-may need to suction
- use caution in pts with limited reserve *
What are the 4 proven factors that affect level of anesthesia?
- ) Baricity of solution
- ) Position of ptâ> during injection and immediately after
- ) Dose of drug
- ) Injection site
What are complications of a spinal?
- failed block
- post-dural puncture headache (PDPH)
- high spinal â >gets too high, canât breath, heart doesnât work right
- nausea: OB pts almost always get nauseated d/t hypotension
- urinary retention
- hypoventilation
- backache: can last a couple years- usually goes away sooner (ligament trauma)
What is the depth to the dura and epidural space from the skin in an average person?
4-5cm
- needle has markings on it to measure depth
(Mid thoracic 3-5 mm wide, lumbar 5-mm wide)
What is different with an epidural block?
- onset of block is slower and less intense
- anesthesia develops in a segmental mannerâ> selective blockade can be achieved
- usually only trying to block sensory- not giving enough to block motor
- can titration- give larger dose for motor block
- LA spreads in both directions in epidural space (epidural space spreads all the way around spinal canal
- epidural anesthesia is diffusion dependent- takes longer, must diffuse to Sa space
What is the approach in an epidural?
- needle should always enter midline regardless of approach (midline or paramedian)- where space is widest
- decreases risk of puncturing epidural veins, spinal arteries, and nerve roots
- inject local into supraspinous and interspinal ligaments
- ligamentum flavum feels ârubberyâ
- use either loss of resistance or hanging drop techniques (drop of water at end of cath- when in epidural space low pressure- drop will suck in toward epidural space)
Why is a thoracic epidural done?
To anesthetize the upper abdomen and thoracic regions
What is different in a thoracic epidural?
- requires smaller dose
- thoracic vertebral spinous processes have more sharp downward angular ion
- insert needle more cephalad
If you know the concentration of LA is 1: 200,000, how do you find out how many mcg/mL that is?
1,000,000/200,000 = 5 mcg/mL
Divide 1 million by the ration to get the # of mcg per mL
What happens with an epidural test dose?
- 3mL of LA with 1:200,000 epi and 1.5% lidocaine (15mcg epi, 15 mg lido) injected prior to full dose
- will have little effect if in epidural space
- if in CSF will rapidly behave like a spinal
- if injected into epidural veinâ> pt will have a 20-30% increase in HR and metallic taste
What are factors that affect level of epidural blockade?
- volume of LA
- age
- pregnancy- vein engorgemnet, smaller space
- speed of injection: alway inject in increments of 3-5 mL Q 3 min- aspirate every time before and after injection
- position
- spread
After loading dose is given, what are ways we can control analgesia?
Maintain with intermittent dosing or continuous infusion.
- intermittent used when high concentrations administered (2% lido or 0.5 % ropivicaine)
- continuous infusionâ> often dilute concentration of LA with low dose lipophillic opioid (fentanyl 1-5mcg/mL)
Dense block = _______ motor involvement
More
What are epidural adjuncts and why are they added?
- epi: increases duration- stays in place
- opioid:
- fentanyl â>early respiratory depression
- duramorphâ> late respiratory depression
- pH adjustment:
- NaHCO3 1mL/10mL of LA
Crossing from epidural to subarachnoid space is influenced by:
- lipid solubility
- molecular weight
What is seen with complications of an epidural?
- back ache (30-40% in O.B)
- PDPH: (1-2%), in wet tap increases incidence to 75%
- trauma with catheter removal
- cath can loop up and knot itself- gets stuck
- make sure cath intact when you pullâ> curved spine helps
What needle tip is associated with a large dural perforation?
17g Touhy needle
