Regional and Neuraxial Anesthesia Flashcards
Anterior or posterior nerve roots carry sympathetic outflow? Motor outflow?
SNS: Anterior
Motor: Anterior
Where do you expect the C8 dermatome to include?
The 5th finger and medial aspect of the arm
A-alpha nerve fibers are responsible for what? Are they large or small and myelinated or unmyelinated?
Motor (thick myelinated)
A-delta nerve fibers are responsible for what? Are they large or small and myelinated or unmyelinated?
Pain & temperature (thin myelinated)
C nerve fibers are responsible for what? Are they large or small and myelinated or unmyelinated?
Pain & temp (small, unmyelinated)
Why do you have differential blockade with spinal anesthesia when using a hyperbaric spinal anesthetic?
Local anesthetic concentration in the intrathecal space at more cephalad levels are lower than in the caudal levels
Where does the sympathetic chain run?
T1-L2
Where does the parasympathetic chain run?
CN III, VII, IX, X, S2-S4
Why would a patient develop cardiac arrest after neuraxial block?
Unopposed vagal influence on the heart
Why would a patient with severe aortic stenosis die after spinal anesthesia?
In patients with AS, LVEDP is high -> spinal anesthesia decreases preload and afterload -> decreased aortic diastolic pressure results in decreased LV perfusion -> MI
How does spinal anesthesia affect inspiratory and expiratory mechanics?
Decreased both (intercostal and abdominal muscles) -> decreases pulmonary reserve
What do local anesthetics bind to?
Intracellular side of the alpha subunit of voltage gated sodium channels during the inactivated or activated states (not resting state)
What does pKa determine for local anesthetics?
Onset of action (although concentration and environmental pH also plays a role)
What determines potency of a local anesthetic?
Lipid solubility
What determines duration of action of anesthetics?
Protein binding, larger doses (longer duration), vasodilatory activity (more vasodilatory = shorter duration i.e. lidocaine)
Rate the following in highest rate of absorption to lowest: IV, caudal, brachial plexus, sciatic, subQ, tracheal, intercostal, paracervical, epidural
IV > tracheal > intercostal > caudal > paracervical > epidural > brachial plexus > sciatic (peripheral nerve) > subQ
What kind of local anesthetic was probably used if a patient has an allergic reaction and what compound is it mostly due to?
Esters because of PABA formation after metabolism; although some amides contain preservatives (methylparaben) which can also cause an allergic reaction
How are ester local anesthetics metabolized?
Metabolized by pseudocholinesterases
What local anesthetic can cause methemoglobinemia?
Benzocaine (independent of metabolism?) and prilocaine (metabolized to O-toluidine derivatives which oxidizes iron to the ferric state)
How are amide local anesthetics metabolized?
Liver
What is transient neurologic symptoms and what local anesthetic is classic for it?
Radicular irritation after spinal anesthesia (i.e. burning and aching in buttocks and lower extremities with NO loss of sensation or motor function); Lidocaine
Intravascular bupivacaine or ropivacaine injections require what treatment?
Intralipids; other local anesthetics are less protein bound and are less responsive to intralipids
How long do you need to wait to give a single shot spinal/epidural in patients with once daily lovenox? Twice daily lovenox? When can you restart the lovenox?
Once: 12 hours after last dose and restart 6 hours after block
Twice: 24 hours after last dose, and restart 24 hours after block
When can you place an epidural catheter with a patient on lovenox for DVT ppx? When can you remove an epidural catheter? When can you restart lovenox after catheter is in? After catheter is removed?
Placement: 12 hours after last dose
Restart: 6-8 hours after catheter placed
Remove: 12 hours after last dose
Restart: 2 hours after catheter removed
How long should clopidogrel be stopped before an epidural is placed?
7 days
What INR and aPTT is the cutoff for epidural placement?
INR should be less than 1.5
aPTT less than 40
How long do you need to wait for epidural placement in a patient with heparin ppx? When can you restart the heparin?
Placement: Anytime; if more than ppx dosing, check aPTT and it should be less than 40
Can restart after 1 hour after catheter placement
The inferior tip of the scapula correlates to what vertebral level? Prominent cervical thoracic process? Superior iliac crest? Posterior superior iliac spine?
- T7
- C7
- L4
- S2
If a patient is given a hyperbaric lidocaine spinal injection and laid supine, what spinal level would the block travel caudally?
T6 (normal kyphosis of the back in the supine patients limit the movement to this level)
Is obesity associated with higher spinal blocks?
No
What does adding fentanyl to spinal anesthesia with bupivacaine do for you?
Intensifies the sensory blockade and allows for faster onset of anesthesia
How do intrathecal fentanyl and morphine differ?
Fentanyl: lipophilic + limited dural crossing and shorter lifespan in CSF + little cephalad spread + early respiratory depression only (within 30 min)
Morphine: hydrophilic + crosses dura slower and longer lifespan in CSF + lots of cephalad spread + 2 respiratory depressions (early and late)
Is pruritis worse with IV or intrathecal administration of opioids?
Intrathecal
Which opioids are more prone to cause urinary retention?
The more lipophilic the drug, the less likely it will cause urinary retention (morphine > fentanyl)
What cardiovascular effects do you see with meperidine?
- Atropine-like structure -> increased HR
- Cardiac depressant effects
- Local anesthetic properties
Intrathecal opioid leads to itching… which would be least effective in treating: ondansetron, propofol, diphenhydramine, naloxone, nalbuphine
Diphenhydramine (intrathecal opioid mediated itching is a central mechanism and does not involve histamine release)
Does the addition of epinephrine increase the density of the block?
No, only prolongs the duration of action
Which local anesthetic can interfere with epidural opioid mediated analgesia?
Chloroprocaine
Large nerve roots of L5-S2 are responsible for what phenomenon?
Sacral sparing in epidural anesthesia
What is the greatest risk factor for development of an epidural hematoma: experience, diabetes, not using cap and mask, betadine instead of chlorhexidine, placement of a catheter vs. single shot
Catheter placements
After a 2cc bupivacaine injection for interscalene block, patient develops a tonic-clonic seizure. Why?
Vertebral artery injection (100% of LA goes to the brain)
After a 2cc bupivacaine injection for interscalene block, the patient develops Horner’s syndrome. Why?
Stellate ganglion block, injecting just off of Chassaignac’s tubercle (C6)
What nerve does an interscalene block sometimes miss?
Ulnar (C8 & T1)
Supraclavicular vs. interscalane, which is better at blocking the ulnar nerve?
Supraclavicular
Which block has the highest incidence of chylothorax?
Infraclavicular
Axillary approach for brachial plexus block misses what nerve?
Musculocutaneous: lateral forearm coverage
How are the nerves oriented around the axillary artery for an axillary nerve block?
Medial: superior lateral
Ulnar: superior medial
Radial: deep to the artery
What nerve is injured in a wrist drop?
Radial nerve (wrist extension)
What nerve is responsible for wrist flexion?
Median nerve
What nerve is responsible for pinching?
Ulnar nerve
For a 3-in-1 nerve block, what nerves are usually blocked and what do they innervate?
Femoral nerve (medial thigh), obturator (medial knee), lateral femoral cutaneous (lateral aspect of thigh), genitofemoral
What nerve is missed with a popliteal block for ankle surgery?
Saphenous nerve (medial malleolus); need to perform a femoral nerve block
What nerve is missed following ankle block if patient has sensation over lateral heel and malleolus?
Sural nerve
What is the order in terms of loss of conductivity after spinal anesthesia?
- SNS and PSNS
- Sensation (C fibers first)
- Motor (A-alpha)
What levels do you have blockade of dermatomes and blockade of motors?
Pain and temperature is lost 2 dermatomes above the level; motor is lost 2 dermatomes below the level
Benefits of spinal/epidural anesthesia over GETA?
- Decreased hypercoagulable state
- Increased tissue blood flow (vasodilation)
- Increased oxygenation (normal ventilation)
- Increased peristalsis (lower dose of opioids)
- Decreased stress response (sympathectomy)
Average duration of action of each local anesthetic intrathecally: Bupivacaine, Ropicavaine,Tetracaine, Procaine, Lidocaine
Bupi: 90-120 min (100-150 with epi) Ropi: 90-120 min (100-150 with epi) Tetracaine: 90-120 min (120-240 with epi) Procaine: 45 min (60 min with epi) Lidocaine: 60 min (90 min with epi)
Most important factors for spinal anesthesia?
- Baricity
- Patient position during and after injection
- Drug dose
Why do we not use lidocaine anymore with spinal anesthesia?
Risk of transient neurologic symptoms and cauda equina syndrome
What sensory level loss has a higher risk of bradycardia?
Sensory level above T6
How big is the epidural space in the lumbar, thoracic and cervical spine?
Lumbar: 5-6mm
Thoracic: 3-5mm
Cervical: 2mm
What is a rough estimate for how much local anesthetic needs to be administered for an epidural?
1 cc of local anesthetic per segment blocked
What segment level is associated with the following cutaneous landmarks: perineum, lateral aspect of foot, umbilicus, tip of xiphoid, nipple line, 5th finger
- S2-S4
- S1
- T10
- T6
- T4
- C8
What is the fastest acting but shortest duration local anesthetic for an epidural?
Chloroprocaine (2-3%)
How early do you need to stop ASA or NSAIDs before an epidural?
No contraindication
What changes in a local anesthetic solution when you add epi?
The pH decreases (more acidic)
What condition would greatly increase the half life of lidocaine?
Liver cirrhosis
Which amide local anesthetic is the least toxic and why?
Prilocaine: extrahepatic metabolism and mostly topical use
What is the max dose of the following ester local anesthetics: chloroprocaine, procaine, cocaine, tetracaine
Chloroprocaine: 12 mg/kg
Procaine: 12 mg/kg
Cocaine: 3 mg/kg
Tetracaine: 3 mg/kg
What is the max dose of the following amide local anesthetics: lidocaine, mepivicaine, prilocaine, bupivacaine, ropivacaine
Lidocaine: 5 mg/kg (7 with epi) Mepivicaine: 5 mg/kg (7 with epi) Prilocaine: 8 mg/kg Bupivicaine: 3 mg/kg Ropivicaine: 3 mg/kg
What is a possible complication from liposuction procedures?
Since they use a max of 55 mg/kg of local anesthetic (lidocaine), sometimes you can have a large accumulation of LA in the adipose tissues that causes toxicity to occur 6-12 hours after the procedure
What toxicity can happen with the preservative sulfite in local anesthetic preparations? EDTA? Methylparaben?
Sulfites: anaphylactoid reactions, arachnoiditis
EDTA: low back pain at epidural injection site
Methylparaben: anaphylactoid reactions
What is the preservative in 2-chloroprocaine and what can it cause?
Sodium bisulfite; can lead to arachnoiditis
What is the maximum recommended dose of cocaine?
1-3 mg/kg
What does adding sodium bicarbonate to lidocaine do?
Decreases the pain on subcutaneous infiltration
It also causes more non-ionized forms which results in faster diffusion of LA through cell membrane (unfortunately has conflicting data)
How is remifentanil broken down?
Nonspecific blood and tissue esterases
Is the response to epinephrine as a test dose different if the patient is awake or under GA?
Yes, BP and HR responses are reduced under GA
What is the dose of intralipid for local anesthetic toxicity?
1 cc/kg bolus then infusion of 0.25 cc/kg/min for 10 min
Which local anesthetics are in EMLA (eutectic mixture of local anesthetics) cream?
Lidocaine and prilocaine
Is the S- or R- enantiomer of bupivacaine more cardiotoxic?
R-bupivacaine is more cardiotoxic (ropivacaine is actually the S-enantiomer)
The pKa of mepivacaine is 7.6. At physiologic pH (7.4), what % of mepivacaine is in the uncharged (unionized) form? 39% or 61%?
pH = pKa + log (ionized/unionized)
- 6 = 7.4 + log (ionized/unionized)
- 2 = log (ionized/unionized) -> thus more ionized than unionized (39% to be exact)
What is the advantage of mixing epinephrine to LA just before injection vs premixed epi-LA vials?
The premixed vials are acidic to ensure that epinephrine does not get degraded -> LA are mostly ionized -> slower onset of action; thus you get a faster block onset by mixing your own epi before use
What proteins is bupivacaine primarily bound to in vivo?
Alpha-1-acid glycoproteins (AAG)
Intrathecal microcatheters with continuous infusions of 5% lidocaine are associated with what complication?
Cauda equina syndrome
What patient position puts them more at risk for transient neurologic symptoms after spinal injection of lidocaine?
Lithotomy
Why would you administer IV lidocaine prior to succinylcholine?
Relaxes bronchial smooth muscles whereas succinylcholine only blocks skeletal muscles at the nicotinic receptor
What is the mg/mL dose of 1% lidocaine? 0.5% bupivacaine?
1% lidocaine = 10 mg/mL
0.5% bupivacaine = 5 mg/mL
What type of nerve fibers (large, small, myelinated, unmyelinated) are more resistant to local anesthetics?
Large myelinated neurons (A-alpha - motor) are most resistant, followed by smaller myelinated (A-delta - pain/temp), then small unmyelinated (C - pain/temp)
After an accidental IV injection of 300 mg mepivacaine, the patient has a tonic-clonic seizure, what is the next best step: chest compressions, intralipids, ketamine, propofol, methohexital
Propofol to terminate seizures
Intralipids are good for bupivacaine and ropivacaine toxicity (highly protein bound)
Does adding epinephrine affect the duration of action for ropivacaine? Bupivacaine? Tetracaine?
Does NOT affect ropivacaine or bupivacaine. It does increase the duration of action of tetracaine
What patient characteristics affect local anesthetic spread in the subarachnoid space?
- Position during and after injection
- Height (taller = more)
- Spinal column anatomy
- Decreased CSF volume (pregnancy, ascites, increased age, etc)
After an intercostal block, what would you worry about if the patient suddenly develops respiratory distress?
- Poor respiratory reserve (secondary to phrenic nerve block - 100% of the time)
- Pneumothorax
