Local Anesthetics

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Anonymous
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305971
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Local Anesthetics
Updated:
2015-08-02 11:16:29
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BC CRNA
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Pharm
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  1. Locals anesthetics produce...
    reversible conduction block of both central and peripheral nerve impulses by blocking the action potential at electrically excitable tissue
  2. Differential blockade is....
    different fibers are affected differently (autonomic, sensory and motor) so there are various amounts of block in various areas
  3. Uses for local anesthetics in our practice
    • Neuraxial block (spinal, epidural)
    • Regional block (upper or lower extremities)
    • Local infiltration (before IV or for post-op pain)
    • IV regional (bier block)
    • Cardiac arrhythmias
  4. 3 layers of nerves
    • Endoneurium: innermost: composed of glial cells, loose connective tissue, capillaries and fibroblasts
    • Perineurium: middle: the individual fibers are gathered into fascicles and surrounded by the perineurium which is made of connective tissue
    • Epineurium: outermost: encases the entire peripheral nerve and is made of dense connective tissue
  5. Nerve fibers are classified according to....
    • Fiber diameter
    • Presence or absence of myelin
  6. The fiber diameter and presence of absence of myelin determine what?
    sensitivity to blockade!
  7. 3 main types of fibers
    • Type A: heavily myelinated (alpha, beta and gamma)
    • Type B: lightly myelinated
    • Type C: non-myelinated
  8. What produces myelin and what does myelin do?
    • Schwann cells produce myelin
    • - Myelin increases conduction velocity
    • - Makes the nerve more susceptible to local anesthetic-induced condition blockade 
    • - Block is only needed at the nodes of Ranvier to work
  9. List the fibers in order of diameter
    • Type A - Alpha
    • Type A - Beta
    • Type A - Gamma
    • Type A - Delta
    • Type B
    • Type C - dorsal root
    • Type C - sympathetic
  10. List the order in which blockade of peripheral nerves usually progresses (differential blockade)
    • - Sympathetic block with peripheral vasodilation and increased skin temperature (autonomic)
    • - Loss of pain and temperature sensation
    • - Loss of proprioception
    • - Loss of touch and pressure sensation**
    • - Motor paralysis

    **warn patients they may still be able to feel someone touching them or pressure, but not pain, so they don't panic if they do feel pressure
  11. Local anesthetics bind to sodium channels in the _______state
    inactivated-closed
  12. Basic MOA for locals anesthetics
    • - Slow rate of depolarization and threshold isn't reached which leads to conduction blockade
    • - Bind to sites on the inner part of sodium channels (inner gate) as well as blocking the external opening
    • - Locals plug up the sodium channels preventing channel opening
    • - The nonionized form passes through the membrane and then becomes ionized, and the ionized form plugs up the sodium channel
  13. TRUE OR FALSE: The nonionized form of local anesthetics are what have an effect
    FALSE! 

    Both the ionized and non-ionized form can have an effect because of the blocking of sodium channels, so both contribute to blockade. However only the non-ionized form can pass through the membrane.
  14. Frequency-dependent blockade
    • - Useful technique to have the sodium channel in the state that they are most likely to be blocked
    • - During onset of blockade, if patient uses extremity, it depolarizes the channels, so the local anesthetic will be more likely to get to the sodium channel and block it
    • - Repetitive stimulation aka repetitive depolarization increase the likelihood of finding an open sodium channel
  15. Bupivacaine
    - Potency and lipid solubility
    - pKa
    - Duration and protein binding
    - Max Dose
    • - Potency: ++++
    • - pKa: 8.1
    • - Duration: ++++
    • - Max dose: 3mg/kg
  16. Lidocaine
    - Potency and lipid solubility
    - pKa
    - Duration and protein binding
    - Max dose
    • - Potency: ++
    • - pKa: 7.8
    • - Duration: ++
    • - Max dose: 4.5 mg/kg (without epi) 
    •                   7 mg/kg (with epi)
  17. Bupi and lido % ionized at pH 7.4
    • Bupi: 83%
    • Lido: 76%
  18. Bupi and lido onset and duration
    • Bupi:
    • - Onset: slow
    • - Duration: 240-480 mins

    • Lido:
    • - Onset: rapid
    • - Duration: 60-120 mins
  19. The lower the pKa the more nonionized fraction that will be present, so onset will be more rapid. What LA is the exception to this?
    Chloroprocaine! 97% ionized but the toxicity is so low, that we have to use high concentrations which causes a more rapid onset d/t mass diffusion or concentration effect
  20. How are locals prepared? How about with epi?
    • - as water-soluble hydrochloride salts with pH 6-7
    • - Epi is unstable in alkaline environments so solutions with epic have pH 4-5
  21. What does adding sodium bicarbonate do to locals?
    • - Speeds onset
    • - Improves quality of block and prolongs duration because increased amount of free base available
    • - Decreases pain on injection!!
  22. Which locals are short, intermediate and long acting and what are their  % protein binding
    • Short:
    • - Procaine - 6%
    • - Chloroprocaine - basically 0
    • Intermediate: (LMP)
    • - Lidocaine - 64%
    • - Mepivacaine - 77%
    • - Prilocaine - 57%
    • Long: (BET)
    • Bupivacaine and ropivacaine - 95%
    • - Etidocaine - 95%
    • - Tetracaine - 75%
  23. Order of tissue absorption for local anesthetics:
    • - IV
    • - Tracheal
    • - Intercostal
    • - Caudal
    • - Paracervical
    • - Epidural
    • - Brachial plexus 
    • - Sciatic
    • - Subcutaneous
  24. Vasoconstrictors effect on local anesthetics
    • Decrease absorption
    • Increase neuronal uptake
    • Enhance quality of block, prolong duration of action and limit toxic effects 
    • *effects are most pronounced with shorter acting LA
  25. Contraindications to epi
    • Unstable angina pectoris
    • Cardiac dysrhythmias
    • History of HTN
    • Uteroplacental insufficiency
    • Peripheral nerve block in areas that may lack collateral flow 
    • IV regional anesthesia
  26. Two locals that can lead to methemoglobin
    Prilocaine (amide) and benzocaine (ester) - produce metabolites that can accumulate in large doses and convert Hgb to methHgb which doesn't bind oxygen
  27. Treatment for methemoglobinemia
    • Methylene blue
    • 1-2 mg/kg of 1% solution IV over 5 minutes
  28. CV side effects of locals
    • - Depressed automaticity
    • - Decreased refractory period
    • - Vasodilation
    • - Brady, heart block
    • - Hypotension
  29. CV toxicity with locals
    • - Decreased contractility
    • - Arrhythmias (decreased conduction velocity and blockade of cardiac sodium channels and ANS)
  30. CV effects specifically related to cocaine
    • - Adrenergic nerve endings usually reabsorb norepi, but cocaine inhibits that repute so it potentiates adrenergic stimulation 
    • - HTN, vasoconstriction
    • - Arrhythmias
  31. Early neurologic signs of local toxicity
    • - Circumoral numbness
    • - Dizziness
    • - Excitation: restlessness, seizure activity --> CNS depression: drowsiness and eventually unconsciousness
  32. Cauda equina syndrome
    • - Repeated doses of 5% lidocaine (also 0.5% tetracaine) through a small bore IV (28g) 
    • - Lithotomy position may also cause it
    • - Small bore catheters off the market
    • - Still see it, but RARELY
    • - Causes high concentrations of anesthetic and permanent damage: diffuse injury, bowel and bladder sphincter dysfunction and even paraplegia
  33. Transient radicular irritation
    • Symptoms: burning pain and aching in lower extremities and buttocks
    • Usually happens 24hrs after recovery from spinal because it takes a while before inflammation starts 
    • Usually have full recovery within 7 days
  34. Treatment for neuro toxicity from locals
    hyperventilation to decrease cerebral blood flow and decrease the amount of drug that reaches the CNS
  35. Respiratory effects of locals
    • - decreased hypoxic drive and response to CO2
    • - post retrobulbar apnea syndrome: LA into optic nerve sheath causes depression of medullary center 
    • - blocks reflex bronchoconstriction associated with laryngoscopy and intubation (Lido 1.5mg/kg usually used)

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