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Nervous System > Ph- Local Anesthesia > Flashcards

Ph- Local Anesthesia Flashcards

1
Q

What are the 3 chemical categories of local anesthetics?

What drugs belong to each category?

A
  1. Esters
    - cocaine
  2. Amides
    - lidocaine
    - mepivacaine
    - bupivacaine
    - prilocaine
    - ropivacaine
  3. Vasoconstrictors
    - epinephrine
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2
Q

What is the goal of local anesthetics?
Are they reversible or irreversible?
What is the mechanism by which they operate?

A

Local anesthetics are used to REVERSIBLY suppress pain sensation by inhibiting nerve conduction in the vicinity of their application.

They act by inhibiting the activity of voltage-gated Na+ channels

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3
Q

Local anesthesia is defined as the loss of ____________ without the loss of _______________.

A

Loss of sensation in a body part without loss of consciousness or CNS control over vital functions

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4
Q

What is the common overall chemical structure that local anesthetics share?

A
  1. hydrophobic group [aromatic]
  2. ester or amide linkage
  3. hydrophilic, ionizable group [ 2 or 3 amino group]
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5
Q

With local anesthetics, what is the hydrophobicity proportional to?

A

Hydrophobicity is proportional to the potency and duration of action of the drug.

The more hydrophobic a drugs:

  1. more delivery and association to target sites/channel proteins [Faster acting]
  2. slower rate of metabolic processing by plasma enzymes or liver enzymes [longer acting]
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6
Q

What does the chemical linkage between the hydrophobic aromatic ring and the hydrophilic amino group of local anesthetics determine?

A

Duration of action:

  1. Esters = hydrolyzed by plasma esterases so they have a short duration of action
  2. Amides = survive the plasma and are degraded in the liver, so they have a longer t1/2

*if concerned of side effects due to circulatory distribution of local anesthetic, use an ester

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7
Q

What is the only local anesthetic agent with nerve impulse blockage AND vasoconstriction?
What are the side effects?

A

Cocaine

  • catecholamine uptake block leads to:
    1. toxic side effects
    2. CNS mood altering properties
    3. cardiovascular side effects
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8
Q

What is the most widely used local anesthetic?

Describe the speed, potency, and length of effect.

A

Lidocaine

  1. fast speed of action
  2. more potent than procaine
  3. longer-lasting than procaine [moderate duration]
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9
Q

What 2 amide local anesthetics are to be avoided in obstetrics?

A
  1. Mepivacaine- toxic to neonates

2. Prilocaine- causes methemoglobinemia which is enhanced in neonates

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10
Q

What amide local anesthetics has greater cardiac toxicity than lidocaine? If this is a concern, which drugs should be used instead?

A

bupivacaine

*if cardiotoxicity is a concern, use ropivacaine instead

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11
Q

How are local anesthetics delivered?

What influence does absorption and distribution through the circulatory system have?

A

They are delivered locally by direct injection or application at the desired site.

Absorption/distribution through the circulatory system is:

  1. NOT important for determining onset of anesthetic activity
  2. CRUCIAL for determining rate of recovery and potential toxicity
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12
Q

What 4 factors determine the systemic distribution of local anesthetics?

A
  1. dosage
  2. vascularity at the site of administration
  3. strength of binding to tissue
  4. presence of vasoconstricting substances
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13
Q

Epinephrine is less effective at increasing the duration of action of _________________, because __________________________.

Which amide drugs are least likely to need epinephrine?

A

vasoconstrictors are less effective at increasing the duration of action of more lipophilic and inherently longer acting substances because they interact more strongly with the tissue and are less susceptible to spread through local blood flow anyway

The long acting amides are bupivacaine and ropivacaine

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14
Q

Describe the 2 main actions of cocaine.

A
  1. anesthetic activity by blocking Na channels

2. vasoconstrictive activity by inhibiting catecholamine uptake and consequently potentiating a-adrenergic signaling

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15
Q

Local anesthetics have an ionizable 2 and 3 amino group that allows the local anesthetic to exist in one of two forms. What are they?
What is the pka of most local anesthetics?
What form is likely to predominate at physiological pH?

A
  1. neutral, deprotonated amine [weak base]
  2. protonated, charged cation [weak acid]

pka is 8-9
Since physiologic pH is near 7, most of the time the drug exists as a protonated cation

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16
Q

What is the rationale for the general amphiphilic chemical structure of local anesthetics?

A
  1. cationic form of the anesthetic is primarily responsible for blockade of Na channel
  2. to cross the hydrophobic plasma membrane to reach the site of action, they must be in neutral form

Charged amine group provides the electrostatic block of ion conduction through the voltage-gated Na channel, and the hydrophobic aromatic group facilitates partitioning into the hydrophobic plasma membrane of target tissue

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17
Q

Why are anesthetics less effective in inflamed areas ?

A

inflammation creates a more acidic so more local anesthetics are in the cationic, protonated state. This means less is able to cross the membrane, lowering their cellular concentration

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18
Q

How are local anesthetics eliminated from the body?

A
  1. Esters are rapidly broken down in the plasma by esterases w/ a half life of less than a minute. They are excreted renally.
  2. Amide-linked drugs travel through the circulation to the liver where they are modified by N-dealkylation and then hydrolyzed by liver microsomal enzymes
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19
Q

When Na channels are blocked, what are the 4 effects on the action potential?

A
  1. greater threshold for generation
  2. reduced rate of depolarization
  3. prolonged AP
  4. reduction in max depolarization
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20
Q

Describe the structure of the voltage-gated Na channel.

What is the target of local anesthetics?

A

3 subunits- A, B1, B2
A = 4 subunits each consisting of 6 membrane-spanning helices
4 domains form a ring at the cell membrane with a single aqueous pore where Na flows through the middle

Between the 5th and 6th transmembrane helix, there is a pore formed. TTX and STX block the outer pore region, anesthetics block the inner pore region

Most local anesthetics block the pore near the intracellular mouth at the sixth transmembrane helix of the fourth domain

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21
Q

What are the 3 main reasons natural toxins like tetrodotoxin, saxitoxin and dinoflagellates are not used in anesthesia despite their ability to block the external Na channel pore?

A
  1. not use-dependent [would block all nerves, not just those that are overfiring]
  2. binding is relatively non-reversible
  3. large-scale production is not possible
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22
Q

What does it mean that anesthetics are “use-dependent”?

Which drugs are less use dependent?

A

The degree of inhibition is directly related to the frequency of nerve stimulation

Cytoplasmic anesthetics are charged and bind open or inactive states.
Lipophilic drugs are less use-dependent because they bind to open, closed AND inactive

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23
Q

What are the 3 states of the Na+ channels?

What states are bound by anesthetics?

A
  1. closed/resting
  2. open/active
  3. blocked/inactive

If the local anesthetic comes from the cytoplasm [aqueous environment-cation] the drug binding site is accessible only in the open or inactivated states
*most important because at physiological pH, most drug exists as cation

If the local anesthetic comes from the membrane [lipophilic environment- neutral] it can bind all three states

24
Q

Which nerve fibers are most sensitive to anesthetic block?

A

Faster = more sensitive to anesthetic block so from most sensitive to least:

Type C fibers [not myelinated]

  1. dorsal root [pain]
  2. sympathetic [postganglionic]

Type B [light myelination]
preganglionic autonomic

Type A [all heavily myelinated]

  1. delta [pain, temp]
  2. gamma [muscle spindles]
  3. beta [ touch, pressure]
  4. alpha [motor, proprioception]
25
Q

Why is it that local anesthetics are able to block pain with minimal effect on motor activity, proprioception, etc?

A

The main pain fibers are dorsal root and delta fibers both which have a fast firing rate.

Motor fibers [alpha] have the slowest firing rate

  1. use dependence, it is much more likely for pain fibers to be in inactivated, or open and thus able to be bound by anesthetics
  2. smaller pain fibers are less myelinated and thus there is a shorter length over which the action potential propogates with more closely spaced channels [more binding sites]
26
Q

Although topical anesthesia is generally not effective for healthy skin, what mixture of drugs can be used?

A

lidocaine and prilocaine form an oil that penetrates the skin

27
Q

What areas of the body are topical anesthetics typically used?
What are the 2 main drugs used?

A
  1. mucous membranes- mouth, nose, pharynx, bronchial tree, esophagus, GU
  2. wound margins [cocaine]

Lidocaine
-give with a small dose of adrenergic agonist [phenylephrine]
Cocaine:
-pro: simultaneously acting as vasoconstrictor and anesthetic
-con: increased risk of cardiotoxicity

28
Q

When is the peak effect of cocaine or lidocaine?

A

2-5 minutes after administration

29
Q

What is infiltration anesthesia?
What drugs are used?
What is co-injected with the anesthetic?

A

Injection of local anesthetic drug into target tissue or desired site of action w/o attention to the course of cutaneous nerves.
Lidocaine and bupivacaine are the most suitable agents
Co-inject of epinephrine prolongs the effect of the drug by 2 fold

30
Q

At what sites should you not use epinephrine co-injection with anesthetics?

A

Sites supplied by end arteries [fingers, toes]

since strong vasoconstriction could lead to severe tissue hypoxia and possible gangrene

31
Q

Large surgical procedures require large amounts of anesthetic at a local site so this can result in _____________. What could these surgeries benefit from?

A

Local –> too much systemic toxicity

Large surgeries benefit from a broader block

32
Q

What are the 2 preps for local anesthesia?

How are they prepared?

A
  1. single vial dose [where you add H20 or dextrose]
  2. multi-dose vials w/ and w/o epinephrine
    - prepared with chloride salts which make the solution acidic [stabilize the dissolved drug]
33
Q

What happens with repeated injections of local anesthetic to the same site?

A

Repeated injections exhaust the buffering capacity of the drug and some effectiveness will be lost

34
Q

What is IV anesthesia used for?
What are the 2 drugs used?
Describe the procedure.

A

IV anesthesia is used to anesthetize a limb

  1. lidocaine
  2. prilocaine

Procedure:

  1. inject the drug distally to a pressure cuff or tourniquet
  2. anesthesia remains effective until circulation is restored fully

Premature release of pressure [under 20 minutes] may result in systemic toxicity

35
Q

What is a field block a cross between?
What procedures is it used for?
What drugs are used?

A

It is a cross between infiltration anesthesia and nerve block.
It is used to anesthetize large cutaneous areas like procedures involving limbs, scalp and abdominal wall

It uses the same amount/type of anesthetic as infiltration anesthesia [lidocaine, bupivacaine] but is targeted to the nerve areas that serve the field of interest distal to the site of injection

36
Q

What is a nerve block anesthesia?
What nerves are blocked?
What determines the duration of anesthesia?

A

Nerve block is when you inject local anesthetic drugs in the neighborhood, large peripheral nerve bundles or nerve plexus

It blocks:

  1. sensory nerves
  2. somatic motor nerves

Duration of anesthesia depends on:

  1. exact location of injection
  2. dose of drugs
  3. physical characterisitcs
  4. use of vasoconstrictors
37
Q

What drugs are used for intermediate length nerve block?

Long duration nerve block?

A

Intermediate nerve block:
1. lidocaine
2. mepivacaine
[these can be extended by adding epinephrine]

Long duration nerve block:
1. bupivacaine

38
Q

What 2 procedures can you think of as specialized forms of nerve block?

A
  1. spinal anesthesia

2. epidural anesthesia

39
Q 
How is spinal anesthesia given?
What is the target of the anesthesia?
What surgeries is it used for?
What is the risk for systemic effects?
What are negative side effects?
A

Anesthesia is injected into the CSF in the subarachnoid space of the spinal canal in the lumbar area [below the end of the cord at L1-L2]

Target of the anesthesia is spinal cord and spinal roots and is used for surgery of the lower abdomen, pelvis and legs

There is a small change in plasma levels, minimizing the risk of systemic effects

Complications:

  1. hypotenstion - due to sympathetic block
  2. respiratory depression
  3. postural headache
  4. postop urinary retention
40
Q

Where do sympathetic fibers in the spinal nerve roots leave the spinal cord?
How does spinal anesthesia affect these nerves?

A

Sympathetic fibers leave the cord from T1-L2.

Anesthesia is injected below L2 but there is considerable upward spread which can induce sympathetic block

41
Q

What determines the height of the “upward spread” of spinal anesthesia?

A
  1. patient’s position

2. density of the drug mixture relative to CSF [baricity]

42
Q

Does upward spread of spinal anesthesia extend higher for sympathetic or sensory block?

A

Sympathetic block often extends several spinal segments higher than the sensory block due to greater sensitivity of pre-ganglionic fibers to anesthetic drugs

43
Q

What is the effect of low concentrations of spinal anesthesia on the cardiovascular system?
What can be done to partially address the problem?

A

greater vasodilation of the venous side compared to the arterial side results in poor venous return and blood pooling –> poorer organ perfusion

  1. fluid infusion
  2. maintaining 10-15degree head-down positoin to encourage venous return
44
Q

What are the 2 most important reasons to take a patients physical position into account for spinal anesthesia?

A
  1. baricity

2. venous return

45
Q

For spinal anesthesia, the general rule is to use ___________ for short procedures and __________ for medium to long procedures.

A

Lidocaine for short procedures
Bupivacaine for medium/long procedures except that it has been associated with a greater risk of cardiac side effects so it can be substituted with ropivacaine

46
Q

The ________ and _____________ determine the direction and extent of migration of local anesthetic within the dural sac. _______ settle to the bottom of the sac, ___________ will rise, ___________ will diffuse largely equally from the point of injection.

A

baricity and postion of the patient determine the direction and extent of migration

Hyperbaric will settle to the bottom
Hypobaric will rise
Isobaric will diffuse equally

47
Q

For patients with weakened cardiac or respiratory systems, is the risk of complications greater for general anesthesia or spinal anesthesia?
What about for a healthy person?

A

Risk is higher for general anesthesia if the person has cardio/resp problems

In a healthy person, general anesthesia carries very little risk and there may be a greater concern with spinal due to:

  1. risk of infection and nerve damage
  2. hypotension
48
Q

In terms of needle placement, how does spinal anesthesia differ from epidural?

A

Spinal = through the ligamentum flavum, dura mater, arachnoid mater into the subarachnoid space

Epidural= just through the ligamentum flavum

49
Q

What is the primary advantage for using epidural anesthesia?

A

a continuous infusion catheter can be placed allowing for regulated long duration anesthetic effect esp in procedures where the length of time of procedure is beyond physician control [labor]

50
Q

What drugs are used for epidural anesthesia?

A

Long acting:

  1. bupivacaine [if no cardiac probs]
  2. ropicacaine [if the patient is at increased risk of cardiotoxicity]

Shorter acting:
1. Lidocaine- can be delivered at continuous or repeated infusion through catheterization

51
Q

Why is there the potential for substantial increase in plasma concentration of local anesthetic agents during epidural injection?
What is the advantage and disadvantage?

A

Epidural space has a rich venous plexus so there is increased plasma concentration increase of local anesthestic

Advantage: acculumated acidity/loss of drug effectiveness is NOT as much of a problem as with infiltration anesthesia

Disadvantage: compared to spinal anesthesia, this has a higher chance of systemic toxicity

52
Q

What are the manifestations of CNS toxicity from local anesthetics at lose dose?
High dose?
What are the 4 main ways to avoid CNS toxicity?

A

Low dose -sleepy, light headed, visual/audio disturbance, restlessness

High dose - convulsion, CNS depression, death

Avoid by:

  1. minimal effective dose
  2. avoid accidental IV injection
  3. avoid highly vascular areas/perfused tissues
  4. pretreat with benzodiazapine to prevent seizures
53
Q

What are the direct and indirect causes of cardiovascular toxicity with local anesthetics?

A

Direct effect on cardiac and smooth muscle membrane
-pacemaker dysfunction
-vasodilation induced loss of cardiac contraction strength and dilation of arterioles
Indirect effect by interfering with autonomic nerves

54
Q

Describe the local anesthetic toxicity on:

  1. smooth muscle
  2. respiration
  3. hematologic
  4. allergic Rxn
A
  1. inhibits contractility of the bowel and intestinal smooth muscle and increases resting tone
  2. respiratory depression from excessive doses
  3. high dose prilocaine is metabolized to an agent that converts hemoglobin to methemoglobin so don’t use in resp/cardiac patients or pregnant
  4. esters are more likely to casue allergic response because they are metabolized to PABA
55
Q

What is radiculopathy?

What is cauda equina syndrome?

A

Radiculopathy- damage to the nerve root during needle placement resulting in:
pain, numbness, tingling, weakness

Cauda Equina syndrome- neuro deficit in lower extremities with numbness or bladder dysfunction

56
Q

Name the drug that:

  1. does not require co-administration of a vasoconstrictor
  2. is the most versatile
  3. carries the highest risk of cardiotoxicity
  4. is not recommended for obstetrics because of neonatal toxicity
  5. is a vasoconstrictor used with most local anesthetics
A
  1. cocaine
  2. lidocaine
  3. bupivacaine [so use ropivacaine instead]
  4. mepivacaine and prilocaine
  5. epinephrine

Nervous System (32 decks)

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