Lecture 11.1: Local Anesthetics

Question 1

Allergic reactions to what type of local anesthetics is most common due to metabolism to allergy-causing compounds?

How can you tell the type of local anesthetic based on its name?

Answer 1

Ester-type (i.e., benzocaine, cocaine, procaine, and tetracaine)

*Will only have one‘i’ in the name; while amides will have at least two ‘i’s

Question 2

Which local anesthetics are more prone to hydrolysis and as a result generally have a shorter duration of action?

Answer 2

Ester-type

Question 3

Use of which substances will reduce systemic absorption of local anesthetics and is useful for those drugs with intermediate or short durations of action (ie., prolongation of action)?

Answer 3

Vasconstrictors (i.e., epinephrine)

Question 4

Which type of local anesthetic is widely distributed after IV bolus administration?

Answer 4

Amide-type

Question 5

How does the metabolism of ester-type local anesthetics differ from the amide-type?

Answer 5

• Ester-type are metabolized in plasma
• Amide-type are metabolized in liver and then excreted in urine as charged substances (CYP450)

Question 6

What is the MOA of local anesthetics?

Where is their receptor site located?

Answer 6

- Block voltage-gated Na⁺ channel currents and stop spread of AP’s across nerve axons

• Receptor site is at inner vestibule of the sodium channel

Question 7

What structural properties of some local anesthetics cause a faster rate of interaction with the sodium channel and more potent actions?

Answer 7

Smaller and more lipophilic

*Tetracaine, bupivacaine, and ropivacaine

Question 8

How does fiber diameter and degree of myelination affect the action of local anesthetics?

List an example

Answer 8

• Preferentially block small fibers
• Myelinated nerves tend to become blocked before unmyelinated nerves of the same diameter

*Preganglionic B fibers are blocked before smaller unmyelinated C fibers

Question 9

How does firing frequency affect the action of local anesthetics, which are preferntially blocked?

List an example

Answer 9

• Those tha fire at higher frequenciesof depolarization are blockedfirst
• i.e., type A delta and C fibers are blocked earlier than large A alpha fibers

Question 10

Which is affected first by local anesthetics injected into a bundle of large mixed nerves (sensory or motor)?

Answer 10

Motor nerves

Question 11

How does the position of sensory fibers within the extremites change from proximal to distal limbs?

Answer 11

• Proximal sensory fibers are located in outer portion of nerve trunk (will be affected first by local anesthetic)
• Distal sensory fibers are located in the core of the nerve (motor fibers will be affected first)

Question 12

What is block anesthesia and its purpose?

Answer 12

• Injection in major nerve trunks
• Purpose is the anesthetize a region distal to the injection site

Question 13

What is intravenous regional anesthesia (Bier block) used for?

How is it done?

Answer 13

• Used for short surgeries (<60 min) involving UE and LE’s
• IV injection of agent is placed in a distal vein while circulation of limb is isolated w/ a proximally placed tourniquet
• Large doses of local anesthetic used; tourniquet remains in place to prevent high circulating drug levels causing systemic effects

Question 14

Which local anesthetic potentiates the effect of NE on α-adrenergic receptors by blocking NET and results in localized vasoconstriction?

Answer 14

Cocaine

*Eliminates need for combining the drug w/ epinephrine

Question 15

Epinephrine administration to prolong the action of local anesthetics should never be injected into what tissues?

What could this result in?

Answer 15

• Those supplied by end arteries
• Fingers, toes, nose, and penis
• Vasocontriction could cause gangrene

Question 16

What are the early signs of CNS toxicity caused by local anesthetics?

Answer 16

- Circumoral and tongue numbness

- Metallic taste

Question 17

High concentrations of local anesthetics in the CNS may have what adverse effects?

Answer 17

• Nystagmus
• Muscular twitching
• Convulsions
• Death by respiratory failure

Question 18

When large doses of local anesthetics are required premedication with parenteral _____________ can provide prophylaxis against CNS toxicity by raising the seizure threshold

Answer 18

Benzodiazepine (diazepam or midazolam)

Question 19

Undesired effects of local anesthetics on the CV system are the result of direct effects on ___________ and from indirect effects on the _____________

Answer 19

Undesired effects of local anesthetics on the CV system are the result of direct effects on cardiac and smooth muscle and from indirect effects on the ANS

Question 20

Local anesthetics block which ion channels in the heart and can have what adverse effects on the heart and blood pressure?

Answer 20

• Block cardiac sodium channels
• Decrease electrical excitability, conduction rate, force of contraction
• Arteriolar dilation —> sytemic hypotension

Question 21

Cocaine is an exception to the local anesthetic effects on the heart, and instead can cause what?

Answer 21

• Vasconstriction –> local ischemia
• HTN
• Cardiac arrhythmias

Question 22

Which local anesthetic is the most cardiotoxic due to its long duration of action?

Answer 22

Bupivacaine

Question 23

Which local anesthetic is a Class Ib antiarrhythmic?

Answer 23

Lidocaine

Question 24

What are the effects of Lidocaine on the heart?

Answer 24

• Suppresses automaticity of condunction tissue by increasing electrical stimulation threshold of ventricle, His-Purkinje system, and spontaneous depolarization of the ventricles during diastole
• Blocks both the initiation and conduction of nerve impulses by decreasing the neuronal membrane’s permeability to sodium ions, which results in inhibition of depolarization w/ resultant blockade of conduction

Question 25

What is Benzocaine used for?

Answer 25

• Only as topical agent
• Dermatologic conditions, hemorrhoids, premature ejaculation, and as anesthetic lubricant (i.e., NG an endoscopic tubes/catheters)

Question 26

Bupivacaine has a tendency to provide more of a (sensory or motor) block?

Answer 26

Sensory

Question 27

Cocaine, as a local anesthetic, is used primarily how?

Answer 27

Topical anesthtic of the upper respiratory tract

Question 28

What is Lidocaine used for clinically?

Answer 28

• Alternative choice for pt’s with allergy to ester-type local anesthetics
• Antiarrythmic agent

*Faster, more intense, longer acting, and more extensive anesthesia than an equal dose of Procaine

Question 29

What is Procaine used for clinically?

Answer 29

Only for infiltration anesthesia (local anesthesia produced by injection of the anesthetic solution directly into the area of terminal nerve endings)

Question 30

Which local anesthetic is metabolized to a para-aminobenzoic acid, which inhibits the action of sulfonamide antibiotics?

Answer 30

Procaine

Question 31

Low concentrations of local anesthetics may produce which adverse effects on the CNS?

Answer 31

• Sleepiness (*CIS Q)
• Light-headedness
• Visual and auditory disturbances
• Restlessness

Question 32

Which local anesthetic is for surface use only?

Answer 32

Benzocaine

*i.e., someone comes in with bad road rash or something

Question 33

Which 3 local anesthetics have long durations of action?

Answer 33

1. Bupivacaine
2. Ropivacaine
3. Tetracaine

Question 34

Which local anesthetic has a short duration of action; used only for infiltration anesthesia?

Answer 34

Procaine