Local Anesthetics

Front 1

Lidocaine
Amide of Ester?
Clinical Use?
Problems?
Misc?

Back 1

Lidocaine - Amide

Clinical Use:
1. Topical
(2‐10%)
2. 5% 
patch 
for
 postherpetic
neuralgia/neuropathic


3. 
Infiltration
(moderate)
 ‐
IV 
regional 
(w/o 
epinephrine)

4. 
Peripheral
 nerve 
block
 (short)
 ‐ 
central 
neural
 block

5. Spinal 
anesthesia (short)


Problems:
1. Cardiovascular depression (in doses producing CNS toxicity)
2. vasodilation


MISC
1. Eutectic
 mix
 with 
prilocaine 
(low
 melting-
point) 
allows
deeper
 topical 
penetration
(5
mm)

Front 2

Mepivacaine
Amide or Ester?

Back 2

Problems:
Toxic to Neonates

Front 3

Bupivacaine
Amide or Ester
Clinical use?
Problems?
Misc?

Back 3

Bupivacaine - Amide

Clinical Use:
1. 
Infiltration 
(long)

2. 
peripheral 
nerve
 block 
(moderate)

3. 
central 
neural 
block 
(long)

4. spinal
 anesthesia 
(long)

- Essentially LONG ACTING

Problems
1. More cardiotoxic than Lidocain
2. Not used in OB due to toxicity to baby
3. Not topical (will absorb systemically)

Front 4

Ropivacaine
Amide or Ester?
Clinical use?

Back 4

Ropivacaine - Amide

Clinical use:
1. Less cardiotoxic than Bupivacine
2. Long Acting

Cannot be use topically due to systemic absorption

Front 5

Prilocaine
Amide or Ester?
Clinical Use?

Back 5

Prilocaine - Amide

Clinical use:
1. IV 
regional 
(alternative
 to
 lidocaine - less
 vasodilation)

2. 
Peripheral
 nerve 
block

3. Central 
neural 
block


Problems
1. Methemoglobinemia

Front 6

Procaine
Amide or Ester
Clinical Use?
Problems?

Back 6

Procaine - Ester

Clinical Use:
1. Infiltration (short)

Problems:
1. slow
 onset
 and short duration
2. Post‐op 
nausea 
(17%)

3. Metabolized to PABA which inhibits sulfonamides

Front 7

Cocaine
Amide or Ester?
Clinical Use?
Problems?

Back 7

Cocaine - Ester

Mechanism:
1. Inhibits reuptake of Norepinephrine - causing vasoconstriction
2. Blocks Sodium channels

Clinical Use:
1. Topical 
(1‐4%) 
in 
upper 
respiratory 
tract

2. 
Surgical
 EARS,
 NOSE, THROAT
3. Vasoconstrictor

Problems:
1. Abuse potential
2. May get systemic absorption

Front 8

Benzocaine
Amide or Ester?
Clinical Use?
Problems?

Back 8

Benzocaine - Ester

Clinical Use
1. Most commonly used over the counter (OTC) topical
2. Low solubility

Problems:
1. Methemoglobinemia

Front 9

Tetracaine

Back 9

Tetracaine - Ester

Clinical use:
1. 
Topical 
(2%)

2. Peripheral 
Nerve
 Block 
(long)

3. 
Spinal 
anesthesia
 (long)


Problems
1. Slower metabolism
2. More systemic Toxicity

Front 10

Undesirable Effects/Toxicity of Local Anesthetics

Back 10

Undesirable Effects/Toxicity or Local Anesthetics

1. CNS
- Stimulation (restlessness, tremor, convulsions) → Depression (Drowsiness, sedation, respiratory failure)
2. Cardiovascular system
- Heart – Decreased conduction, Decreased force of contraction, Decreased excitability
3 Rarely, low concentrations may cause cardiovascular collapse
- Bupivacaine is more cardiotoxic than lidocaine
- Vascular smooth muscle – relaxation → Decreased BP
4. Smooth Muscle
- Decreased contraction of bowel and uterine smooth muscle
5. Hypersensitivity Reactions:
- Dermatitis, Asthmatic attacks (Primarily seen with esters, but occasionally with amides), also reaction to vasoconstrictors
6. Nausea – Procaine has a 17% incidence of postoperative nausea

Front 11

Treatment of Systemic Toxicity

Back 11

Treatment of Systemic Toxicity with Local Anesthetics

1. Stop injection
2. Provide Oxygen
3. Tracheal intubation and control of ventilation if necessary
4. Suppress seizure activity – thiopental or midazolam
5. Treat ventricular dysrhythmia and CVS support

Front 12

Factors affecting the usage of local anesthetics?

Back 12

Factors affecting the usage of local anesthetics:
1. The time of onset is affected by the pKa of the substance
2. The potency of a local anesthetic is determined mainly by the lipid solubility of a substance
- In general, the more hydrophobic, the more potent the substance.
3. The duration of action is determined by protein binding and lipid solubility

Front 13

Where are Amides metabolized?
Where are Esters metabolized?

Back 13

Amides are metabolized in the liver - degraded by hepatic cytochrome P450s (problem with patients with severe hepatic disease)

Esters are metabolized in plasma by plasma esterases (Plasma cholinesterase)
- Note that esters also have the PABA metabolite (procaine) - thus they are more likely to cause allergic reactions than sulfonamide

Front 14

Infiltration Anesthesia

Back 14

Infiltration Anesthesia

- Intradermal or subcutaneous injection
- Anesthesia without disrupting normal bodily functions
- Epinephrine will prolong the duration of action (never use in tissues supplied by end arteries - e.g. fingers, toes, ears, penis)
- Patient may experience pain immediately after injection
- Useful for minor surgery

Short Duration: Procaine
Moderate Duration: Lidocaine, Prilocaine
Long duration: Bupivacaine

Note that Cocaine and Benzocaine are typically not used for infiltration anesthesia.

Front 15

Intravenous Regional Anasthesia

Back 15

Intravenous Regional Anesthesia

- Intravenous administration into tourniquet occluded limb
- Use is limited by potential risk (systemic circulation, tourniquet pain)

- Cardiovascular collapse has been reported with Bupivacaine

- Lidocaine without epinephrine is most frequently used
-- Prilocaine causes less vasodilation so it may be even better.

Used primarily for surgeries of the forearm and hand

Front 16

Peripheral Nerve Blockade

Back 16

Peripheral Nerve Blockade

- Used to produce greater areas of anesthesia than infiltration

Determinants of onset include:
- Proximity of injection to nerve (never inject directly into the nerve)
- Concentration of drug volume injected
- Degree of ionization (sodium bicarbonate can be added to raise pH and speed onset)

- Lidocaine, Bupivacaine (Lidocaine has significantly shorter duration of action than Bupivacaine)

Front 17

Central Neural Blockade

Back 17

Central Neural Blockade - Either epidural or spinal (intrathecal)

- Can block autonomic nervous system (vasodilation) sympathetic blockade!
- Epidural associated with greater incidence of systemic toxicity because more drugs used for longer period, more risk of systemic effects.

- Baricity of solution determines direction of migration of drug

Complications
- Infection, hematoma, trauma to cord, and postural headache

Duration of action of lidocaine is shorter than bupivacaine < tetracaine

Front 18

Epidural Anesthesia

Back 18

Epidural Anesthesia

Bupivacaine is used for long duration surgeries
- Cardiotoxicity is a concern

Front 19

Spinal Anesthesia

Back 19

Spinal Anesthesia
- Valuable during surgery involving the lower abdomen, lower extremities, and perineum

Procaine and Lidocaine are short duration
Bupivacaine is longer duration