Week 6: Local Anesthetics

Question 1

Benzocaine

1. Onset and pKa
2. Use

Answer 1

1. UNIQUE: Weak acid: pKa = 3.5, very fast onset

2. Topical use only!!

Question 2

Cocaine

1. Onset and pKa
2. How does it work?
3. Use and Route

Answer 2

1. pKa = 8.6, very fast onset
2. Blocks nerve impulses and causes local vasoconstriction by inhibiting local NE reuptake
3. Local anesthesia in nasal passages when LA and vasoconstriction is desired. **Don’t use IV –> euphoria due to blockade of dopamine reuptake in the CNS

**ONLY naturally occurring LA

Question 3

2-Chloroprocaine

1. Onset and pKa
2. Duration
3. Maximum Single Dose for Infiltration

Answer 3

1. pKa = 8.7, 6-12 minutes
2. 30-45 minutes
3. 600mg

Question 4

Tetracaine

1. Onset and pKa
2. Duration
3. Maximum Single Dose for Infiltration

Answer 4

1. pKa = 8.6, 10-15 minutes
2. 60-180 minutes
3. 100 mg (topical)

Question 5

Lidocaine

1. Onset and pKa
2. Duration
3. Dose
4. Maximum Single Dose for Infiltration

Answer 5

1. pKa = 7.7, 2-4 minutes
2. 60-120 minutes
3. 1.5%-2.0% for regional blocks for surgery, 1% with epi is good for local infiltration
4. 300 mg

Question 6

Mepivacaine

1. Onset and pKa
2. Duration
3. Maximum Single Dose for Infiltration

Answer 6

1. pKa = 7.7, 2-4 minutes
2. 90-180 minutes
3. 300 mg

Question 7

Prilocaine

1. Onset and pKa
2. Duration
3. Maximum Single Dose for Infiltration

Answer 7

1. pKa = 7.7, 2-4 minutes
2. 60-120 minutes
3. 400 mg
   * *First synthetic LA

Question 8

Etidocaine

1. Onset and pKa
2. Duration
3. Downfall

Answer 8

1. pka = 7.9, 2-4 minutes
2. 240-480 minutes
3. No useful for peripheral nerve blocks because prolonged motor block outlasts sensory block

Question 9

Bupivacaine

1. Onset and pKa
2. Duration
3. Maximum Single Dose for Infiltration

Answer 9

1. pKa = 8.1, 5-8 minutes
2. 240-480 minutes
3. 175mg

Question 10

Ropivacaine

1. Onset and pKa
2. Duration
3. Maximum Single Dose for Infiltration

Answer 10

1. pKa = 8.1, 2-4 minutes
2. 240-480 minutes
3. 200 mg

Question 11

Levobupivacaine

1. Onset and pKa
2. Duration
3. Maximum Single Dose for Infiltration

Answer 11

1. Onset like bupivicaine (5-8 minutes)
2. 240-480 minutes
3. 175 mg

Question 12

Lipid Emulsion

1. Use
2. Dose
3. Upper limit

Answer 12

1. Treatment of LAST
2. 20% Lipid Emulsion Therapy
   Bolus: 1.5ml/1 minute
   Infusion: .25ml/kg/min
   **May repeat bolus 1-2x if needed or increase infusion to .5ml/kg/min
3. Upper limit: 10ml/kg over 30 minutes

Question 13

Key Points of Chloroprocaine

Answer 13

• Clorinated procaine -> more rapidly metabolized by plasmacholinesterase
• Minimal placental transfer even with 40% decrease in PC: Good for OB

Question 14

Key Points of Tetracaine

Answer 14

• Too toxic for peripheral blocks; Only used for long acting spinal blocks
• Theoretically, mixing with lidocaine can be used for peripheral blocks to give fast onset with long duration and less toxicity

Question 15

Key Points of Lidocaine

Answer 15

• Vasodilation properties

- Cardioprotective qualities from a metabolite formed by oxidative dealkyation

Question 16

Rank the Amides for Rate of Liver Metabolism

Answer 16

Prilocaine>Lidocaine> Mepivicaine> Ropivicaine>Bupivicaine

Question 17

Key Points for Mepivicaine

Answer 17

• Lacks vasodilation of lidocaine

- ***TOXIC to neonates- DO NOT use in OB

Question 18

Key Points for Prilocaine

Answer 18

• Lacks vasodilation and limits CNS toxicity
• **Can cause methemoglobinemia -> usually with doses 8mg/kg or with liver problems-> treat with methylene blue (1-2mg/kg)
• Not commonly used for peripheral blocks

Question 19

Key Points for Bupivicaine

Answer 19

• CardioTOXIC: Difficult to dissociate from Na+ channels
• Used for peripheral blocks (less than .5%), continuous infusion (less than .1%) with opioids, and can be used in OB (less than .25%)

Question 20

Key Points for Ropivacaine

Answer 20

• S-enatiomer of bupivicaine

- VERY popular for peripheral blocks

Question 21

Metabolism of Amide LAs

Answer 21

Metabolized in the liver by deakylation of an ethyl group from the tertiary amine and hydroxylation
**Hepatic blood flow and liver function are important
Renal clearance of unchanged LA is about 3-5%

Question 22

Metabolism of Ester LAs

Answer 22

Hydrolyzed by circulation pseudocholinesterase (plasma cholinesterase)
PC is made in the LIVER:
Affected by liver disease, high BUN, pregnancy, chemo drugs, atypical PSE, genetic makeup

Question 23

Blood Flow and LA Absorption Rates From Fastest to Slowest

Answer 23

1. Intravenous
2. Tracheal
3. Intercostal
4. Caudal
5. Paracervical
6. Epidural
7. Brachial Plexus
8. Subarachnoid, Sciatic, Femoral
9. Subcutaneous

*I Think I Can Push Each Bolus SSlowly For Safety

Question 24

Adverse Effects of LA

Answer 24

• Allergic reaction
• Systemic Toxicity
• Neural Tissue Toxicity
• TNS (transient neurologic syndrome)
• Cauda Equina syndrome
• Anterior spinal artery syndrome
• Methemoglobinemia

Question 25

Allergic Reaction with LA

Answer 25

- Rare -> usually symptoms from systemic toxicity - If present, most often to the preservative: METHYLPARABEN - Esters> Amides for true allergy with NO cross sensitivity between classes

Question 26

Systemic Toxicity

Answer 26

LAST: Starts with CNS symptoms: drowsiness, paresthesias in the mouth and tongue, tinnitus and auditory hallucination Then Muscular spasm Then seizures, coma, respiratory arrest, and cardiac arrest **as patient gets worse, more acidotic -> makes LAST worst

Question 27

Neural Tissue Toxicity

Answer 27

Nerve damage: Very rare event with epidural and spinals | **Lidocaine more concerning in the spinal because increased concentrations of Ca+ may be mechanism for toxicity

Question 28

TNS

Answer 28

Transient Neurologic Syndrome: Pain in low back, butt for 6-36 hours after spinal **Lidocaine implicated Often resolves in 7 days after treatment with NSAIDS

Question 29

Cauda Equina Syndrome

Answer 29

Large sensory, bowel, bladder dysfunction and paraplegia Implicated with 5% lidocaine with patient in lithotomy position Could be from LA pooling at the nerves Can be serious, but most often presents as urinary retention

Question 30

Anterior Spinal Artery Syndrome

Answer 30

LE paresis with variable sensory deficit after resolution of regional Maybe caused by thrombus or spasm of anterior spinal artery *Epi is implicated, more prevalent with elderly and PVD Presents similar to epidural hematoma

Question 31

Methemoglobinemia

Answer 31

Oxidation of Hb to MetHb **Implicated with benzocaine, prilocaine, lidocaine Treat with methylene blue (1-2mg/kg) Don't see symptoms until 15% are converted to MetHb

Question 32

Cocaine Toxicity

Answer 32

Symptoms: coronary vasospasm, MI, dysrhythmias | - Can be seen up to six weeks after use

Question 33

Key Points for Tumescent Technique

Answer 33

- Subcutaneous placemet of large volumes of diluted Lido with epi - Tumescent lido with epi mas = 35-55mg/kg because 1 gm of fat absorbs 1 mg lido and acts as a tissue buffering system - Complications: Lido toxicity, cardiac depression and decreased contractility

Question 34

What is the order of loss of nerve sensations?

Answer 34

1. Autonomic functions 2. Pain 3. Cold 4. Warmth 5. Touch 6. Pressure 7. Vibration 8. Proprioception - ------ Want to be here --------------- 9. Motor function (ALWAYS LAST)

Question 35

Three factors that determine duration of block

Answer 35

1. Lipid solubility 2. Vascularity of tissues 3. Presence of vasoconstrictors (prevent vascular uptake of the LA molecules)

Question 36

Type A Nerve Fibers 1. Function 2. Myelination 3. Sensitivity to Block

Answer 36

1. Fast afferent and efferent to muscles for pain, tactile, proprioception 2. Myelinated 3. Alpha are least sensitive -> delta are most sensitive of type A fibers

Question 37

Type B Nerve Fibers 1. Function 2. Myelination 3. Sensitivity to Block

Answer 37

1. Pre-ganglionic autonomic 2. Some myelination 3. More resistant to blocks

Question 38

Type C Nerve Fibers 1. Function 2. Myelination 3. Sensitivity to Block

Answer 38

1. Slow, dull pain 2. Non-myelinated 3. More sensitive to block