Local anesthetics

Question 1

3 parts of LA

Answer 1

1. lipophilic group: usually benzene ring
2. hydrophilic group: tertiary amine
3. intermediate bond: hydrocarbon chain

Question 2

intermediate bond

Answer 2

1. Ester: -CO-

2. Amide: -HNC-

Question 3

Ester LA

No ‘i’ prior to -caine

Answer 3

1. Procaine
2. Cocaine
3. Chloroprocaine
4. tetracaine
   pseudocholinesterase

Question 4

Amide

(contain an ‘i’ before -caine

Answer 4

1. lidocaine
2. bupivacaine
3. ropivacaine
4. mepivacaine
5. prilocaine

Question 5

Uptake: from fastest to slowest

Faster uptake–>higher plasma concentrations–>increasing chance of local anesthetic toxicity

Answer 5

Intravenous > tracheal > intercostal > caudal > paracervical > epidural > brachial plexus > sciatic > subcutaneous

Question 6

factors influencing absorption and onset of action

Answer 6

1. lipid solubility–>penetrate nerve membrane–>potency

2. pKa: lower–>more un-ionized–>speed of action

Question 7

biotransformation & excretion

Answer 7

Ester: pseudocholinesterase —>water-soluble–>urine
Amide: liver P-450–>kidney

Question 8

enhance LA effects

Answer 8

1. epinephrine: 5mcg/ml (1:200,000) to 20mcg/ml (1:50,000)–>vasoconstricting
2. opioids
3. alpha-adrenergic agonists: clonidine
4. steroids

Question 9

informed consent: brachial plexus

Answer 9

bleeding, infection, nerve damage, persistent paresthesia or weakness, shortness of breath, and local anesthetic toxicity

Question 10

paresthesia

Answer 10

a sensation of pricking, tingling, or creeping on the skin that has no objective cause

Question 11

Intralipid (20% lipid emulsion)

Answer 11

bolus of 1.5 mL/kg over 1 min–> 0.25 mL/kg/min x 30-60 min–>repeated boluses as needed for persistent asystole.

Question 12

post pain control vs surgical anesthesia

Answer 12

1. Bupivacaine & ropivacaine: long duration–>postop pain

2. Lidocaine & mepivacaine: surgical anesthesia (not post op pain d/t short duration)

Question 13

dyspnea

Answer 13

1. phrenic nerve: close to brachial plexus–>anesthetized–>unilateral diaphragm paralysis
2. pneumothorax: MC in supraclavicular
3. LA toxicity

Question 14

Increase risk of cardiovascular toxicity of bupivacaine

Answer 14

1. pregnancy: progesterone
2. hypoxemia
3. respiratory acidosis (not alkalosis)
4. hyperkalemia (rather than hypo)

Question 15

What percentage of amide LA are excreted unchanged in urine

Answer 15

<5%

Question 16

Local anesthetic systemic toxicity (LAST)<—unintended intravascular injection or absorption

Answer 16

1. intercostal > caudal epidural > lumbar epidural > brachial plexus > subcutaneous
2. Risk factors: age, cardiac disease, hepatic dysfunction, hypoxemia, and acidosis
3. CNS is more sensitive than cardiovascular system, except bupivacaine & etidocaine

Question 17

LA CNS toxicity

Answer 17

1. benign: perioral numbness, tinnitus
2. On cnetral inhibitory pathways: shivering, muscle tremors, tonic-clonic seizure
3. depressant: coma, hypoventilation, respiratory arrest

Question 18

Cauda equina syndrome (CES)

Answer 18

1. bowel & bladder dysfunction
2. perineal sensory loss
3. LE motor weakness
   Note: d/t diffuse injury to CE
   Risk factors: high dose & consentration, restricted distribution, vasoconstrictors, LA (lidociane highest)

Question 19

Transient neurologic symptoms (TNS)

Answer 19

Pain
1. uni- or bi-lateral
2. buttocks, LB, LEs
3. onset: usually 12-24 hrs after SA or EA
4. duration: 6hrs to 4 days
5. risk factors: lidocaine, lithotomy, obesity, knee arthroscopy, outpt surgery
Note: - neurologic PE

Question 20

Cardiac Toxicity

Answer 20

1. CNS excitatory–>increase: HR, BP
2. Direct cardiac–>arrhythmia, prolongation of PR & QRS, depression of SA & AV nodes, QT prolongation, VT, torsade de pintes, VF

Question 21

Regional anesthesia

Answer 21

1. performed after Std ASA monitoring

2. Available: 20% intralipid emulsion, AMBU bag, O2

Question 22

ASRA

Answer 22

www.asra.com

Question 23

Allergy to LA

Answer 23

1. Amide: extremely rare
2. Ester: cross-reactivity to p-aminobenzoic acid (PABA)–>allergy (Ester LA metabolized to PABA)
3. Preservatives: parabens

Question 24

preservatives/additives

Answer 24

1. Methylparaben: in some multi-dose formulation of LA–>paraben–>allergy
2. Metabisulfate: –>urticarial, flusing, pruritus, airway obstruction
3. EDTA: i.t. –>nerve injury; Epid–>back muscle spasm
4. Epi: 1) prolongation; 2) test dose
5. Na2CO3: faster onset

Question 25

Epinephrine test dose

Answer 25

1. epinephrine: 1:200,000 2. Intravascuar injuection 1) increased T vave 2) increased/decreased HR 3) increased BP

Question 26

Methemoglobinemia

Answer 26

1. Prilocaine-->O=toluidine in liver-->oxidize HgB 2. usu. seen >600mg 3. cyanosis refractory to O2 4. metHgb >15%--->anxiety, dyspnea, HA, weakness, n/V-->50%-->confusion, seizures, arrhythmias, hemodynamic instability, death 5. metHgb: co-oximetry 3. treated: methylene blue

Question 27

ECG w/ LAST

Answer 27

Initial ECG changes indicating cardiac toxicity from local anesthetics include peaked T waves, elevated ST segments, and a prolongation of PR and QRS intervals. Next, depression of the SA and AV nodes is observed. Finally, the QT interval is prolonged, leading to ventricular tachycardia, torsade de pointes and ventricular fibrillation.

Question 28

_____________ of amide local anesthetics is the most important factor determining clearance from plasma.

Answer 28

Protein Binding